- Capacitors, Coils (aka, Inductors), Diodes, Resistors, Transistors
- Switches & Relays
- Charging System
- Starter Relay, Starter Motor and Solenoid
- Engine Ignition System
This document is part of a series of airhead electrical systems documents. You can find the others here.
- Electrical Basics
- /5 Series Circuits
- /5 Series Electrical Components
- /6 Series, 1975-76 Circuits
- /7 Series, 1977 R100RS Circuits
- /7 Series Electrical Components (See: /6 Series, 1975-76 Electrical Components)
This document describes the various electrical components used in the /6 Series of BMW “airhead” motorcycles and summarizes how they work. The /6 series was manufactured from 1974-1976. I include a number of references in the Resources section containing much more detail about how the /5 and /6 Series components work and the differences compared to later airhead series.
The /6 series is an evolutionary step forward from the /5 series. Consequently, many of the /6 series components function in the same way as the /5 series. For this reason, this document includes links to the appropriate section of the /5 Series Electrical Components document referenced above. For each component, I’ll indicate if the /6 series component is the same as the /5 series, or what modifications occurred in the /6 series. Where new components were added in /6 series, I’ll discuss their function.
The /7 series uses almost identical electrical components to the /6. There was a change to the ignition system later in the /7 series that replaced the mechanical points with an electronic ignition system using an ignition control unit (ICU) and a Hall effect sensor to replace the mechanical points.
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”]
- Airheads Beemer Club: (Requires membership (JOIN) to read full content)
[Various articles written by members about electrical system & components]
- 12 Volt Relay Guide
[Good overview of various standard relays showing internal construction
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.
Here is a Haynes Manual wiring diagram with the wires removed showing all the major components. Some of these major components include other components such as resistors, capacitors, relays and transistors.
It’s easier to follow along if you click the picture above to enlarge it. Move your mouse around to scroll across the enlarged image.
As mentioned earlier, I provide links to the relevant section of the “/5 Electrical Components” document and describe any differences in the component in the /6 seriesl
Capacitors, Coils (aka, Inductors), Diodes, Resistors, Transistors
There is no difference in how these fundamental components work, so click the link to the section of the /5 document to learn all about them.
Coil (aka Inductor)
Difference Between a Coil and a Straight Piece of Wire
Magnetic Field Interaction with Coil Magnetic Field
Switches & Relays
The functions of switches and relays are quite similar for the /6 series compared to the /5 series. However, there are some additional switches used on the /6 listed below.
Here is a link to the /5 information about switches and relays
List of Switches
Many of the switches used on the /6 series are the same as those on the /5 series. Here is a list of the differences.
- The combination switches on the handlebars have a different form factor than those used on the /5 series.
- The ignition switch uses a standard ignition key instead of the “nail” style used on the /5 series.
- There is a separate headlight ON-OFF-PARK switch in the right combination switch instead of being integrated with the ignition “nail” switch used on the /5 series.
- There is Starter Cut-out interlock switch activated by the clutch lever.
- The design of the neutral switch change starting 09/1975 for the 1976 model year. See the later note for details.
- A brake fluid low level indicator switch is added for models having a front disk brake.
Below is a list of switches on the /6 series.
NO – Normally Open, NC – Normally Closed. See the “Switches & Relays” link above for details about the different kinds of switches.
- Ignition Switch (key actuated switch)
- Headlight OFF-PARK-ON Switch (flip switch)
- Front Brake Lever Switch (push button switch on R60, pressure switch on other models)
- Rear Brake Lever Switch (push button switch)
- Horn Button (push button switch)
- Turn Signal Switch (rocker switch)
- High/Low Switch (rocker switch integrated with High Beam Flash Switch)
- High Beam Flash Switch (push button switch, NO, integrted with High/Low Switch)
- Oil Pressure Switch (pressure switch, NC)
- Brake Fluid Low Level Switch (pressure switch, NC)
- Contact Breaker, aka “Points” (cam operated switch with spring to close)
- Starter Cut-out Switch (push button switch, pushed in when clutch lever is pulled, NO)
- Neutral Switch (push button switch. Switch type changed in 09/1975, 1976 MY)
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.
Types of Relays
The relay functions are similar, but some relays have a different internal design. Here is a description of what a relay does and the various ways relays function.
List of Relays
The following components contain relays which are indicated by the red circle.
- Starter Relay (NO relay)
- Starter Solenoid (NO relay)
- Voltage Regulator (NC relay)
- Turn Signal Relay (NO relay)
- Headlight Relay (NO relay but with a special diode)
- Horn (Mechanical flapper relay)
You probably noticed that the contact terminals of the starter relay have different numbers than the DIN Terminal Code Standards of (85) and (86). If you look at the DIN standard for terminals, you will see that (31b) is negative, or ground from a switch or relay. So, it’s a better description of the wire that connects to this contact as it comes from the starter button which is a push button switch that creates a path to ground. The power into the relay contacts comes from the ignition switch after the fuse, so labeling it (15) is more specific as to the source of the input current to the contacts. Once again, there is some interpretation allowed when applying the DIN terminal standards.
The battery size increased for the /6 series compared to the /5 series, but the function is the same.
The charging system is the same design using a 3-phase ac current alternator. But, the /6 series alternator generates more power (280 w) than the /5 series alternator (180). Here is a link to the /5 Series Charging System
The charging system is responsible for charging the battery. It includes a 3-phase ac alternator, a diode board, also known as a rectifier, and the voltage regulator. The battery is also part of the charging system since it is the recipient of the output from the diode board. Refer to the [Charging Circuit] section of the [/6 Series Circuits] document to see how these components are connected.
The /6 series alternator has an added “center-tap” in addition to the three phases used by the /5 series alternator. The /6 series center-tap provides more current boosting the alternator output. Adding the center tap changes the diode board along with the stator and the stator cover.
Another change that occurred starting in 09/1975, the start of the 1976 model year, is the diameter of the stator increased from 105 mm to 107 mm. The change happened in the last model year of the /6 series motorcycles.
The /6 Series alternator generates 3-phase alternating current. This requires three coils to be wound on the stator. The stator wires are arranged in what is called the “Y” configuration as shown below. And, the center of the “Y” includes a fourth wire called the center tap. This wire is called “Y” in the circuit diagrams.
Each phase is one leg of the “Y” circuit. Note that one end of each coil is connected together at the junction of the three branches. This common connection is called the “center tap” or “neutral” connection. The center tap is also a terminal in the /6 series stator design. The “Y” nomenclature invokes the notion that each phase is 120 out of phase with it’s neighbor.
The other end of each coil and the center tap exits the alternator via a terminal on the front cover as shown on this the Haynes Manual wiring diagram symbol for the alternator.
The stator cover is different on the /6 series since it includes an extra terminal for the center tap “Y” connection as shown below.
The /6 series rotor has the same design as the /5 rotor
Diode Board, aka Rectifier
The /6 series diode board is configured with 11 diodes. Eight of them create a full wave rectifier and there are two more diodes for the center tap “Y” current flow. These convert the ac current from the alternator into dc current that charges the battery.
The /5 series diode board has nine diodes. Other than the three added diodes for the center tap current, the /6 diode board works just like the /5 board. You can read about how the diode board converts ac current to dc in the links below to how the /5 board works.
The voltage regulator used on the /6 series is an electro-mechanical device just like the one used in /5 series. It operates the same way.
You can read about how the /6 voltage regulator works to regulate the current flow to the battery based on the state of the batteries charge by clicking on the links to the /5 sections below.
Operation of Voltage Regulator Relay
Engine Off Current Flow
Engine Start Current Flow
Low Battery Current Flow
Almost Fully Charged Current Flow
Fully Charged Current Flow
Starter Relay, Starter Motor and Solenoid
The starter relay, starter motor and solenoid are part of the starter circuit which is described in the [Starter Circuit] section of the [/6 Series, 1975-76 Circuits] document. This circuit includes the battery, the starter button, the starter relay, and the starter motor and solenoid. This circuit uses one manual switch, the starter button, and two relays, the starter relay and the starter motor solenoid to connect power from the battery to the starter motor. For the starter relay to work, the ignition switch and kill switch must be ON. In addition, the transmission must be in neutral, or if it’s in gear, the starter cut-out switch must be ON which requires the clutch lever to be pulled in to disengage the transmission from the engine.
The /6 series starter motor and solenoid work the same as the /5 series. The /6 series starter relay is different than the one used on the /5 series. The /6 series relay does not include the transistor that prevented activating the starter motor when the engine is running.
Starter Relay Design
The starter relay used in the /6 series works the same way as the one in the /5 series with one exception. The /5 relay included a transistor that prevents the starter from engaging the flywheel when the motor is running. This was a nice feature, but could lead to a problem when the battery was low and it was cold, known as the “cricket problem”. The /6 relay eliminated the cricket problem, but does not completely eliminate engaging the starter motor when the engine is running.
The /6 uses two interlock switches, the neutral switch (also used on the /5 series, but wired differently on the /6) and an additional “starter cut-out switch” which is normally open, but closes when the clutch lever is pulled in (i.e., when the clutch disengages the transmission from the engine).
On the /6 series, it is possible to engage the starter with the engine running and the bike in neutral. And you can engage the starter when the bike is in gear by pulling the clutch lever in. This can be helpful if the bike stalls in gear as you can pull the clutch and restart without finding neutral.
Despite the two starter motor interlock switches, the neutral switch and the starter cut-out switch, you can damage the flywheel and/or the starter motor on the /6 series if you press the starter button while:
- the engine is running and the transmission is in neutral
- the engine is running, the transmission is in gear and the clutch lever is pulled in.
Operation of Starter Relay
Refer to the [Starter Circuit] section of the [/6 Series, 1975-76 Circuits] document for a description of the wiring that connects to the starter relay. Here is a diagram of the /6 series starter relay.
Terminal (86) gets power from the GREEN–Blue wire which comes from section (15u) on the bottom of the Connector Block. If the kill switch is ON then the GREEN–Blue wire has power. When the starter button is pushed on the right combination switch, it completes a ground path using the BLUE–Yellow wire attached to terminal (85) which energizes the relay coil and closing the relay.
When the relay closes, a fair amount of current from the (+) battery terminal flows from one of the 2.5 mm2 RED wires connected to terminal (30) through the relay contacts to the larger diameter (2.5 mm2) BLACK wire the starter solenoid.
The starter relay (D+) terminal is used only for connecting two BLUE wires together so they act as a single wire. These wires carry power from the charging indicator bulb in the instrument cluster to provide power to the alternator rotor windings when the engine is off. See the [Alternator] section for more details about how the charging indicator bulb affects operation of the alternator.
Starter Motor and Solenoid
The starter motor and starter solenoid work the same way on the /6 series as on the /5 series.
Engine Ignition System
The /6 series uses the same ignition system as the /5. There were some ATU (advance unit) differences, but both ignition systems function the same way.
Condenser, aka Capacitor
2019-01-22 Edited document and section links
2019-08-20 12 volt relay guide
2021-04-30 Add /7 Series, 1977 R100RS Circuits link.
Absolutely amazing piece of research and a compendium for the future. Thank you for all the hard work that went in to this.
I have been following your posts for a few years. Without a doubt the best documented, researched and presented instructions for all Airhead enthusiasts. Invaluable asset for anyone interested in these wonderful machines. Thanks Brook
Those are very kind words, and I thank you. I’m pleased this documentation of my projects has been helpful to you.
I’m grateful to have stumbled upon your high quality work!
Thanks for a fantastic research. I’m looking for help with an intermittent issue with my R75/6 where all of the electrics just stop working and the bike rolls to a standstill. Then after a few moment the electrical system comes back on. I’ve noticed that when this last happened I was pushing the bike home I had left the ignition in the on position and all the power came back on. At my whits end as I have no background with electrics. I’ve been told that this could be a problem with the starter relay used on the /6. Can you give me any directions or advice otherwise I can see this old warhorse become put at the back of the garage and left. I’ve owned this bike for 7 years and am the second owner having bought it of a good family friend who at the age of 82 said it was getting a bit heavy and now at the age of 89 is still using a 125 daily. Any help or direction would be much gratefully received.
You are reading the correct document to help you find the problem, assuming your /6 is not a 1974 model as the wiring is a bit different. I show the RED wire path, which I repeat below, so I hope that helps you.
So, there are two things to trace and look at. I’ll start with the simplest first.
1. An intermittent battery ground will do this.
a Confirm your ground cable is clamped to the battery terminal tightly and there is no corrosion on the cable and the bolt securing it.
b Confirm the other end of the battery ground cable is tightly secured by the transmission bolt.
c If there is any corrosion on the battery ground cable terminals, that can progress under the insulation, so it’s not visible, and eat through the wires and give an intermittent ground.
2. The battery (+) power is conducted from the battery by RED wires. If there is a break along this path, you loose power throughout the bike. So, you need to check all the wires and terminals for corrosion, broken terminals or damage. The path from the battery goes as follows:
a. Battery to Diode Board (B+) terminal. With engine cover off, it’s the large terminal on the right side of the board.
b. Diode board (B+) terminal to Stater Relay terminal (30).
c. Internal to the Starter Relay is a wire that connects to the other spade on terminal (30). It it’s broken, you can’t see it, but no power gets past the starter relay. To diagnose that, when the bike looses power, bang on the starter relay (which of course is under the tank, so that’s fun 🙂 to see if power comes back.
d. Starter Relay terminal (30) to the connector board section (30). This board is inside the headlight relay. The wire from the Starter Relay will have two RED wires on the terminal. The other terminal will have one RED wire and it goes to the Ignition Switch terminal (30). It’s possible the ignition switch is not making solid contact with terminal (30). Again, trying banging on the switch to see if it brings the power back on. Also, try jiggling and turning the key to see if gets the power back on.
So, that’s the diagnosis road map.
I hope this helps. Best of success getting to the bottom of your problem.
Hi Brook. Many thanks for getting back to me. Your clear and concise instruction has given me the get up and go to approach the job at hand. Thanks for the simple step by step approach I was concerned that you may have overloaded me with technical terms as is the case when I’ve tried to read other blogs.
I’ll get back to you with my findings but for now keep you and yours safe and health in these strange times. Kind Regards. Neal.
Hi Brook. I followed your advice and have used the bike for a few months and about 1500 miles just to make sure no issues arise. I can confirm that the culprit was the 42 year old Starter Relay. Many thanks for your help and wonderfully interesting and informative site. Many thanks, Neal.
You’re welcome and I’m glad you found where the Gremlin was lurking.
Don’t know if you’ll see this and respond, but I was wondering if you can tell me what the 0.75 Black wire attached to terminal 87 is on the starter relay? I have the same wire on my bike but it doesn’t seem to make a difference if it’s connected or not.
Btw, I did review the starter section of the /6 wiring Series, 1975-76 Circuits document, but it didn’t include this wire…
I should also mention that I have the Endralast 450w charging system and their electronic ignition (so no diode board).
I also don’t have any of the blue wires on my bike that attach to the D+ terminals.
I assume you have a 1975-76 /6 model bike. BMW added the black wire on terminal (87) in the later part of the /6 series, but it’s not used until about the 1978 year of the /7 models. For that reason, I left it out of the /6 wiring circuit diagrams.
It’s purpose is to turn off the headlight when starting the engine. In the 1978 /7 series and later, the headlight is on when the ignition is on and to provide more power to the starter motor, BMW changed the starter wiring so power from the starter relay flows through the ground wire to the headlight. This means there is not voltage difference across the two headlight wires, so it goes out.
If I recall the EME 450w alternator supplies power to the charge light indicator in the instrument cluster, but since it uses a permanent magnet rotor, there is no need for power on the D+ terminals. The D+ creates a magnetic field in the rotor coil of the stock Bosch alternator by passing the charge light indicator current through the rotor coil so the alternator can generate current when the bike starts.
I hope that helps.
Ah, thank you so much!
Yes, my bike is an early 75 (Jan) and I’ve noticed there are a few left over things in my harness (if I remember correctly there was an unconnected wire in the headlight shell that seems to be part of a change to the brake light).
And now, apparently, here are new unused things as well.
In any event I’m trying to track down the root-cause of my plugs continually fouling over the course about 600-miles.
I’ve spent quite long time fussing with the Bings (complete rebuild and cleaning) and endless hours balancing them, and eventually decided to replace them with Mikunis. Valves are adjusted, timing is right on at 34deg (per EME recommended setting)
So now I’m looking at the wiring and wondering if I have a weak spark due to a wiring issue or a fault (I did find some melted insulation on the Green/Blue lead that goes from the voltage regulator to the coil…
In any event, I’m working my way through the ignition section of the harness right now, so your help is hugely appreciated.