This document is part of a series of airhead electrical system documents. You can find the others here.

• Electrical Basics
• /5 Series Electrical Components
• /5 Series Circuits
• /6 Series, 1975-76 Electrical Components
• /7 Series, 1977 R100RS Circuits
• /7 Series Electrical Components (See: /6 Series, 1975-76 Electrical Components)

This document describes the various electrical circuits used in the BMW /6 Series motorcycles and shows the wiring connections. Rather than start with a completed wiring diagram showing all circuits and wires, I construct the wiring for each circuit one at a time. I think this makes it easier to understand a circuit and how it fits within the entire wiring system.

I include a number of references in the Resources section containing much more detail about how the /5 & /6 Series components work and the differences.

Resources

Here is a list of resources I used to help me prepare this material.

• Chicago Regional BMW Owners Association (CHITECH): Electric Manual: R-Models, 1955 — 1990
  [A well respected guide to airhead electrical systems]
• Robert Fleischer: Critique of the Chitech BMW Electric School Manual
  [Robert’s review of the CHITECH Electrical manual with many useful notes]
• Robert Bosch: DIN Terminal Codes
  [Explanation of the DIN terminal numbers used to identify the purpose of electrical terminals]
• Robert Fleischer: Metric & American Wires, Colors, Bosch wire & Connection Codes, Sources & Wiring, Schematic Diagrams
  [American vs. DIN wire sizes, DIN colors and links to wiring diagrams]
• Karl Seyfert, MOTOR Magazine: Understanding European DIN Wiring 
  [More about DIN electrical standards]
• Haynes-Wiring Diagrams: BMW 2-valve Twins, ’70 to ’96
  [I used images from Haynes electrical diagrams to show my circuits and schematics of components]
• Duane Auscherman: BMW Motorcycle /5 Electrical
  [A nice collection of documents about airhead electrical systems and components]
• Duane Auscherman: Electrical Service Bulletins
  [A helpful collection of BMW service bulletins pertaining to airhead electrical systems and components]
• Robert Fleischer: Electricity 101+ for BMW Airhead Motorcycles
  [Electricity and electrical system fundamentals explained]
• Robert Fleischer: Electrical Hints, Problems, Fixes
  [A series of notes to help diagnose airhead electrical problems}
• Robert Fleischer: Headlight Switches & Relay Operations
  [Details about various headlight switch and relay combinations used in airheads]
• Robert Fleischer: The Alternator Charging System
  [Details about the versions of the airhead charging systems and components]
• Metroplex Alternator & Starter: Charging System Operation 
  [A treasure trove of information about charging systems. Clear, concise and very understandable]
• Anton Largiader : Airhead Alternators
  [Another useful resource about the airhead charging system]
• Robert Fleischer: Starting and Starter Motor Problems
  [The starting system and motor including Bosch and Vario]
• Robert Fleischer: The Slash 5 (/5) starter relay ‘cricket’ noise & starter problem
• Robert Fleischer: Diode Boards & Grounding Wires On BMW Airhead Motorcycles [Details about how they work, changes in design and issues seen]
• Robert Fleischer: Ignition (System)
  [Details about how the various airhead and third party ignition systems work]
• Robert Fleischer: How Spark Plug Ignition Systems Work
  [Details about how ignition systems work]
• Robert Fleischer: Bosch Metal Can Mechanical Voltage Regulator, Clean & Adjust
  [Good detail on internal design, how to identify failure, how to fix, how to adjust]
• Robert Fleischer: Headlight switches & relays operations
  [Emphasis on the various wiring designs for the headlight switch and headlight relay starting with early /6 models. Minor amount about the /5 models]
• Electronics Tutorials:
  [Details about how transistors work and how they are used as “switches”]
  • Bipolar Transistor
  • NPN Transistor
  • PNP Transistor
  • Transistor as a Switch
• Airheads Beemer Club: (Requires membership (JOIN) to read full content)
  [Various articles written by members about electrical system & components]
  • Tech Tips (See TT-12 Engine Electrical System and TT-61 General Electrical System)

NOTE:
As a convention, I use BOLD CAPITAL LETTERS to indicate a solid wire color and use the same color for the letters. If the wire has a stripe, I use bold Initial capital letter for the stripe color with the letters the same color as the strip, e.g., RED, GREEN–Red.

Links to sections in this and other documents are shown with Blue Bold Underline.

What Makes a Closed Circuit?

Although all the electrical components are wired together and to the battery, no electrical current flows through the wires or components when the ignition switch is turned off. That’s because there isn’t a complete path, or closed circuit, that goes from the (+) battery terminal through the wires and components of the circuits back to the (-) battery terminal, or ground.

NOTE:
Okay, I kinda lied didn’t I? On models with a clock, electrical current is flowing to it all the time as long as the battery is charged. But with that exception, no current flows thought the rest of the wires and components when the ignition is turned off. Only the R90/S came standard with a clock, but it can be added to of the /6 series bikes as an option.

For electrical current to flow, there has to be a continuous conducting path from the battery (+) to the battery (-), or ground terminal. Beside the ignition switch, there are other switches, and relays inside various components, that are normally “off” so current can’t flow through the component.

A relay is an electrically operated switch that connects and disconnects two, or more, external terminals of a component so current can flow between them. Frequently, a relay is used to create a complete path, or closed circuit, between the (+) and (-) battery terminals. When the ignition is off, no current can flow to the switching part of these relays so they can’t complete the path to the (-) battery terminal. Therefore when the ignition switch is off, all the circuits become open circuits.See the relays section of the components document for details about how relays work.

/6 Series Wiring Diagram Components

You can find detailed descriptions of the various electrical components in the 5 Series Electrical Components document. Many of them are the same as those used in the 6 Series. Those that changed in the 6 series include the following:

• Turn Signal Relay
• Alternator (more power)
• Left Combination Switch
• Right Combination Switch
• Instruments and Instrument Lights
• Instrument Cluster Flex-board
• Headlight Relay (added for /6 models)
• Starter Relay (removed transistor starting interlock function)
• Connector Block Inside Headlight Shell (added for /6 models)

Let’s start with a wiring diagram for the 6 series bikes that doesn’t show the wires, just the electrical components. I extracted this image, and all the others in this document, from the Haynes manual.

NOTE:
It’s easier to follow along if you click the picture above to enlarge it. When it opens, click once to magnify. You can use click to adjust the magnification. Move your mouse to navigate within the enlarged view.

Note in the upper right hand corner I show the black wires connecting the points, condenser, coils and spark plugs, and you will see some wire stubs at the terminals of various component. But, it’s pretty much a blank sheet that I will fill with the appropriate wires for each circuit so it’s easier to see what goes where. As I describe the circuit, you will see the logic behind the terminal numbers and wire colors.

Connector Block Inside Headlight Shell

The /6 series added a wiring Connector Block inside the headlight shell. The Connector Block organizes the wiring system.

As shown above, each section of terminals on the Connector Block is color coded to match the wire color of the wires that are attached to the terminals and includes the terminal number for the wires. For example, RED wires are connected to terminal (30) per the DIN standards. You can see there is a block of four terminals in the RED section of the Connector Block marked (30). 

NOTE:
All the Connector Block terminals of the same number are connected together on the backside of the board. So any terminal in the same numbered section of the Connector Block acts like any other terminal and all of them act like a single wire.

The top section of the Connector Block has two sections labeled (15) and (15u) with a 8 amp fuse between them. The (15) section is GREEN and the (15u) section is GREEN-Black. The terminals in the GREEN section are connected through the fuse to the terminals in the GREEN-Black section. The Black stripe indicates the wire is after a fuse.

And at the bottom left side of the Connector Block, there is another section marked (15u) with GREEN–Blue wire colors. These terminals are NOT the same as the (15u) section at the top right corner of the Connector Block. The terminals in the GREEN–Blue (15u) section are not after the fuse. They are part of the kill switch wiring. Don’t get confused if you are poking around inside the headlight shell.

That said, each section of the Connector Block shows the wire colors as well as the section number, so be sure the wire color you are plugging in matches the colors shown in the section of the Connector Block you plug it into.

NOTE:
The Haynes manual shows these terminals as (15a) which is actually the GREEN-Blue section (15u). Section (15a) is nowhere to found on the Connector Block section labels. There are other mistakes in the Haynes and Clymer wiring diagrams, so be cautious.

Just underneath the top (15)-(15u) section are two sections labeled (58) and (58u) with GREY and GREY-Black color code respectively. There is another 8 amp fuse between them.  The terminals in the GREY section are connected through the fuse to the terminals in the GREY-Black section.

NOTE:
Most wiring diagrams do not show the (15u) or (58u) designation for terminals on the Connector Block. Instead, the GREEN–Black (15) and GREEN (15u) terminals are all shown connected to terminal section (15) of the Connector Block and the GREY–Black (58) and GREY (58u) terminals are all shown connected to the terminal section (58). 

Also, the wiring diagrams shows the (15u) and (58u) terminals on the left side of the Connector Block, but they are located on the right side of the actual Connector Block.

Don’t let these differences between the wiring diagram and the actual layout of the Connector Block confuse you.

Instrument Cluster Flex-board

The /6 series added a flex-board Inside the instrument cluster that provides connections to the bulbs inside the cluster via a plugable connector in the main wiring harness. The back of the instrument cluster has set of 12 pins that connect to a large rubber connector with 12 sockets. Not all the pins-sockets are used. This diagram below shows wire colors, pins-sockets and the paths on the flex-board to the light bulbs inside the instrument cluster.

This table shows pin number, wire color and purpose of each pin/socket of the Flex-board.

PIN    Wire Color            Purpose                                        
1        BLACK–White       Turn Signal Indicator Power (+)
2        BROWN–Green    Oil Pressure Switch Ground Path (-)
3        GREY-Black         Speedometer-Tachometer Illumination Power (+)
4        BLUE                    Alternator Power from Diode Board (+)
5        BROWN–Blue       Low Brake Fluid Switch Ground Path (-)
6        BROWN-Black     Neutral Switch Ground Path (-)
7        BROWN                Speedometer-Tachometer-High Beam-Turn Signal Ground (-)
8        WHITE                  High Beam Indicator Power (+)
9        NOT USED
10      NOT USED
11      NOT USED
12     GREEN–Blue Brake Fluid-Neutral-Alternator-Oil Pressure Power (+)
NOTE: The diagram above mistakenly shows this wire as
GREEN-Black.

NOTE:
Copper foil inside a plastic membrane is used to create an electrical path between the bulbs and the pins. The exposed foil folds over the edge of the bulb socket holes and contacts the terminals on outside of the bulb holders. The foil breaks over time and can cause intermittent lighting and/or failure of one or more bulbs. You can repair the foil or there is a very nice replacement for the entire Flex-board with LED bulbs available from KAT-DASH. (http://katdash.com/)

RED Battery (+) Wires

DANGER:
Should you be poking around next to any of the RED wires with a screw driver, socket driver or other metal tool AND happen to touch any exposed terminal (30) with a connected RED wire and the frame or the grounded case of a component at the same time, ALL the power in the battery will immediately flow through your tool. You will melt tools, wires and potentially destroy components. That is why you should remove the battery (-) ground cable from the tachometer drive bolt on the transmission BEFORE you poke around next to the electrical contacts and wiring with metal tools.

You can do a lot of component fault isolation WITHOUT having power connected to it. In those cases where you need power when testing a component, such as a relay, be very careful to avoid touching a terminal AND a ground wire or any metal on the frame, engine, transmission, etc. with the battery (-) terminal connected to the tachometer drive bolt on the transmission.

Of course, if you have a volt meter, you can connect it between any terminal (30) and a ground to get a voltage measurement without fear of damage.

By convention, current flows from the (+) battery terminal and back to the (-) battery terminal (called the ground) to make complete a circuit. If there is not a complete circuit, no current flows through a wire

NOTE:
Historically, and incorrectly, it was assumed electricity flowed from the (+) to (-) terminals of a battery. This direction of electric current flow is called “conventional current flow”. Later, electrons were discovered, and it was shown they are what moves when an electric current flows in a wire, so the actual direction of “electron flow” is from (-) to (+).  I use conventional current flow [(+) to (-)] when I describe how dc circuit flows in the wiring diagrams. 

So, the place to start is by drawing the wires connected to the (+) battery terminal.  Notice that any component terminal that connects to the (+) battery terminal is identified as terminal (30) according to the DIN standard for terminal numbers. Also, RED insulation in the DIN standard indicates a wire is directly connected to the (+) battery terminal.

NOTE:
In a one case, the starter solenoid, a large BLACK wire directly connects the (+) battery terminal to the terminal on the starter solenoid. Other than that, all direct paths to the (+) battery terminal use a RED wire.

Another deviation is some after-market alternator-to-diode board cables for the (U, V, W) alternator phases include a RED wire, but it is not directly connected to the battery (+) terminal.

Battery (+), Starter Solenoid & Diode Board RED Wires

The diagram below shows two of the wires attached to the (+) battery terminal.

NOTE:
It’s easier to follow along if you click the picture above to enlarge it. When it opens, click once to magnify. Move your mouse to navigate within the enlarged view.

There is a large BLACK cable from the battery (+) terminal to the screw terminal on the front of the starter solenoid. There is also a smaller RED wire on that terminal the goes to one of the two male spade terminals which is on the left side of the diode board under the front engine cover. These two male spade terminals that are (30/B+) and are common so they act as a single wire.

Diode Board, Starter Relay & Connector Block RED Wires

Here are a couple more of the battery (+) RED wires coming from the diode board terminal (30/B+).

NOTE:
It’s easier to follow along if you click the picture below to enlarge it. When it opens, click once to magnify. Move your mouse to navigate within the enlarged view.

NOTE:
BMW uses the two Starter Relay (30) terminals as a common connector to distribute the current flow from the battery (+) terminal to all the other circuits on the bike. There is an internal jumper wire inside the Starter Relay that bridges the two terminal (30) male spades. That jumper can fail and if it does, the bike will loose all power since the current flowing from the battery (+) terminal to the first terminal (30) can’t get to the second and onward to section (30) on the Connector Block inside the headlight shell.

Corrosion on the starter relay terminals that can cause intermittent failure. If brake fluid leaks from the master cylinder reservoir mounted on the spine tube directly above the starter relay, the fluid can cause intermittent connections and terminal corrosion.

Connector Block, Ignition Switch & Light Relay RED Wires

As shown in the diagram below, A RED wire from section (30) on the Connector Block inside the headlight shell goes to terminal (30) of the ignition switch inside the headlight shell. A second RED wire from ignition switch terminal (30) goes to terminal (30) on the headlight relay which is also inside the headlight shell. Since these two RED ignition switch wires attach to the same ignition switch terminal, they act as a single wire.

NOTE:
It’s easier to follow along if you click the picture above to enlarge it. When it opens, click once to magnify. Move your mouse to navigate within the enlarged view.

Connector Block to High/Low/Flash Switch RED Wire

In the diagram below, another RED wire in section (30) on the Connector Block inside the headlight shell goes to the (30) terminal of the left side combination switch on the handlebar.

Since the RED wire goes to the High/Low/Flasher switch in the left side combination switch on the handlebar, it has power when the ignition switch is off. This is done so you can flash the high beam with the ignition off.  See the [Headlight High/Low/Flash Switch High-Beam Flasher] section below for details about the high beam flasher wiring, and how later versions of the combination switch do not support high beam flash unless the ignitions switch is ON.

Ignition Switch ON Circuit

When the ignition switch is turned on (not to the park position), it’s terminal (30) connects to terminal (15) and terminal (56) via the internal switch contacts inside the ignition switch.  I show the internal connections by the RED path inside the ignition switch in the diagram below.

NOTE:
It’s easier to follow along if you click the picture above to enlarge it. When it opens, click once to magnify. Move your mouse to navigate within the enlarged view.

When the ignition switch is on, the power from the RED wire at terminal (30) goes to the GREEN wires connected to terminal (15) and terminal (56) of the ignition switch. Therefore, when the ignition switch is turned on, the GREEN wires are directly connected to the battery (+) terminal AFTER the ignition switch. But they carry no current after the ignition switch is turned on unless there is a path back to the (-) battery terminal. I refer to the path back to the (-) battery terminal as the ground path.

According to the DIN terminal standard, terminal (15) is “switched positive after the battery (ignition switched output)”.  So, whenever you see terminal (15) on a component, you know current can only flow to that terminal when the ignition switch is turned on. And, it will have a Green wire(s) attached to it.

The GREEN wire connected to ignition switch terminal (15) goes to section (15) of the Connector Block inside the headlight shell.  The GREEN wire connected to ignition switch terminal (56) goes to terminal (56) of the left combination switch mounted on the handlebar. This is the input to the yellow headlight OFF-PARK-ON flip switch that turns on the parking or headlights. I’ll cover how the GREEN wire connected to terminal (56) of the left combination switch works with the headlight OFF-PARK-ON switch later.

Kill Switch Wiring

The kill switch stops the engine from running, prevents the starter from working and prevents a number of instrument cluster indicator lights from working. It was not used on the /5 series models.

As shown in the diagram below, another GREEN wire from section (30) of the Connector Block inside the headlight shell goes to the kill switch, which is a red handled flip switch, inside the combination switch mounted to the right handlebar. A GREEN–Blue wire comes out of the kill switch and goes to section (15u) of the Connector Block.

NOTE:
The Haynes manual wiring diagram incorrectly labels the section of the Connector Block that has the GREEN–Blue wires as (15a); it is actually section (15u). 

Another confusion factor is the Connector Block has two (15u) sections. The one with the GREEN-Blue wires is at the bottom of the Connector Block. The other (15u) is at the top left side of the block and has GREEN–Black wires.

And another source of confusion is that the (15u) and (15) sections at the top of the Connector block are shown in the Haynes Manual as section (15) with GREEN and GREEN–Black wires, which is incorrect. There are a number of labeling errors in the Haynes wiring diagrams.

When the kill switch is in the ON position (centered), power goes into it from the GREEN wire and out of it via the GREEN–Blue wire. When the kill is in the OFF position (flipped up or down), no power goes through the kill switch so the engine, starter and some instrument cluster indicator bulbs don’t operate.

A second GREEN–Blue wire connected to section (15u) of the Connector Block inside the headlight shell goes to terminal (86) of the Starter Relay. A second GREEN–Blue wire is connected to starter relay terminal (86), so both wires act as if they are a single wire. The second GREEN–Blue wire goes to a terminal on the left side coil to power the coils and the ignition system. When the engine starts, the points provide the path back to the (-) battery terminal completing the circuit so current flows through the coils when the points are closed and stops flowing through the coils when the points open.

If the kill switch is OFF, there is no power to the coils so the engine can’t run.

Instrument Cluster Bulbs (Charging, Neutral, Oil Pressure, Brake Fluid) Wiring

Another GREEN–Blue wire comes off the GREEN–Blue wire to Starter Relay terminal (86) that is part of the kill switch circuit to provide power to the charging, neutral, oil pressure and low brake fluid bulbs in the instrument cluster as shown in the diagram below.

NOTE:
If the kill switch fails, the ignition system and the charging, neutral, oil pressure and low brake light fluid lights in the instrument cluster aren’t powered. If all these lights go out and the bike won’t start when the ignition is turned on, either the kill switch is OFF or it has failed. 

An emergency repair can be made. Find the GREEN and GREEN–Blue wires going to the kill switch inside the headlight shell from the cable that comes into the headlight shell from the right side control switch. Remove them which will expose a GREEN terminal and a GREEN–Blue terminal on the Connector Block. Connect a jumper wire to the GREEN and GREEN–Blue terminals. This by-passes the kill switch. Now power will go to the coils and the instrument bulbs when you turn on the ignition key and the bike will run.

Oil Pressure Bulb Ground Path Wiring

The ground path for the oil pressure bulb is shown in the diagram below. A BROWN–Green wire goes from the other terminal of the oil pressure bulb to the oil pressure switch mounted on the left side of the engine block.

The oil pressure switch case acts as the ground path via the engine and transmission back to the (-) battery terminal completing the circuit.

Neutral Bulb Ground Path Wiring

The diagram below that shows the Neutral Switch path to ground. I removed the BROWN–Green wire from the oil pressure switch for clarity .

A BROWN-Black wire goes from the other neutral bulb terminal to terminal section (85b) on the Connector Block. A second BROWN-Black wire goes from another terminal in section (85b) to the neutral switch on the rear of the transmission. A BROWN wire from the other neutral switch terminal goes to frame ground that provides a path to the (-) battery terminal.

NOTE:
Terminals on the left side of terminal section (85b) in the diagram above have BROWN–Yellow wires and they are connected to those on the right side of section (85b) that have BROWN–Black wires through a Diode as shown on the Haynes Manual diagram. But, on the Connector Block, there is a section labeled (LKK). The LKK terminals have the BROWN–Yellow wires shown on the right side of terminal section (85b) in the Haynes Manual diagram. So, the Haynes Manual diagram doesn’t explicitly call out the LKK section terminals. 

I’ll explain the purpose of the diode when I discuss the Starter Interlock switches later.

When the transmission is in neutral, the neutral switch is on so the bulb lights, and when the transmission is not in neutral the neutral switch is off and the bulb goes out.

NOTE:
There was a change in the transmission shift cam plate starting in 09/1975, or the 1976 model year. The 1974-75 cam used a hill at neutral to push the plunger of the switch to close the switch and complete the circuit. So, this switch is a Normally Open switch. When the cam moves from neutral, the switch plunger extends all the way out and opens the switch turning off the neutral light. In 1976 the shift cam plate changed so neutral used a valley and the neutral switch was changed to a Normally Closed switch. When the transmission is in neutral, the switch plunger extends all the way closing the switch and completing the circuit. When the cam moves away from neutral it pushes the plunger into the switch opening the switch turning off the light. If you install the wrong neutral switch the neutral light will not work correctly.

Charging Indicator Bulb BLUE Wire Path

I removed the BROWN–Green wire that goes to the oil pressure switch and the BROWN-Black wires used with the neutral switch to simplify the diagram below. It shows the path of the Blue wire to the alternator charging, or “GEN” bulb.

NOTE:
The Blue wire acts like a ground path in that it completes the circuit for the charging indicator bulb so it can light, but it’s not actually a direct path to the battery (-) terminal, as explained below.

The BLUE wire from the other side of the bulb doesn’t go directly to ground. Instead, it ultimately goes to the diode board so it can carry the output dc voltage from the alternator. The reason for this is that a bulb will not light if there is no voltage difference across it. So, when the alternator is producing enough power, the voltage on the Blue wire becomes (+) 12v which is the same voltage as the GREEN–Blue wire connected to the other bulb terminal, and the alternator charging light goes out.

As shown in the diagram above, the BLUE wire from the alternator charging bulb first goes to the (D+) terminal of the Starter Relay. A second BLUE wire from the Starter Relay (D+) terminal goes to the (D+) terminal of the Diode Board. A second wire from the (D+) terminal of the Diode Board goes to the (D+) of the Voltage Regulator. I’ll explain how the Voltage Regulator works later. The alternator dc output voltage comes from the (D+) terminal of the diode board and flows back to the charging light bulb.

Brake Fluid Low Level Bulb Ground Path Wiring

I removed all the wires for the ground path of the other instrument bulbs so I can show the ground path for the brake fluid level bulb more clearly in the diagram below.

The BROWN–Blue wire from the other brake fluid level bulb terminal goes to the brake fluid level switch mounted on the frame spine tube under the gas tank. The other terminal of the brake fluid level switch has a BROWN wire that goes the frame ground providing the path back to the (-) battery terminal.

NOTE:
The brake fluid level indicator bulb is required since the fluid reservoir is out of sight underneath the gas tank. Adding the fluid level switch to the reservoir gives notice when the fluid level has dropped too low in the reservoir. Starting in 09/1980, or the 1981 model year, the brake fluid reservoir was moved to the right handlebar lever, like most other manufacturer’s did, and the brake fluid indicator bulb was removed from the instrument cluster.

Headlight OFF-PARK-ON Switch & Relay Wiring

The yellow headlight OFF-PARK-ON switch is integrated into the left side combination switch mounted on the handlebar. When the ignition switch is turned on, power flows to the headlight OFF-PARK-ON switch via the GREEN wire connected to terminal (56) of the headlight OFF-PARK-ON switch as shown in the diagram below.

When the headlight OFF-PARK-ON switch is turned to the headlight position, power flows through the switch and exits on the GREEN–Violet wire to terminal (86) of the headlight Relay inside the headlight shell.

NOTE:
A headlight relay is used on the /6 series but not on the /5 series. However, all power to the headlight still flows through the headlight High/Low/Flash switch inside the combination switch on the left handlebar. 

Terminal (85) of the relay goes to ground via a BROWN wire to section (31) of the Connector Block inside the headlight shell. One of the other terminals in section (31) goes to the frame ground next to the coils completing the path to the (-) battery terminal.

Therefore, when the ignition switch is turned on, power will flow through terminals (86) and (85) of the headlight relay closing the relay. This completes the path between terminal (30) with the RED wire from the battery (+) terminal and terminal (87).  Headlight relay terminal (87b) also gets power for the parking lights from the RED wire as explained later.

Headlight HIGH-LOW-FLASH Switch Low Beam

A YELLOW–White wire from the headlight relay terminal (87) goes to terminal (56) of the HIGH-LOW-FLASH switch inside the left side combination switch mounted on the handlebar as shown below.

NOTE:
Terminal (56) of the HIGH-LOW-FLASH switch on the left handlebar is shown with either (56) or (58) assigned to it depending on which wiring diagram you look at. Based on the DIN standard, (56) indicates a “spot light” while (58) indicates a “license plate lights, instrument panel” terminal. I think (56) is the correct number based on the purpose of that terminal which is to power the headlights.

The HIGH-LOW-FLASH switch is typically in the centered, low beam position, as I show in the diagram above. I show how the power flows through the switch when it is in the low beam position and then exits on terminal (56b) to the YELLOW wire connected to it. The other end of that YELLOW wire goes to section (56b) on the Connector Block inside the headlight shell. Another YELLOW wire connected to section (56b) goes to the low beam filament of the headlight. The BROWN ground wire from the headlight bulb that goes to a terminal in section (31) of the Connector Block in the headlight shell and then onward to the frame ground.

This design allows the headlight to go on when the engine is off provided the headlight ON-OFF switch is in the headlight position.

Headlight HIGH-LOW-FLASH Switch High Beam

Changing the HIGH-LOW-FLASH switch to the high beam position directs the power from the YELLOW–White wire from the headlight relay to exit the high/low/flash switch on terminal (56a) on a White wire. The White wire goes to section (56a) of the Connector Block inside the headlight shell. A second White wire in section (56a) goes to the high beam filament of the headlight bulb as shown in the diagram below.

Headlight HIGH-LOW-FLASH Switch High Beam Flasher

The original left combination switch has a RED wire from the battery connected to terminal (30) of the HIGH-LOW-FLASH switch. This switch had nine wires. This allows the high beam to be flashed on even when the ignition switch is off.  The power from the RED wire at terminal (30) exits the switch on the WHITE wire at terminal (56a) and goes to the headlight high beam filament via the Connector block section (56a) when the momentary switch is pushed closed.

NOTE:
The original left combination switch with nine wires is no longer available. The replacement switch (part# 61 31 1 243 248) has eight wires and does not include the RED wire. Therefore, if your bike has the eight wire switch, you loose the high beam flasher function when the ignition is OFF. You can only flash the high beam when the ignition is ON.

GREEN-Black Wires After Fuse

As shown in the diagram below, the GREEN wire from ignition switch terminal (15) goes to the left side of terminal section (15) of the Connector Block in the headlight shell.

The left side terminals are connected to the terminals on the right side via an 8 amp fuse. The right side terminals of the Connector Block have a GREEN-Black color and the wires connected to the right side are also GREEN-Black. The Black stripe indicates these are wires after a fuse. So when you see any GREEN-Black wires, you know they are after a fuse, after the ignition switch and connect to the (+) battery terminal. No current flows through the GREEN-Black wires unless the ignition switch is on AND the fuse is not blown AND there is a path back to the (-) battery terminal through a component.

NOTE:
As shown in the Connector Block section above, there is a (15u) terminal section and a (15) terminal section at the top of the actual Connector Block . The Haynes Manual does not show the (15u) section in their version of the wiring diagram, instead showing both the GREEN (15u) terminals and GREEN–Black (15) terminals all in section (15).

Brake Light Switches and Horn Power Wiring

As shown in the diagram below, one of the GREEN-Black wires brings power to the front brake light switch of the front disk brake which is mounted to the front of the brake fluid reservoir, and to the rear brake light switch and also the horn.

A GREEN–Red wire leaves the front disk brake and rear brake switches and goes to the dual filament rear bulb taillight bulb to power the brake light filament. These two GREEN–Red wires from the front and rear brake switches are connected together inside the tail light housing.

If this is the R60 with drum brake, there is a GREEN-Black wire that brings power to the front brake light switch at the handle bar. The other side of the switch has a GREEN–Red wire that connects to the GREEN–Red wire from the rear brake switch and continues to the dual filament rear tail light bulb’s brake light filament. These two GREEN–Red wires from the front and rear brake switches are connected together inside the tail light housing.

NOTE:
Although the diagram above shows both the front drum brake and disk brake GREEN–Black wires and GREEN–Red Wires, only the appropriate ones exist depending on if the bike has the drum or disk front brake.

Brake Light Switches Ground Path Wiring

Shown below is the BROWN ground wire from the taillight bulb to the frame ground that completes the path to the (-) battery terminal.

Horn Ground Path Wiring

The horn ground wiring path goes through the horn button on the left side combination switch on the handlebar. The horn operates with the button completes the ground path back to the (-) battery terminal. I removed the GREEN–Red wires and BROWN ground wire from the brake light switches and taillight bulb for clarity in the diagram below.

There is a BROWN–White wire that goes from the other horn terminal to terminal section (H) of the Connector Block inside the headlight shell. Another BROWN–White wire connects to the other Connector Block (H) terminal and goes to terminal (50) of the left side combination switch. A BROWN wire connects to terminal (85) of the left side combination switch and goes back to the Connector Block to one of the terminals of terminal section (31). One of the terminal section (31) terminals goes to the frame ground completing the path back to the (-) battery terminal.

Volt Meter Power & Volt Meter, Clock Ground Path Wiring

The volt meter is powered by a GREEN-Black wire as shown in the diagram below.

There is a BROWN wire from the (-) volt meter terminal to the negative clock terminal. A second BROWN wire from the clock goes to one of the terminals in the section (31) of the Control Block completing the ground path to the (-) battery terminal.

Turn Signal Relay & Handlebar Switch Wiring

A GREEN-Black wire from section (15) of the Connector Block connects to terminal (49) of the turn signal relay inside the headlight shell as shown in the diagram below.

The /6 series turn signal relay has two contacts that close, terminal (49a) and terminal (KBL), when the turn signal switch selects either the left or right turn signals.  The contact attached to terminal (49a) sends power to the two turn signal bulbs on one side of the bike and the second contact attached to terminal (KBL) sends power to the turn signal indicator bulb at the bottom of the instrument cluster.

There is a transistor inside the turn signal relay. It senses the lower current draw if one of the turn signal bulbs is out. In that case, the indicator bulb in the instrument cluster flashes once and then stays off. But, the remaining turn signal bulb continues to flash.

Unlike the /5 series turn signal relay whose flash rate depended on the resistance of the two turn signal bulbs and their wiring including the ground path, the /6 relay flash rate is load (resistance) independent. You can add extra lights without changing the flash rate.

There is a GREEN–Yellow wire from turn signal relay (49a) that brings power to the right side combination switch as shown in the diagram below.

No current flows through the GREEN–Yellow wire until the turn signal switch is moved to select the right or left turn signals.

Right Turn Signals Wiring

NOTE:
For pilots and boat operators, the convention is red is left side, or port, and green is right side, or starboard.  The way to remember this is left, port and red have fewer letters than green, right and starboard. Similarly, the DIN wiring code uses the red stripe to indicate left side wires. But, they use black stripes for the right side wires. However, the way to remember this is the same, red and left have fewer letters than black and right.

The right side turn signals are selected when the turn signal switch is pushed down connecting the GREEN–Yellow wire on terminal (49a) to terminal (R) of the turn signal switch. A BLUE-Black wire connected to the turn signal switch (R) terminal connects to section (R) of the Connector Block inside the headlight shell. The other terminals of section (R) have BLUE-Black wires going to the left front and rear turn signals as shown in the diagram below.

Left Turn Signals Wiring

Similar to the right side turn signals, when the turn signal switch is pushed up, the GREEN–Yellow wire on terminal (49a) connects to terminal (L) of the turn signal switch. A BLUE–Red wire connected to the turn signal switch (L) terminal connects to section (L) of the Connector Block inside the headlight shell. The other terminals of section (L) have BLUE–Red wires going to the right front and rear turn signals as shown in the diagram below.

Turn Signals & Turn Signal Relay Ground Path Wiring

The ground path for the turn signals on the /6 series uses separate BROWN wires to each turn signal bulb as shown in the diagram below.

The rear turn signal bulb ground wires connect to the tail light bulb ground inside the rear tail light housing. A BROWN wire from the tail light housing goes to the frame ground providing a path to the (-) battery terminal.

The front turn signal bulbs go to Connector Block section (31) that has a wire that goes to the frame ground providing a path to the (-) battery terminal.

There is also a BROWN wire from terminal (85) of the turn signal relay shown in the diagram above. It goes to section (31) of the Connector Block inside the headlight shell and then to the frame ground completing the ground path for the turn signal relay electromagnetic switch.

Turn Signal Indicator Bulb Wiring

The second contact of the turn signal relay is connected to terminal (KBL) with a BLACK–White wire attached to it. This wire goes to the turn signal indicator bulb in the instrument cluster via pin (1) of the instrument cluster cable.

The ground path for the indicator bulb goes to the BROWN wire connected to pin (7) of the instrument cluster cable. That wire goes to the brake fluid level switch under the gas tank and from there to the frame ground that completes the path to the (-) battery terminal.

Ignition Switch PARK Circuit

The ignition switch has a position for turning on the parking lights, but not starting the bike. The RED wire connected to terminal (30) of the ignition switch goes to terminal (58) of the ignition switch when the switch is in the park position as shown in the diagram below.

GREY-Black Wires After Fuse

Ignition switch terminal (58) has a GREY wire that goes to section (58) of the Control Block inside the headlight shell as shown in the diagram above. There is an 8 amp fuse, that receives power for ignition switch terminal (58), between the left and right side terminals of section (58). GREY-Black wires attach to the right-side terminals. The Black stripe indicates these wires are after the fuse.The wires that power the parking lights are GREY–Black indicating they are after the fuse.

NOTE:
The actual Control Block has a section (58) and a section (58u) that are across from each other on either side of the Control Block, but the Haynes manual does not show section (58u). Section (58) of the Control Block has the GREY wires while section (58u) has the GREY–Black wires. And, the GREY and GREY–Black wires are the opposite sides of the Control Block from where these wires are shown in the Haynes manual. Don’t let these differences confusing when you compare the wiring diagram to the actual Control Block wiring inside the headlight shell.

Front & Rear Parking Lights Wires

One GREY-Black wire from section (58) of the Control Block goes to the running light filament of the dual filament tail light bulb. A second one goes to the front headlight parking bulb as shown in the diagram below.

The ground path for the dual filament rear bulb goes the frame ground via a mounting bolt for the coils under the gas tank. The front ground path goes to the headlight ground that continues to a terminal in section (31) of the Connector Block inside the headlight shell and then via another wire from section (31) to the frame ground.

Instrument Cluster, Clock & Volt Meter Bulb Wires

The instrument cluster illumination bulbs at the top of the speedometer and tachometer get power from a GREY-Black wire from a terminal of section (58) of the connector block that connects to pin (3) of the instrument cluster as shown in the diagram below.

If this is an R90S or another /6 series bike with the optional clock and volt meter installed, then a GREY-Black wire from a terminal of section (58) goes to terminal (58) of the clock and a second wire goes from clock terminal (58) to volt meter terminal (58) so the clock and volt meter are illuminated when the parking lights are turned on.

The ground path for the two instrument cluster illumination bulbs connect to terminal (7) of the instrument cluster. A BROWN wire from terminal (7) goes to the brake fluid level switch and from there to the frame ground.

All Ignition Switch PARK Light Bulb Wires

The diagram below shows all the parking light wires and ground path wires including the power from terminal (58) of the ignition switch.

Alternative Ways To Turn Parking Lights On

There are two other ways power can be supplied to the parking light bulbs, from the headlight switch which has a parking light position, and when the headlight switch is ON while the ignition switch is ON.

Headlight OFF-PARK-ON Switch PARK Position Wires

Beside the ignition switch parking selection that sends power to the parking lights, the yellow headlight OFF-PARK-ON switch in the left combination switch can turn on the parking lights. When the ignition switch is ON, power from the GREEN wire connected to terminal (15) of the ignition switch goes to terminal (15) of the yellow headlight OFF-PARK-ON switch on the left combination switch as shown in the diagram below.

When Park is selected on headlight OFF-PARK-ON switch, power goes through the switch and exits on a GREY wire connected to terminal (58) of the headlight OFF-PARK-ON switch. The wire goes to section (58) of the Connector Block and then through the Fuse to the GREY-Black wires connected to the parking light bulbs as shown in the diagram below.

Headlight Relay Parking Lights Terminal (87b)

When the ignition switch is turned on, power goes through the switch to terminal (56) and out the GREEN wire to terminal (56) of the yellow headlight OFF-PARK-ON switch in the combination switch on the left handlebar, as shown in the diagram below.

If the headlight is selected with the yellow switch, power goes through the switch to the GREEN–Violet wire connected to switch terminal (56). The wire goes to terminal (86) on the headlight relay which closes the relay. The headlight relay has two sets of power out terminals, terminal (87) and terminal (87b). When the relay closes, power from the battery (+) terminal flows through the RED wire connected to terminal (30) to both these terminals. Terminal (87) sends power to the headlight as explained in the Headlight OFF-PARK-ON Switch PARK Position Wires section while the power going thought terminal (87b) and the GREY wire connected to it goes to section (58) of the Connector Block inside the headlight shell. The power through Fuse to power all the parking light bulbs as shown in the diagram above.

Charging Circuit

The /6 series uses an alternator, not a generator. A generator produces dc current while an alternator produces ac current. Since the battery is a dc storage device, the charging circuit has to convert ac current to dc current before sending it to the battery.  A diode board does this conversion.

As the motor RPM increases, the alternator voltage also increases. If the voltage gets too high it will damage the battery so a voltage regulator monitors the alternator output voltage and limits it to a maximum of about 14.1 volts.

The BMW alternator uses a magnetic field generated in a moving coil of wire by electricity flowing through the rotating coil. This rotating magnetic field induces a magnetic field and electrical current flow in a stationary coil of wire. The stationary coil of wire is called the stator and the rotating coil is called the rotor. The rotor is attached to the front end of the crankshaft which spins the rotor’s coil and magnetic field inside the stator’s coils inducing electrical current in the stator’s coils. This induced current in the stator’s coils flows to the diode board that converts it to dc current and then to the battery to charge it.

How The Alternator Rotor Coil Gets Current When The Engine is Off

I will start with the wires that power the alternator rotor coil. When the engine is not running and the ignition is on, the current flowing through the charging indicator bulb in the headlight shell continues though the rotor coil on it’s way to the (-) battery terminal. That small current flow is enough to create a small magnetic field in the rotor coil when the engine is not running.

NOTE:
If the red kill switch is in the OFF position (flipped up or down), then there is no power to the charging indicator bulb in the instrument cluster so the alternator can not work since there is no current flow through the rotor. That said, you can’t start the bike nor can the ignition system work.

I showed the wires from the charging indicator bulb previously in the [Alternator Bulb Ground Path Wiring] section. Here is one of those diagrams showing the BLUE wires.

The BLUE wires in the diagram above include the wire from the charging indicator bulb. It goes to the (D+) terminal of the starter relay, then to the (D+) terminal of the diode board and then to the (D+) terminal of the voltage regulator. I describe how the voltage regulator works in the components document. Without going into the details, the charging indicator lamp current flows through the voltage regulator and exits via the voltage regulator (DF) terminal.

There is a BLUE-Black wire from the (DF) terminal of the voltage regulator that goes to the (DF) brush terminal of the alternator rotor as shown below.

The current from the charging indicator bulb flows through the alternator rotor coil and exits the rotor coil via the (D-) brush terminal which is grounded to the alternator housing creating the ground path back to the (-) battery terminal. The rotor coil (D-) terminal also has a BROWN ground wire that goes to the (D-) terminal of the voltage regulator. Although this is a ground wire, the ground path for the charging indicator bulb is via the alternator housing. I’ll explain the purpose of the BROWN ground wire later.

It is this small current flow through the rotor coil when the engine is not running that allows the alternator to create power when the bike is first started. After the engine starts and reaches idle RPM, the charging indicator light goes out. The power generated by the alternator goes to the diode board and flows back out through the (D+) terminal then via the BLUE wire to the starter relay (D+) terminal and back to the charging indicator bulb. The alternator (+) voltage is applied to the same blue wire coming from the charging indicator light and when the alternator voltage reaches about 12.2 volts, there is not enough voltage difference across the charging indicator light filament to get it to light, so the bulb goes out.

You can visualize this as one stream of water flowing out of the charging indicator bulb unimpeded when the engine is not running. When the engine is at idle RPM, a second stream of water flows in the opposite direction toward the charging indicator bulb and it is strong enough that the net flow out of the charging indicator bulb reaches zero so the bulb goes out.

Generating Power From the Alternator

The alternator includes both a rotating coil of wire, the rotor, and a stationary set of coils of wire, the stator. The alternator creates ac current in each of the stator’s coils of wire. Since the stator has three separate coils of wire, placed 120 degrees from each other, the alternator creates three separate ac current flows 120 degrees apart. This is called 3-phase ac current.

Stator Phases and Center Tap (U, V, W, Y) Wires

The /6 series stator has four connections (U, V, W, Y). Three wires, (U, V, W) each carry current for one phase of the three phases of ac current the alternator produces. The fourth wire, (Y), is called the center tap. The /5 series did not have the center tap on the alternator. Adding it to the /6 series increased the power output from the 180 watts of the /5 alternator to 280 watts for the /6 alternator. Also, the diameter of the alternator increased to 107 mm in the /6 series from the smaller 105 mm used for the  /5 series.

NOTE:
The change to the larger diameter /6 280 watt alternator was not uniform. Therefore, it’s possible to find a 1975 /6 with the smaller 105 mm alternator instead of the lager 107 mm. Both will produce 280 watts if they have the center tap (Y) terminal and the associated diode board.

In the diagram below, three BLACK wires (U, V, W) from the stator go to the back of the diode board and the center tap wire (Y) connected to the (Y) terminal on the front of the stator housing goes to the (Y) terminal the right side of the diode board as you face the front of the motorcycle.

NOTE:
Some replacement wires for the three (U, V, W) stator wires that go to the back of the diode board have colored insulation on them. This is for convenience in showing the ends of each of the three wires and does not indicate the purpose for the wires. In particular, one of the wires has red insulation, but that wire does NOT go to terminal (30) and should not be confused with any of the wires that go directly to the (+) battery terminal.

Diode Board Wires

I describe how the diode board works in the components document. The diode board creates two dc outputs from the ac input it receives on the (U, V, W, Y) terminals from the alternator. In the diagram above, the first output is from diode board terminal (B+). The RED wire connected to this terminal goes to terminal (30) on the starter solenoid. The large BLACK wire attached to the starter solenoid terminal goes to the (+) battery terminal and the alternator current on this wire is what charges the battery.

The output from diode board terminal (30) connects to a RED wire that goes to terminal (30) of the starter relay. A second RED wire attached to starter relay terminal (30) goes to section (30) of the Connector Block which has terminals that connect to the ignition switch and to the the High/Low/Flash switch in the left handlebar combination switch.

The (D+) terminals of the diode board have the two BLUE wires attached to it. One of those wires goes to the (D+) terminal of the starter relay and then back to the charging indicator light to turn off the light when the alternator produces enough voltage as I described earlier. The other BLUE wire goes to the (D+) terminal of the voltage regulator. It provides an input to the voltage regulator from the alternator which the relay inside the voltage regulator uses to limit the maximum voltage produced by the alternator so it can’t damage the battery. I describe how the voltage regulator works in the /6 Series Electrical Components document.

Starter Circuit

The starter circuit applies power to the starter motor and solenoid to turn the engine over. If the engine ignition circuit is working and the correct fuel-air mixture reaches the cylinders, the engine starts.

BMW uses a very large starter motor of almost 1 horse power (HP). The current flow into the starter motor can be quite large requiring a large diameter copper wire to carry that much current and not melt the wire. It’s impractical, and dangerous, to have that wire go all the way to the handlebar mounted starter button. Therefore, the handlebar starter button activates a relay inside the starter relay and it in turn activates an even larger relay, called the starter solenoid, that attaches directly to the starter motor. If you think of a relay as an electrically operated switch, then BMW uses three switches in series to power the starter motor; the handlebar push button switch, the starter relay switch and the starter solenoid switch. Failure of any of the three switches will prevent the starter motor from operating.

Starter Relay Power Wires

I’ll start with the power into the starter relay that I showed earlier in the section about the GREEN–Blue kill switch wires as shown in the diagram below.

As shown above, a GREEN–Blue wire from section (15u) goes to terminal (86) of the starter relay. It supplies power to one side of the electrical relay switch inside. Terminal (30) of the starter relay has a RED wire that is a direct path the (+) battery terminal.

NOTE:
The GREEN–Blue wire is part of the kill switch circuit. Therefore if the kill switch is off, the starter will not work as there is no power going to the electromagnetic relay inside the starter relay.

Starter Button Wires

As shown in the diagram below, a BLUE–Yellow wire goes from terminal (50) of the starter switch in the combination switch mounted to the right handlebar to section (85) of the Connector Block. A second BLUE–Yellow wire from section (85) of the Connector Block inside the headlight shell goes to terminal (85) of the starter relay.

Starter Relay Interlock Switches Wiring

The /6 series has two interlock switches to prevent damage to the starter motor; the neutral switch and the starter cutout switch which is operated by the clutch lever. These switches are in the ground path back to the (-) battery terminal. If both switches are open, then there is no path back to the (-) battery terminal. Therefore, if the starter button is pushed and both interlock switches are open, no current can flow through the starter relay’s electromagnet to close the internal switch and no power goes to the starter solenoid so the starter motor can’t spin.

Starter Cut-Out Switch Wires

There is a BROWN–Yellow wire attached to terminal (85) of the starter button. It goes to section (85b) of the Connector Block inside the headlight shell. A second BROWN–Yellow wire attached to section (85b) goes to the starter cut-out switch as shown in the diagram below.

The other terminal of the starter cut-out switch has a BROWN wire that connects to a terminal in section (31) of the connector block. That section has a BROWN wire that goes to the frame ground completing the path to the (-) battery terminal as shown in the diagram below.

Neutral Switch Wires

The neutral switch has a BROWN-Black wire that goes to a terminal on the right side of section (85) on the Connector Block and the neutral bulb has a BROWN-Black wire that connects to the other terminal in section (85) as shown below.

NOTE:
The Haynes manual shows terminals across from the BROWN–Yellow in section (85b). These have BROWN–Black wires attached to them. But, that section of the Connector Block is labeled section (LKK) where the BROWN–Black wires connect. Once again. the Haynes diagram is only approximately correct.

NOTE:
There is a [Diode] between section (85b) and section (LKK) of the Connector Block (on the Haynes diagram, between the left and right side (85b) terminals). It is mounted on the back of the Connector Block. The purpose of this diode is to prevent the neutral light from being on when in the transmission is in a gear and the clutch lever is pulled in.

If the diode fails open (open circuit so not current flows between section (85b) and (LKK)), the bike won’t start in neutral, but will start when you pull the clutch lever to close the starter cut-out switch.

If the diode shorts (conducts current both ways), the neutral lamp will illuminate every time the clutch lever is pulled backwards. The bike will continue start in neutral.

The other neutral switch terminal has a BROWN wire that goes to the frame ground completing the path back to the (-) battery terminal completing the circuit.

Starter Relay Interlock Switches Operation

The diagram below shows the complete starter button circuit including the two ground paths through the neutral and starter cut-out switches.

The neutral switch and the starter cut-out switch create a parallel circuit between the starter button and the starter relay coil power. Therefore, if either switch is closed, the starter relay coil gets a ground path via terminal (85) when the starter button is pushed so the starter relay closes and sends power to the starter solenoid.

If the bike is in neutral, then the neutral switch is closed. When the starter button is pressed on the right handlebar combination switch, the starter coil relay ground path is through the neutral switch and the starter relay coil closes and sends power to the starter solenoid.

If the bike is not in neutral, but the clutch lever is pulled so the clutch is disengaged and the transmission is disconnected from the engine, then the starter cut-out switch is closed and the starter relay coil ground path is through the starter cut-out switch’s ground path and the starter relay coil closes and sends power to the starter solenoid.

If the bike is not in neutral and the clutch is lever is not pulled, then both the neutral switch and the starter cut-out switch are open and no power flows to the starter relay coil when the starter button is pushed so the starter motor can’t turn. This protects the starter motor from being inadvertently engaged with the flywheel when the motor is running.

Starter Button Activation Of Starer Relay

As shown in the diagram below, when the starter button in the right side combination switch is pressed and either (or both) the neutral switch and starter cut-out switch are closed, current can flow into the starter relay on the GREEN-Black wire connected to terminal (86) following the GREEN path inside the starter relay to terminal (85) where it exits on the BLUE–Yellow wire through section (85) of the Connector Block to terminal (50) of the starter button and follows whichever of the two ground paths (neutral switch or starter cut-out switch) is available to the frame ground and then to the (-) battery terminal.

The starter relay closes it’s switch arm across the switched contacts connecting terminal (30) to terminal (87) as shown by the RED path inside the starter relay in the diagram above. This sends power from starter relay terminal (87) to terminal (50) of the starter solenoid via a larger diameter BLACK wire.

Starter Motor and Starter Solenoid Wires

The starter solenoid is another relay with larger current carrying capacity than the starter relay. The starter solenoid case is grounded to the engine case via its mounting bolts. Therefore as current flows into the starter solenoid via terminal (50), as shown by the BLACK lines inside the solenoid in the diagram below, the relay is energized since the other side of the relay is connected to the case of the solenoid.

The solenoid relay closes, as shown by the RED arrow in the diagram above, sending power directly from the battery (+) terminal from the black wire connected to terminal (30) of the starter solenoid into the starter motor (indicated by the RED path) and the starter motor starts to spin. There are some other details about how the starter solenoid works that I describe in the /6 Series Electrical Components document.

Engine Ignition Circuit

The engine ignition circuit creates a spark inside the cylinders to ignite the fuel-air mixture. This circuit is powered when the ignition switch is ON and the kill switch is ON as shown previously and again in the diagram below.

Ignition Switch and Kill Switch Wires

When both switches are ON, battery power flows through the ignition switch to terminal (15) where a GREEN wire goes to section (15u) at the top of the Connector Block. A second GREEN wire from section (15u) goes to the kill switch. A GREEN–Blue wire leaves the kill switch and goes to the other section (15u) at the bottom of the Connector Block with the GREEN–Blue wires (not to be confused with the section (15u) at the top of the connector block with the GREEN wires). A GREEN–Blue wire on the other (15u) terminal at the bottom of the Connector Block goes to terminal (86) on the starter relay. A second GREEN–Blue wire attached to start relay terminal (86) goes to a terminal on the left of the two coils that are mounted on the spine tube under the gas tank.

Coil, Condenser & Points Wires

I created the simplified diagram below to discuss the coil, condenser and points wires on the /6 series ignition system. All the GREEN wires from the ignition switch and section (15) of the Connector Block that come from the ignition switch and go to the kill switch are compressed into a single wire. All the GREEN–Blue wires from the kill switch to the Connector Block and eventually to the coil terminal into a single wire.

The GREEN–Blue wire to the left side coil terminal (shown as the right side coil in the diagram above) connects to the coil primary windings. Current flows through the left coil’s primary windings to the other primary terminal of the left coil. A short, black wire attached to this terminal connects to the adjacent primary terminal on the right side coil. Current flows through the right coil’s primary windings and exits on the other primary terminal.

That coil primary terminal has another black wire attached that goes to one side of the condenser (aka, capacitor) that is located inside the front engine cover. The other side of the condenser has the wire that goes to one side of the points. The other side of the points is grounded via the points plate and mounting screws to the engine completing the ground path to the (-) battery terminal. When the points close, there is a path to the (-) battery terminal so current flows through the coil primary windings, the condenser and the points. However, current does not flow into the capacitor since the path through the points is a short circuit.

I describe how the ignition coil works in the /6 Series Electrical Components document. Briefly, when current flows through the primary coil windings, it generates a magnetic field. When the points open, current stops flowing through the primary windings of the coils. This causes the magnetic field to collapse creating a very high voltage in the secondary winding in each ignition coil. Each ignition coil secondary winding is connected to a spark plug. The voltage created in the secondary winding is high enough to cause a spark between the electrodes of each spark plug igniting the fuel-air mixture in one of the cylinders.

One other effect of the collapsing primary coil magnetic field is to create a high voltage across the points. It’s high enough to cause a spark between the point contacts which will damage them over time. The capacitor slows the voltage rise across the points long enough to prevent the spark across the points while the points are open. It also increases the voltage generated in the secondary windings of the ignition coils.

Revisions

2019-01-22  Edited document and section links
2019-01-23  Clarification about section (15) & (15u) errors in Haynes manual and duplicate (15u) sections with different wire colors on the Connector Block
2019-07-19  Fixed error about where GREEN wire goes inside headlight shell
2020-02-10  Fixed error in Neutral Switch section about diode in connector block inside headlight shell
2021-01-25  Edited charging indicator bulb ground path text
2021-03-22  Format editing
2021-04-16  Fix broken picture links in starter relay section
2021-04-30  Add /7 Series, 1977 R100RS Circuits link
2021-08-09  Noted error in instrument flex-board picture wire color
2023-01-04  Correct left/right error about 15u, 58u location on connector block & wiring diagrams.