- What Makes a Closed Circuit?
- 5 Series Wiring Diagram Components
- Battery (+) Wires
- Ignition Switch Circuit
- Ignition Switch-Park and Headlight Switch
- Turn Signal Circuit
- Charging Circuit
- Starter Circuit
- Engine Ignition Circuit
This document is part of a series of airhead electrical system documents. You can find the others here.
- Electrical Basics
- /5 Series Electrical Components
- /6 Series, 1975-76 Circuits
- /6 Series, 1975-76 Electrical Components
This document describes the various electrical circuits used in the 5 Series 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 independently. I think this makes it easier to understand a circuit and the entire wiring system.
I include a number of references in the Resources section containing much more detail about how the 5 Series components work and the differences compared to later airhead series.
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 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]
- 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]
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 an 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.
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. And since the /5 series didn’t come with a clock, no electrical current flow through any of the /5 series wires when the ignition is off.
For electrical current to flow, there has to be a continuous conducting path from the battery (+) to the battery (-), or ground, terminals. Beside the ignition switch, there are other switches, and relays inside components, that are “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, from the (+) to (-) battery terminals. When the ignition is off, no current can flow to the electrical switch 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 relays work.
5 Series Wiring Diagram Components
You can find detailed descriptions of the various electrical components in the 5 Series Electrical Components document.
Let’s start with a wiring diagram for the 5 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.
It’s easier to follow along if you click the picture above and enlarge it. The cursor will display with a magnifying glass. Click the picture to enlarge and move your mouse to move around the enlarged diagram.
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.
Battery (+) Wires
By convention, current flows from the (+) battery terminal and back to the (-) battery terminal (called the ground) to complete a circuit.
Historically, and incorrectly, it was assumed electricity flowed from the (+) to (-) terminals of a battery. This direction of electric current flow is called “conventional currrent 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 indicates a wire is directly connected to the (+) battery terminal. In a couple of instances, a non-red wire is directly connected to the (+) battery terminal.
The diagram below shows the wires directly attached to the (+) battery terminal.
It’s easier to follow along if you click the picture above and enlarge it. The cursor will display with a magnifying glass. Click the picture to enlarge and move your mouse to move around the enlarged diagram.
Starting with the (+) battery terminal, a RED wire goes to terminal (30) on the starter relay. That terminal has two other RED wires connected to it, so that terminal is a common connection and all of the wires act as if they are a single wire. One of those wires goes to terminal (30) on the diode board and the other goes to terminal (30) / (51) on the ignition switch.
In this case, terminal (51) is an internal connection inside the ignition switch. It’s marked (30)/(51) on the BMW wiring diagrams to signify the internal side of the terminal is a path inside the switch. You will also find this terminal marked “30/51” on the ignition switch board inside the headlight shell. For our purposes, the (30) means the RED wire is directly attached to the (+) battery terminal.
The wire to terminal (30)/(51) of the ignition switch brings power to the ignition switch, That wire and a GREEN–Red wire are attached to the same terminal, so this terminal creates a common connection and the wires act as a single wire.
The GREEN–Red wire goes to the left handlebar switch connecting to terminal (30), indicating that this terminal is directly connected to the battery (+) terminal, even though it’s NOT a RED wire. The GREEN–Red wire provides power to the high beam flasher function of the left control switch. When that push button switch is closed, the headlight will flash even though the ignition switch is off. Later I’ll show the wires that go to the left handlebar switch.
Going back to the battery (+) terminal, there is another wire that is BLACK going to terminal (30) on the starter motor solenoid. This is another case where the wire between terminal (30) on two components is NOT RED. You can rely on the terminal number to indicate the purpose of the wire according to the DIN terminal standard. Since the starter motor draws a lot of power, it’s directly connected to the battery via the starter motor solenoid. The solenoid is a relay capable of passing a lot of current to the starter motor without burning up.
One last point to make here is that at the moment, even if I showed all the other wiring including ground wires, there is no conducting path between any of these components and the (-) battery terminal, or ground. Therefore at this point with the ignition switch OFF, no electricity is flowing through any of the wires shown above.
Should you be poking around next to any of these wires with a screw driver, socket driver or other metal tool AND happen to touch any terminal (30) and the frame or 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.
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.
Ignition Switch Circuit
When the ignition switch is turned on, terminal (30) is connected to terminal (15) via the internal switch contacts of the ignition switch. I show by the RED path inside the ignition switch.
It’s easier to follow along if you click the picture above and enlarge it. The cursor will display with a magnifying glass. Click the picture to enlarge and move your mouse to move around the enlarged diagram.
Eventually I’ll show how a complete circuit back to the (-) battery terminal is created for the components that get power when the ignition switch is turned on.
An internal connection inside the ignition switch goes to a light switch built inside the ignition switch. The light switch is normally open when you press the “nail” key into the ignition switch to turn it on so no current flows to the lights when the ignition is on. Turning the ignition key turns on the light switch and allows power from terminal (30) to go to the lights as will be explained later.
There are two GREEN wires connected at terminal (15) so it is a common connection and the wires act like a single wire. When the ignition switch is turned on, the GREEN wires are directly connected to the battery (+) terminal AFTER the ignition switch. Therefore they carry no current UNTIL the ignition switch is turned on AND there is a path back to the (-) battery terminal.
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.
Starting with the right-hand GREEN wire, it goes to the ignition coils that are part of the engine ignition system. As you trace the path through the two coils you can see a black wire connects from the last coil to the capacitor, or condenser. Notice that the contact breaker, or points, have a ground symbol. With the points are closed, there is a complete path to the battery (-) terminal. Therefore, when the ignition switch is turned on, AND the points are closed, current starts to flow through the primary windings of the coils building up a magnetic field in the coil windings. The magnetic field is at its maximum when the current flow reaches it’s maximum. The maximum current flow through the coils can be computed using Ohm’s law. For a fully charged battery at 12.6 volts, and a total resistance of 3 ohms through both coils, the current flow will be I=E/4, or I=12.6/3 or 4.2 amps.
It is possible for the points to be closed, or open, when you turn on the ignition switch. If the points are open, then no current will flow through the coils, capacitor and points. The ignition system operation is explained later.
GREEN-Black Wires After the Fuse #15
The left GREEN wire from terminal (15) of the ignition switch goes to one of the 8 amp fuses, which I refer to as Fuse #15. Let’s see where the wires on the other side of Fuse #15 go.
Fuses were NOT included in the early /5 bikes. They were added in the 1972 model year. So on the non-fuse /5 bikes, the GREEN–Black wires I show in the following diagrams will be GREEN wires in an early /5 bike.
The wires on the other side of the fuse are GREEN–Black. The black stripe indicates they are after the 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.
The right hand GREEN–Black wire after the fuse goes to the terminal block inside the headlight shell and then to the horn where it connects to terminal (15). That horn terminal has a second GREEN–Black wire attached to it so horn terminal (15) is a common connection between the wires and they act as a single wire. The second wire from the horn terminal (15) goes to terminal (15) on the starter relay. Again that terminal acts as a common connection to a second GREEN-Black wire attached to starter relay terminal (15). That second wire goes to the rear brake light switch.
Note that none of these electrical components have a direct path back to the (-) battery terminal as internal switches in the components are open so no current flows through the wires connected to them when the ignition is turned on. I’ll show the ground path to the (-) battery terminal for these components later.
Let’s look at the left GREEN–Black wire connected to the fuse and see where it goes. I removed the right GREEN–Black wire to simplify the diagram.
The left GREEN–Black wire goes to three instrument bulbs in the headlight shell. Note the black dots at wire junctions indicating that the wires going to each bulb are connected together. The wires to the bulbs are in parallel. If one wire, or the bulb filament (which is a wire) break, current stops flowing on that branch but can continue to flow through the other two branches. This means if a bulb burns out, the other bulbs can stay lit.
The charging indicator, neutral and oil pressure bulbs connected to a GREEN–Black wire have another connection from the bulbs that I have not shown yet. You will see later how the second connection to these bulbs establishes a path back the the (-) battery terminal creating a closed circuit so they light up when the ignition switch is turned on.
After the instrument bulb connections, the left GREEN-Black wire continues to the turn signal flasher relay at terminal (31) which is a common connection point for a second GREEN–Black wire that connects to the front brake light switch.
Terminal (31) in the DIN standard is used for a connection to a ground. But this terminal on the turn signal flasher relay is not immediately connected to a ground. As you will see in a later diagram, the other GREEN–Black wire attached to terminal (31) of the turn signal flasher does get to a ground via the front brake light switch. From the standpoint of the flasher relay, its’s terminal (31) is the path it uses to get to a ground, hence its designation as terminal (31). Also note there is a (15) designation next to this terminal just inside the outline of the flasher relay. I suspect that notation is there to indicate this terminal connects to a GREEN–Black wire which is what typically attaches to the (15) terminals of components. Fun, isn’t it? 🙂
So far, we have traced all the GREEN–Black wires connected to the ignition switch that carry power to components when the ignition switch is turned on. Now, lets look at the other connections to the instrument bulbs and trace those wires. As you will see, these wires create a complete circuit since they connect to the (-) battery terminal allowing the bulbs to light when the ignition switch is on.
The diagram below shows both GREEN–Black wires after Fuse #15.
Instrument Bulbs Path to Ground – Charging Indicator Bulb
The path from the charging indicator bulb to ground is the most complicated of the three instrument bulbs. This is due to BMW using the current flowing through this bulb to make the alternator function. You can learn about that in the 5 Series Electrical Components document and in the charging system section later in this document.
The instrument bulbs are connected to the GREEN–Black wire on the left of Fuse #15, so to simplify the diagram, I’m only showing that wire in the diagrams below.
A BLUE wire connects to the other side of the charging indicator bulb and runs to the (D+) terminal of the starter relay. This terminal is a common junction connecting with a second BLUE wire that goes to the (D+) terminal of the diode board as shown below.
The (D+) terminal on the diode board is also a common point with another BLUE wire that goes to the (D+) terminal on the voltage regulator as shown below
The operation of the voltage regulatory directs the current from the charging indicator bulb to the (DF) terminal of the voltage regulator when the alternator is not running. A BLUE-Black wire from the regulator (DF) terminal connects to the (DF) brush terminal of the alternator rotor as shown in the diagram below.
The other brush on the alternator rotor is marked (D-) and has a BROWN wire that connects to the (D-) terminal of the voltage regulator as shown above. The (D-) brush of the alternator rotor is grounded by the alternator housing mounting bolts to the engine block creating a path to the (-) battery terminal. So, at last, the path from the charging indicator bulb is grounded and the bulb can light when the ignition switch is on.
The current from the charging indicator bulb creates enough magnetic field in the rotor to make the alternator work when you start the bike. Also, as the alternator puts out more power, eventually the charging indicator bulb will go out. More details are in the Charging Circuit section elsewhere in this document.
Instrument Bulbs Path to Ground – Neutral and Oil Pressure Bulbs
In the diagram below, I removed the wires from the charging indicator bulb to simplify the diagram. You can see the second wires from the neutral and oil pressure bulbs and how they connect to ground via switches.
The BLUE–Black wire from the neutral bulb goes to the neutral switch. The case of the neutral switch creates a ground through the transmission to complete the path to the (-) battery terminal. The neutral switch is closed when the transmission is in neutral. When turning the ignition on, the bulb will light if the transmission is in neutral. It the transmission is in gear, the bulb won’t light.
The BROWN–Green wire from the oil pressure bulb goes to the oil pressure switch on the engine. The metal case of the oil pressure switch is part of the ground path to the (-) battery terminal via the engine case. The oil pressure switch is closed unless there is sufficient oil pressure to open it. When turning the ignition on, the oil pressure bulb will light since there is no oil pressure and the switch is closed.
Front and Rear Brake Light Switches – Path to Ground
I removed the second wires from the neutral and oil pressure bulbs to simplify the diagram and show both GREEN–Black wires from Fuse #15 because the front brake light switch is connected to one of these and the rear brake light switch is connected to the other.
As shown below, the other side of the front brake light switch has a GREEN–Red wire that goes to the connection block inside the headlight shell.
The GREEN–Red wire on the other side of that connection of the terminal block goes to the rear tail light bulb. This bulb has two filaments, one for the brake light and the other for the running/parking light. I’ll talk about the wiring to the running/parking light later.
The rear brake light switch is operated by the rear brake foot pedal. As shown below, that switch has a GREEN–Red wire that has a common junction with the GREEN–Red wire from the front brake light switch. Therefore, there are two parallel paths to the rear brake light bulb, one from each switch. If either the front or rear brake switch is closed, current can flow to the tail light bulb on either path so it will light.
The two filament tail light bulb uses it’s base as the ground. A BROWN wire connects to the rear tail light bulb base. It runs to one of the ground wires attached to the frame at one of the coil bracket bolts. This completes the path to the (-) battery terminal so the tail light bulb will light whenever the front or rear (or both) brake light switches are closed.
Turn Ignition Switch Off
I removed all the wires except for the (+) Battery, the GREEN and the GREEN–Black wires to simplify the diagram. When the ignition switch is turned off, it connects terminal (31) to terminal (15) and prevents current flow from the (+) battery terminal coming in on terminal (30) from entering the ignition switch as shown in the diagram below. That means the light switch is not powered either since it gets power when the ignition switch is in the ON position via an internal connection to terminal (30).
Therefore, components attached to the GREEN, GREEN–Black, GREEN–Red wires as well as the GREY and GREY-[stripe] wires (which I’ll show later) have no power. This causes the various internal relays in the electrical components to open so there is no path back to the (-) battery terminal. No power can flow through the RED wires connected to the (+) battery terminal since there is no longer a path to the (-) battery terminal.
Notice that ignition switch terminal (15) has one wire going to the the coils. So when the ignition switch is OFF the coils are shorted to ground. Any energy stored in the magnetic field of the coils is safely discharged to the (-) terminal of the battery as the magnetic field in the coils collapses.
Ignition Switch-Park and Headlight Switch
The ignition switch includes the switch for the parking lights and headlights. By twisting the “nail” ignition key, the light switch is closed so power from terminal (30) can flow out of terminal (58) (first click) for the parking lights and terminal (56) (second click) for the headlight. As previously described, turning the ignition switch on powers a number of electrical components via terminal (15), but not the running lights until the light switch is turned on. For simplicity I show only the wires associated with light switch terminals, (56) and (58) in this section.
Parking Light Circuit
As shown below, the light switch is turned to the first position to turn on the parking lights powering terminal (58).
As shown above, the GREY wire from ignition switch terminal (58) goes to the other 8 amp fuse, which I call Fuse #58 since it’s received power from ignition switch terminal (58). This means all the components connected to the wire on the other side of the fuse are protected from short circuits and subsequent damage. It also means if this fuse is loose or has blown, NONE of the downstream components will work. So a good way to diagnose a blown lighting system fuse is none of the running lights (front parking light, instrument illumination, rear running light) work.
GREY-Black Wires After the Fuse #58
Similar to the first fuse which had a GREEN wire coming in and GREEN–Black wires going out, the second fuse has a GREY wire coming in and GREY–Black wires going out. The black stripe indicates these wires are protected by a fuse.
Here is the right GREY–Black wire after the fuse.
It goes to the rear tail light/brake light bulb which has two filaments. This wire connects to the parking light filament. I’ll show the ground wire connection from the bulb later.
The middle GREY–Black wire after the fuse goes to the front parking light bulb.
The left GREY–Black wire after the fuse goes to the instrument illumination bulbs.
Each bulb is on a parallel path so a failure of one bulb (or it’s GREY–Black wire) will not affect the other bulb.
Parking Light Ground Wires
Here are the BROWN ground wires from the parking light bulbs.
The instrument illumination bulb bases are grounded by the head light shell to complete the path to the (-) battery terminal. The front parking light and the rear tail light are both grounded at terminal (31) of the ignition switch. This terminal is a common connection for three BROWN ground wires. One of those wires goes to a coil bracket mounting bolt completing the path to the (-) battery terminal.
Turning the ignition “nail” switch to the second position connects terminal (56) to terminal (30) on the ignition switch so current from the (+) battery terminal flows out terminal (56).
The headlight low and high beam are NOT protected by a fuse. If Fuse #58 blows, the headlight will still work.
The parking light circuit continues to get power from terminal (58) when the ignition “nail” switch is in the second position, but I only show the GREY parking light wire to Fuse #58 and removed the GREY–Black and BROWN ground wires to simplify the diagram.
Terminal (56) on the ignition-light switch has a YELLOW–White wire that goes to terminal (56) on the left handlebar control. The headlight high and low beam functions are explained in the following Left Handlebar Switch section. I also show the headlight ground path to the (-) battery terminal in that section.
Left Handlebar Switch
The left handlebar switch controls the high and low beam of the headlight and includes the horn push button. As shown previously, it has a GREEN–Red wire that goes directly to the (+) battery terminal so you can flash the high beam with the ignition off. The other functions, low beam, constant high beam and horn don’t work until the ignition is turned on.
Low Beam Circuit
As indicated by the schematic representation of the internal handlebar switch contacts, terminal (56) is connected to terminal (56b) when the switch is in it’s neutral position. I show that by continuing a YELLOW wire through the switch in the diagram below. That means when the ignition switch is turned to the first position, the headlight low beam will come on. I’ll show the ground connection from the headlight bulb later.
High Beam Circuit-Flash and Continuous
The left handlebar switch controls the high beam. Rotating the switch down flashes the high beam. I removed the low beam wire from the diagram below to simplify it.
In this position, terminal (30) of the left handlebar switch with the GREEN–Red wire is connected to terminal (56a) as I show by the internal WHITE wire through the switch.
When the switch is rotated down to flash the high beam, terminal (56) and (56b) are no longer connected so the low beam filament goes out when the high beam filament goes on.
The WHITE wire connected to (56a) goes to the high beam filament of the headlight bulb. A second connection from the high beam terminal of the bulb socket goes to the instrument high beam bulb so it lights when the high beam is lit.
Rotating the switch up keeps the high beam on. In this position, the left handlebar switch connects terminal (58) with the YELLOW–White wire of terminal (56a). Terminal (56a) has the WHITE wire going to the high beam filament of the headlight bulb. I show that with the white lines inside the handlebar switch. When the handlebar switch is in the high beam position, it disconnects terminal (56b) so the low beam filament goes out in the headlight bulb.
Headlight Ground Wires
The diagram below shows the BROWN ground wires for the headlight bulb filaments. I added the low beam wires back into the diagram.
The headlight bulb ground is from its base to the BROWN wire that also connects to the base of the parking light bulb and then to terminal (31) on the ignition switch plate. There is a BROWN wire from the ignition switch plate terminal (31) that goes to the frame via one of the coil bracket mounting bolts connecting with the (-) battery terminal completing the circuit.
The high beam indicator bulb in the headlight shell is grounded to the headlight shell from the base of the bulb providing a path to the (-) battery terminal.
I added the GREEN–Black wires from Fuse #15 to show how the horn gets power after the ignition switch is turned on. A GREEN–Black wire from the terminal block inside the headlight shell goes to terminal (15) on the horn.
Horn terminal (15) is a common connection with another GREEN–Black wire that supplies power to the starter relay and the rear brake light. If that wire comes loose at the horn, these “down stream” components will NOT get power and will not operate. However, if the horn fails, that does not affect the common connection on terminal (15) so both components will continue to get power from the GREEN–Black wire
The only thing missing for a complete circuit to the horn is the ground path back to the (-) battery terminal. As shown below a BROWN wire goes from the other horn terminal to the “H” terminal on the ignition switch inside the headlight shell.
The “H” terminal on the ignition switch board inside the headlight shell is just a common terminal and does not connect to any other wiring on the ignition switch board.
The left handlebar switch “H” terminal has a BROWN wire that goes to the “H” terminal on the ignition switch board.
The horn button is part of the ground path when it touches the handlebar to complete the path to the (-) battery terminal. When you press the horn button, you are completing the ground path to the (-) battery terminal but you are NOT applying power to the horn from the (+) battery terminal. The power to the horn is supplied by the GREEN–Black wire on terminal (15).
The same technique of closing a switch to complete the ground path is used in many places including the neutral, oil pressure, front brake light, rear brake light switches.
Although the horn ground path looks to be convoluted on the diagram, its compact in reality. Since the left handlebar switch wires go into the headlight shell, it’s a short path to the ignition board inside the shell and then down to the horn. This is an example of how a wiring diagram can make it look like the path of the wires is more complicated than it needs to be, when in fact the physical wiring is straight forward.
Turn Signal Circuit
To simplify the diagram, I’m just showing the GREEN–Black wire from the left side of Fuse #15.
Turn Signal Relay (Flasher)
Terminal (31) on the turn signal flasher relay has a GREEN–Black wire that supplies power to the relay. The relay is designed to connect then disconnect terminals (31) and (56a/56b) which causes the turn signal bulbs on one side to turn on / off. The arm of the relay that bridges terminals (31) and (56a/56b) is made from a bi-metalic strip of two metals. As current flows through it, it heats up and one of the metals expands more than the other. This causes the arm to bend until the arm springs off the internal creating and open circuit. This stops current flow through the arm letting it cool and straighten out until the contacts touch again. This cycle repeats flashing the turn signal lights.
The DIN standard defines terminal (31) as a ground connection, but the GREEN–Black wire is not a ground wire. Ground wires are typically either BROWN or BROWN with a stripe. I checked other BMW supplied wiring diagrams and they show this same terminal number, so this seems either incorrectly labeled or just confusing.
That said, the other GREEN–Black connected to terminal (31) on the turn signal relay continues to the front brake light switch and then to ground on the other side of that switch. Therefore, you could make the case that this terminal “faces” ground from the turn signal relay point of view while its other terminal, (56a/56b) does not. Other than this notion, I have no explanation for why this terminal number is shown as (31).
One other observation about terminal (56a/56b). The earlier 5 Series (before the fuses) had a different relay and terminal (56b) was used. The turn signal rely on the later 5 Series with fuses shows terminal (56b) but it is not used.
A GREEN–Yellow wire connected to terminal (56a/56b) is power out of the relay to terminal (54) of the turn signal switch. In a moment you’ll see how the power flows through the turn signal relay based on the turn signal switch position.
Left Turn Signals
The turn signal switch rocks up or down to select the left or right turn signals.Terminal (L) is the output from the turn signal switch to the left side bulbs via a BLUE–Red wire that connects to the terminal block and to the front and rear turn signals from the other side of the block.
I’ll show the ground paths back to the (-) battery terminal later.
Right Turn Signals
I removed the left turn signal wires to simplify the diagram. When the turn signal switch is moved to the right turn signals, the “R” terminal of the switch has power. The BLUE–Black wire from that terminal goes to the connector block in the headlight shell. The wires from the other side of the block go to the right side turn signal bulbs as shown below.
I’ll show the ground paths back to the (-) battery terminal later.
Turn Signal Ground Path
I show both turn signal wires in the diagram below. The front turn signal bulbs gets a ground path to the (-) battery terminal via the base of the bulb, the metal turn signal housing, the turn signal stalk, the headlight ear, the top fork brace, the steering head bearings and then to the frame. This circuitous ground path is one reason “flaky” turn signals are not uncommon on the 5 series and why a ground wire from each front turn signal bulb socket was added in the 6 series.
The rear turn signal bulbs are grounded with BROWN wires that connect to one of the coil bracket bolts to the frame as shown below.
Turn Signal Indicator Bulb
The turn signal indicator bulb in the headlight shell is connected to the left turn signal BLUE–Red and right BLUE–Black wires from the connector block inside the headlight shell as shown below.
So, how does the indicator bulb light when there is no ground wire connected to the bulb?
When the turn signal switch is turned in one direction, say left, there is power from the (+) battery terminal in the BLUE–Red wires including the BLUE–Red wire going into the indicator bulb. But, there is no power in the BLUE–Black wires including the one going to the indicator bulb. Now, follow the BLUE–Black wire from the indicator bulb back to the connector block and out the other side, you will see that it gets to a ground via the right side front and rear turn signal bulbs. I show that in the diagram below by changing the wire color of the right side BLUE–Black wires to BROWN to indicate they act as the ground path back to the (-) battery terminal when the left turn signal is on in the diagram below.
When the right turn signal is on, then the power gets to the indicator bulb via the BLUE–Black wire and the BLUE–Red wire from the indicator bulb to the left side turn signal bulbs becomes the ground path back to the (-) battery terminal.
So, if the left turn signal is on and the right side BLUE–Black wire from the bulb becomes the ground path back to the (-) battery terminal, why don’t the bulbs on the right side also flash? Current is flowing through the BLUE–Black wires and through the right side bulb filaments to get to the (-) battery terminal so they should light up shouldn’t they?
Well, the right side bulbs will light only if there is a voltage “difference” across the filaments. But, the BLUE–Black wires connected to one side of the bulbs is at the same voltage level as the ground of the right side bulbs, so in fact there is no voltage difference across the bulb filaments. Therefore they don’t flash. Kinda clever, isn’t it, and not something you would expect based on the wire colors.
This circuit keeps the battery charged. The 5 series introduced an alternator for charging the battery instead of the generator used in the preceding /2 series. An alternator is a more efficient way to produce electrical power than a generator as it is more compact, lighter and less expensive to build per generated watt of power.
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.
Powering Alternator Rotor Coil 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.
I showed the wires from the charging indicator bulb previously in the Instrument Bulbs Path to Ground – Charging Indicator Bulb 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, it flows to 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 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 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.
The stator has three connections (U, V, W), each of which carries current for one phase. The three BLACK wires from the stator go to the diode board as shown in the diagram below.
Some replacement wires for the stator connection to 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. For example, 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.
Although the wiring diagram identifies U, V and W stator wires and the diode board has corresponding U, V and W terminals, it does not matter which stator wire connects to which diode board terminal. For example you can connect stator wire U to diode board terminal V and mix up the other two wires as well and it will make no difference to the diode board.
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 and W terminals from the alternator. The first output is from diode board terminal (30). The RED wire from this terminal goes to terminal (30) on the starter relay which is a common terminal for two other RED wires. One of those goes to the battery (+) terminal. The alternator current on this wire is what charges the battery.
The second output from the diode board comes from the (D+) terminal. It goes through 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 5 Series Electrical Components document.
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.
I’ll start with the power into the starter relay that I showed earlier in the section about the GREEN-Black wires from Fuse #15 as shown in the diagram below.
As shown above, one of the GREEN–Black wires from Fuse #15 goes to terminal (15) of the starter relay. It supplies power to one side of the electrical relay switch inside. The other side of the electrical relay is connected to terminal (31b) of the relay. To energize the electromagnet and close the relay, a BROWN-Black wire connected to terminal (31b) helps create the ground path back to the (-) battery terminal. It connects to terminal (31) on the right handlebar switch as shown in the diagram below.
The right handlebar switch terminal (31) connects to the other side of the starter push button and goes to the ignition switch and then to the frame ground via one of the coil mounting bolts as shown below.
As shown below when the handlebar starter push button is pushed, it completes the ground path to the (-) battery terminal so current can flow through the starter relay.
As shown below by the GREEN path inside the starter relay, the starter relay is energized which closes it’s switch arm across the switched contacts. This connects terminal (30) to terminal (87) as shown by the RED path inside the starter relay sending power from starter relay terminal (87) to terminal (50) of the starter solenoid via a BLACK wire.
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 5 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 turned on as shown previously and in the diagram below.
One of the GREEN wires from terminal (15) of the ignition switch goes to one of the primary leads on one of the coils. The other terminal of primary winding of the first coil has a black wire that connects to the second coil’s primary winding. There is wire from the other side of the second coil’s primary winding that goes to one side of the capacitor and and connects via a second wire to the contact breaker (or points). When the points close, there is a path to the (-) battery terminal so current flows through the coil primary windings, 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 5 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 create 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.
2018-09-28 Moved all pictures to Flickr & changed links. Minor edits.
2019-01-01 Fix broken picture link. Fix broken document links. Edit grammar & spelling.
2019-01-11 Changed fuse numbers from #1 to #15, and #2 to #58 to match use.
2019-01-19 Edit typos and formatting
2021-03-15 Correct typo in “Low Beam Circuit”; change (56a) to (56b).