Electrical Basics

This document provides some basic electrical information and shows how to interpret the symbols used on many BMW airhead motorcycle wiring diagrams. It supports these other electrical documents I wrote.

The 5 Series Circuits document traces various circuits using a Haynes Manual wiring diagram. My goal is to remove the clutter you see on a typical wiring diagram and focus in on individual circuits, one at a time, to help you understand what they connect to and how they operate.

The Electrical Components document provides a functional description of how various electrical components work. This can help you diagnose problems with the components when you are having electrical problems.


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

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


The following defines some fundamental terms you need to be familiar with when reading about BMW motorcycle electricity.


Electrons are the negatively charged part of an atom. Electrons will move from a place where there are more of them to any place where there are less of them if there is a “conducting” path between those two places.


Electrons have a charge. Charge can be negative, as is the case for an electron, or positive, as is the case for another part of an atom called the proton. In electricity, electrons are able to flow from a location of high negative charge to a place of lower negative charge if there is a conductor between these locations. It turns out, electrons are able to move easily but protons are not as they are held tightly by strong forces inside the nucleus of the atom. Therefore, motorcycle electrics is concerned with electron flow, not proton flow.

When you read about transistors and other kinds of semiconductors, you will read about “holes” and electrons. A hole is left behind when an electron moves from one part of a semiconductor to another part. If electrons leave a part of the semiconductor material, then the material has a number of holes and acquires a positive charge. In this instance, electrons flowing in one direction create negative charge in the part of the semiconductor they flow to while positive holes appear to flow in the opposite direction to create positive charge in another part of the semiconductor. In this situation, negative electron flow creates both a negative current flow and a positive current flow within a semiconductor.

Conductors and Insulators

A conductor is a material that allows electrons to flow easily between the atoms in a material. An insulator is a material where the atoms hold their electrons tightly and the electrons don’t easily flow between the atoms. That said, if enough energy is available, even the electrons in an insulator will flow between the atoms. At that point, the insulator no longer “insulates”, which is usually a bad thing.


Corrosion means a metal has combined with another element through a chemical reaction. A common example of this is steel and iron rusting. The soft flaky rust is not iron, but iron oxide, a chemical combination of iron and oxygen.

Copper is used in the wires and component terminals of a BMW motorcycle. Copper corrosion can be copper oxides and/or copper sulfates giving a blue or green tint to the copper. Although copper wire and terminals are conductors of electricity, corroded copper is an insulator and does not conduct electricity well. A common problem with 40 year old electrical systems is the wires and terminals have corroded and the insulating property of the corrosion stops electricity from flowing in the wires and at the terminals.

In some cases, corrosion extends into the wire past the insulation and can completely eat away the copper wire underneath the insulation. The wire will intermittently make electrical contact and eventually will fail completely. Visually, you won’t see any problem with the wire and may not know it has failed. Diagnosing this kind of failure can be frustrating.

Voltage, Current and Resistance

Voltage is a measure of the potential difference in charge between two locations. It is measured in Volts (V). It’s similar to the difference in potential energy between the top and bottom of a water tank.

Electrical current, or just current, is the rate electrons are flowing in a conductor. It is measured in Amperes, or Amps (A). It is similar to the rate of water flowing out of the bottom of the tank in gallons per minute (GPM) due to the height of the water in the tank. The higher the water in the tank the faster it flows. The same is true for current flow. The higher the voltage, the more current will flow.

Resistance restricts the flow of electricity just as surface friction at the walls of a pipe, or a partially closed valve, restricts the water flow in the pipe. The higher the resistance of the connection between the (+) and (-) terminals of battery, the less current will flow between them. If the resistance is infinite (a broken wire for instance), no current can flow between the terminals of a battery.

By CONVENTION, current flow, or just current, is from the (+) terminal (positive charge) to the (-) terminal (negative charge) of a battery.

Electrons actually flow from the negative to the positive terminal. So ELECTRON FLOW is opposite to CURRENT FLOW. The convention originates from early electricity theory that got the flow backwards. But, it was so ingrained in the literature, that it was best to continue with the convention. Again, electrical current is ASSUMED to flow from the (+) to the (-) terminal of a battery. The (-) terminal is also called the “Ground”.

And, to add to the confusion, pre-1975 Triumph and BSA British motorcycles use the opposite convention for current flow. So the ground is the (+) terminal in British wiring diagrams. All you can say about a convention is know what it is and follow it, and don’t get mixed up about which convention the wiring system uses. 🙂

How Are Voltage (E), Current (I) and Resistance (R) Related

There is a simple equation, which is known as “Ohm’s Law”, that relates voltage (E), current (I) and resistance (R):

  • E=I x R

Since this is an algebraic equation, there are two other forms of the equation:

  • I = E / R
  • R = E / I

Using the second equation, if there is a voltage of 12 V pushing against a resistance of 10 ohms, then there will be a current of 1.2 amps flowing in the conductor. If the resistance is reduced to 1 ohm, then the current increases to 12 amps. So, can you answer these questions?

  • If the resistance is 0 Ohms, how much current will flow?
  • What happens to a wire when that much current flows through it?

DC and AC Electricity

Direct current, or dc, goes in only one direction like a stream flowing downhill. With one exception, the alternator which generates alternating current, all the wiring and components in the motorcycle are dc.

Alternating current, or ac, is different since the flow reverses directions periodically. In your home wiring, which uses ac current, the direction of the current flow changes 60 times a second. So, ac voltage at a terminal will alternate between (+) and (-). The voltage and current in a wire varies over time and looks like a sine wave.

AC Voltage Sine Wave

AC Voltage (or Current Flow) Vary Over Time

To get more power from an ac motor or an ac alternator, it’s common to have overlapping voltage (and current) sine waves. These are known as phases. The ac alternator in the BMW motorcycle generates three phase ac power. Each phase is offset 120 degrees.

AC 3-Phase Sine Waves

3-Phase ac Current and Voltage Overlap Creating More Power

Since all the other components of the motorcycle use dc power, the ac power from the alternator is converted into dc as will be explained later.


These days, by convention, the ground is the negative (-) terminal of the battery. (See the note above on British wiring systems that use the positive terminal (+) for the ground.) Any conductor with a direct path to the (-) negative terminal of the battery is a ground. A ground on a wiring diagram can be shown by a symbol that looks like this, or something similar.

Frame Ground

Ground Symbol

In many BMW motorcycle wiring diagrams, this symbol means a physical metal-to-metal connection between a wire or metal portion of a component to the frame, engine case, transmission case of other metal part that make a continuous path to the (-) battery terminal. Therefore, the (-) battery terminal has to connect to the engine, frame and/or transmission. BMW airheads use the speedometer cable bolt on the transmission to attach the (-) battery cable.

This bolt is easy to strip and the thick ground wire puts stress on it. A common “upgrade” is to attach the battery (-) cable to a bolt on the frame such as one that attaches the coil bracket to the frame.

Learning The Language of Wiring Diagrams

Let’s talk about the language of wiring diagrams as “spoken” for BMW airhead motorcycles.

A wiring diagram is the blueprint of the electrical system. It shows components in the approximate location on the motorcycle. It shows wires and where they go. And often, it shows details about the electrical connections inside some of the components.

Wiring Diagram 5 Series 1969-1973

5 Series Wiring Diagram (1970-1973) [Source: Haynes Manual]  {CLICK TO ENLARGE}{CLICK TO ENLARGE}

The result is many wires, unfamiliar symbols and “hieroglyphics” that looks like a big bowl of spaghetti. But, there is a “method to the madness” of wiring diagrams. They have a language and when you understand that language, they become a valuable tool.


The lines on the diagram represent wire. Wire comes in different sizes. There are two standards commonly used to define the size; in the U.S., the American Wire Gauge (AWG), and in Europe, the DIN standards. DIN uses “cross-sectional area” in mm². BMW motorcycle wiring diagrams use the DIN standards for wire size and other details.

6 Series-Starter Relay

Wiring Diagram Showing Wire Size in Square Millimeters (Source: Haynes Manual)

In the diagram, the left-most two RED wires are 2.5 mm² cross-sectional area and the right-most two are 0.75 mm². If you are using standard US wire, then you need to find the equivalent AWG gauge that provides the same, or larger cross-sectional area. It is important to understand that wire cross-sectional area INCREASES as AWG numbers DECREASE. That’s the opposite of the DIN specification where a larger number indicates larger cross-sectional area. And, the DIN sizes will not match AWG sizes. So, pick the AWG number that is smaller when the DIN value falls in between two AWG numbers. Here is a reference that shows the metric mm² value and the AWG values it is closest to.

Metric Cross-sectional Area vs. AWG Size

Here is a table that shows acceptable AWG sizes you can substitute for DIN sizes.

DIN mm²   AWG
0.5              20
0.75            18
1.0              16
1.5              14
2.5              12
4                 10
16                4

BMW Wire Colors

The wires have colored insulation, either a single solid color, or a solid color with a different color stripe. Some diagrams (such as the ones in the Haynes manual) show the actual wire colors while many others use two letter abbreviations, next to or along the wire, that define the colors.

When the wiring diagram uses two letter abbreviations they are for the German words for the colors. That makes it a little more convoluted until you get familiar with the German words for colors. Here is a table to help translate the abbreviations.

German DIN Wire Color Abbreviations

DIN Wire Color Abbreviations in English

BMW wire colors indicate something about what the wires connect to and/or what the wires are used for. This link from the Airheads Beemer Club (ABC) shows the various wire colors, including wires with two colors, and what they are typically used for on a BMW airhead. That said, there are exceptions to the color code schemes that you will find now and then.

You have to be a member of ABC to view this article. Why not join?
–> (http://www.airheads.org/)

Here are what the solid colors are used for.

  • RED Indicates power directly from the (+) terminal of the battery. It is “live” even when the ignition switch is off. It is not fused.
    These wires can be DANGEROUS. If you short circuit one of the RED wires directly to ground with a wrench, pliers, screwdriver or anything metal, all the power in the battery IMMEDIATELY flows through the wire and whatever creates a complete path to the (-) battery terminal which is almost every metal part on the bike. That will melt things and can damage electrical components.
  • GREEN Indicates power (+) AFTER the ignition switch and before Fuse #1. If the ignition switch is OFF, there is no power in these wires. For this reason, they are safer than the RED wires, but are still dangerous if you short circuit them when the ignition is ON.
  • BROWN Indicates a ground wire. This is a direct connection to the (-) terminal of the battery. There often are one, or more, physical connections of brown wires to the frame and sometimes to the engine and transmission. The assumption is the frame is connected to the battery (-) terminal at the speedometer drive cable bolt on the transmission that secures the speedometer drive cable. Since the engine and the transmission are fastened together, there is a ground path from the engine case to the transmission case.
  • YELLOW Connection to the headlight low beam.
  • WHITE Connection to the headlight high beam.
  • GREY Indicates power (+) after the ignition switch and before Fuse #2. If the ignition switch is OFF, there is no power in these wires. For this reason, they are safer than the RED wires, but are still dangerous if you short circuit them when the ignition is ON. These wires supply power to the parking and instrument illumination lights.

In addition to the wires with solid colors, some wires have two colors, a solid color and a stripe color. The purpose, or type of power, the wire is used for is indicated by the solid color, and a secondary attribute is indicated by the colored stripe.

For instance, a GREENBlack wire (solid color is GREEN, stripe is black) carries power from the ignition (indicated by the solid GREEN) after a fuse (indicated by the black stripe). Whenever you see a GREENBlack wire you know it is from the (+) terminal of the battery and is protected by a fuse. That means if this wire shorts to ground for some reason, the components it is connected to are protected since the fuse will “blow” and break the circuit very quickly before the large current flow in the wire that results from the short can damage any component the wire connects to.

Here are some of the solid-stripe wires:

  • BROWN-[any stripe] (+) wire to a normally open switch. The other side of the switch will have either a BROWN wire, or a physical metal connection to ground via a fastener or the switch body connection to the engine or transmission case.
  • BROWNYellow: Clutch switch.
  • BROWNBlack: Neutral switch.
  • BROWNGreen: Oil pressure switch.
  • BROWNWhite: Horn switch.
  • BROWNBlue: Low brake fluid switch.
  • BLUERed Left turn signals.
  • BLUEBlack Right turn signals.
  • BLUEYellow Starter switch (button).
  • GREENBlack (+) fused power from the ignition switch after Fuse #1.
  • GREY-Black Parking and instrument lighting after Fuse #2

Component Terminal Numbers

Where wires connect to components, you will see a number next to that connection. These numbers are defined by a DIN standard, (DIN 72552). Here is a link to the terminal numbers and what they mean published by Bosch.

For example, here is a starter relay for a /6 series airhead from the Haynes manual.

6 Series-Starter Relay

6 Series Starter Relay (1975-76) (Source: Haynes Manual)

Starting from the left, we see terminal (30) with two RED wires connected each of which is 2.5 mm² cross-sectional area. Next is terminal (85) with a 0.75 mm² BLUEYellow wire, then terminal (86) with two 1.0 mm² GREENBlack wires, (87) with two BLACK wires and finally terminal (D+) with two BLUE 0.75 mm² wires attached. From the DIN standard for terminal numbers:

  • (30) – Line from battery positive terminal (direct)

The next three are found in the “Switching Relay” category since the Stater Relay is a switching relay.

  • (85) Output, drive (end of winding negative or ground): The output side of the relay coil.
  • (86) Input, drive (start of winding): The input side of the relay coil.
  • (87) Input: On the switched side of the relay coil, note the other side is terminal (30). In the case where the other side of the switched connection through the relay is (30), then (87) is really the output connection, not the input. When the relay closes, battery power flows from (30) through the switched relay contacts and out terminal (87). [Even standards have subtle deviations :-)]

The last terminal is found in the “Alternators and Voltage Regulators”” category.

  • (D+) Alternator positive terminal. A look at the diagram shows this terminal does not interact with the relay in the starter relay. It’s just a point to connect two wires from the alternator output.

This is really helpful when you are looking at a component. All the terminals with (D+) are going to be connected together. Looking at the wiring diagram, you find that the starter relay, diode board and voltage regulator all have (D+) terminals and they are all connected together with BLUE wires that carry alternator output current.

I’ll discuss the 85, 86 and 87 terminals in more detail in the “Relays” portion of the “How Components Work” section.

Common Connections and Crossing Wires

Places where two (or more) wires physically connect together is called a common connection. You can think of this point as if all the wires are the same wire as far as current flow is concerned. Here is an example where multiple ground wires connect to the frame.

Common Connection – Frame Ground (Source: Haynes Manual)

Note the large black circle where the four wires cross and on the wire at the top of the symbol for a ground. That means these wires are physically connected together. In this case, via a ring terminal that is bolted to the frame. The ring terminal creates the electrical common point.

The symbol for a ground is a series of horizontal lines, stacked on top of each other forming a striped triangle. When you see this on a wiring diagram, it usually means the ground is made via a physical connection to the frame, engine block or transmission. A bolt or the part itself being in contact with these components creates the connection to the battery ground terminal.

You will see other instances of wires crossing each other, but there is no common connection between them. If there isn’t a large black dot where the wires meet, then there is no electrical connection between the wires.  They cross due to how the diagram was drawn. Here is a typical one.

Wires Crossing but Not Sharing a Common Connection                                 (Source: Haynes Manual)

Many wires are touching each other as they cross over, but these are not common connections. They don’t have the large round black circle.

Example of Two Common Connections (Large Black Circle) (Source: Haynes Manual)

In this image, there are two common connection points and all the rest are not. The black circle where the three 0.75 mm² GREENRed wires join and the other one with the two GREENBlack wires crossing each other with a black circle are common connections so the wires are electrically connected.

Another example is a terminal on a component where two wires connect. The electrical current flows though both wires as if they are the same wire. The current does not need to flow through the component for the current to flow in both wires.

6 Series-Starter Relay

Common Connections at Component Terminals           (Source: Haynes Manual)

Terminal (30) has two wires attached to it, so they act as if they are a single wire. The same is true for the two wires connected to (86), (87) and (D+).

Note that the (D+) terminal acts only as a common connection point for the two blue wires. The starter relay schematic shows that the (D+) terminal does not connect to any part of the relay mechanism, but two (D+) terminals are internally connected together. So, the (D+) terminals are acting as a common connection point for these two wires. You could easily assume, incorrectly, that the (D+) wires must be connected to some active part of the relay, but they aren’t.

By the way, if the internal connection between the two (D+) terminals fails inside the relay, then these two terminals are no longer a common point and current can’t flow in between the wires. This results in multiple component failures that are relying on the current in one (D+) wire to flow thorough the other. There will be more discussion about this in the “Circuits” section.

Here is another interesting use of a terminal as a common connection point, but the terminal is attached to a relay switched input terminal.

6 Series-Starter Motor & Solenoid

Common Connection on an Active Terminal (Source: Haynes Manual)

Terminal (30) has two wires, a BLACK and a RED. Terminal (30) is also an input to the switched relay contacts so when the relay closes, current flows into the starter motor. But, since the two wires are connected at terminal (30), current flowing in either the black or red wires can flow through the other one even when the relay is open. This is a case where an active relay terminal also acts as a cross connect so it is serving two functions at the same time: input to the relay switched contacts and a common connection point for two wires.

Circuits, Open and Closed

A circuit is a path from the (+) to the (-), or ground, terminal of the battery. As shown in a typical wiring diagram, such as the one above for the /5 series, there are many possible circuits within the wiring system. A circuit can be “open” or “closed” at different times.

When you look at the diagram, you may see a  path between the (+) and (-) terminal of the battery through various components, but the path through a component may not be complete. For switches and relays, internal electrical paths maybe “open” when there is no internal connection. In that case, the circuit is “open” and electricity won’t flow. For example, when a switch closes, a complete path from the battery (+) to (-) terminals is created. The previously open circuit is now “closed” and electricity flows through the terminals of the switch.

Look at the fragment of the /5 wiring diagram below showing a simplified view of the horn circuit.

Series Circuit

5 Series Simplified Horn Circuit

Although the wires connected to the components create a complete path between the (+) and (-) battery terminals,this is an open circuit until BOTH the ignition switch and the horn button are turned on. When the ignition is on, that switch is closed, but only when the horn button is pushed does the horn circuit become a closed circuit allowing current to flow through all the wires and components of the horn circuit.

The battery (-) terminal is connected to the transmission speedometer drive bolt so the metal frame, engine and transmission cases act like a wire going to the battery (-) terminal. That’s indicated by the ground symbol on the battery (-) terminal and I show that by the dotted line from the horn button to the (-) battery terminal. 

Some components have an internal electrical relay. This acts like a manual switch, but is turned on and off by an electrical current flowing through the electromagnetic coil of the relay. If the relay is open then no current flows through the electromagnetic coil and the circuit is open so no current flows through the component. See the Components document for more about relays.

Circuits, Series and Parallel

When all the components in a path are connected sequentially in a closed electrical current, this is called a series circuit since all the components are in series. Here is an example using a simplified portion of the 5 Series wiring diagram of the horn circuit.

Series Circuit

Series Circuit

There is a complete circuit since all the components have a path from the (+) battery terminal back to the (-), or ground, battery terminal. When both the ignition and horn switches are closed, electrical current flow through all the components in the closed circuit.

In a series circuit, the current flow is the same through each component. To see why that is so, remember that each component, including the wire, has a resistance, Rc. The sum of the component resistances equals the total resistance along the path, Rt.

Rt = R(ignition switch) + R(fuse#1) + R(horn) + R(horn button) + R(wire)

Let’s say that Rt is 10 Ohms. From Ohm’s law we know that:

I = E / Rt.
Plugging in what we know for E and Rt,  I = 12.6 / 10.
So, I is 1.26 amps and every component in this series circuit has 1.26 amps flowing through it.

The other fact about a series circuit is that each component will reduce the voltage as the electrical current flows through it. Further, the sum of all the voltage reductions caused by each component, also known as the “voltage drop”, equals the voltage between the (+) and (-) battery terminals, or about 12.6 volts for a fully charged battery. So in equation form where Et is the total voltage drop around a series circuit:

Et = E(ignition switch) + E(fuse#1) + E(horn) + E(horn button) + E(wire) = 12.6 volts

When there are two, or more, paths this is called a parallel circuit as shown below.

Parallel Circuit

Parallel Circuit with Two Paths Between (+) and (-) Battery Terminals

Again, path 2 is complete path between the (+) and (-) battery terminals. It is an open circuit until BOTH the ignition switch and rear brake light switch are closed. When that is true, it is a closed circuit and electrical current will flow through it.

Since both paths start at the (+) battery terminal and end at the (-) battery terminal, the sum of the component voltage drops around each circuit will be the same, or about 12.6 volts for a fully charged battery.  But, the current flowing through each path does not have to be the same, and in fact, rarely is the same. This is easy to understand if you use Ohm’s law and appreciate that the total resistance of all the components in Path 1, R-t1, is likely to be different than the total resistance in Path 2, R-t2. Since the voltage drop is the same for both paths, 12.6 volts, then from Ohm’s law:

I = E / R
I-t1 = 12.6 / R-t1
I-t2 = 12.6 / R-t2
since R-t1 does not equal R-t2, I-t1 is different from I-t2.

Notice that if the ignition switch is closed and the rear brake light switch is closed, Path 2 is a closed circuit. So electrical current will ONLY flow through:

  • the RED wire from the battery to the ignition switch,
  • the GREEN wire from the ignition switch through Fuse #1,
  • the GREENBlack wire from Fuse #1 to horn Terminal (15),
  • the GREENBlack wire from horn terminal (15) through the rear back light switch,
  • the GREENRed wire through rear tail light bulb,
  • finally, the BROWN ground wire back to the (-) battery terminal.

BUT, there is no electrical current flowing through:

  • the horn, or
  • the horn button (until that button is pressed).

Further, as soon as the rear brake light switch is open, NO current flows through any of the wires or components in either Path 1 or Path 2.

Using Circuit Diagram Fragments To Find Electrical Problems

Why is all that important?

If you are trying to isolate an electrical problem in the tail light, and you have this circuit diagram fragment in front of you, a test could be to try the horn.  It it works, you know all components in Path 1 work. The tail light problem has to be isolated to the components in Path 2 that are UNIQUE to Path 2, including:

  • the GREENBlack wire between horn terminal (15) and the rear brake light switch,
  • the rear brake light switch,
  • the GREENRed wire from the rear brake light switch to the tail light bulb,
  • the bulb,
  • and the BROWN ground wire and connection to the frame.

If the horn does not work when you push the horn button, then it’s likely the problem is with components shared by both Path 1 and Path 2, including:

  • the battery,
  • RED battery cable to the ignition switch
  • the ignition switch
  • the GREEN wire from the ignition switch to Fuse #1
  • Fuse #1
  • The GREEN-Black wire from Fuse #1 to horn terminal (15)

I said “likely”, because its possible you have more than one problem, although that’s not as likely as failure of a single component.

I hope this introduction to the “language” of wiring diagrams makes them less intimidating when you look at one. There is a “method to the madness”.


2019-01-22 Added links to other electrical documents. Update Resources. Formatting.
2020-12-28 Minor corrections.
2021-03-16 Correct minor typos.
2021-08-28  Correct wire color error of parallel circuit in “Circuits, Series and Parallel”.

19 thoughts on “Electrical Basics

  1. thanks for the extensive explanation of not only how the electrical system works in terms of fundamentals (electrons/protons) but also how it specifically applies to BMW wiring—which will hopefully in my case help un-riddle the horn and persistent battery drain issue on my 1975 r75/6 that is currently bedeviling me.

    • Hi Brant,

      I hope you get to the cause of your problem. I created this material because I hoped it would help folks when trying to find the cause of an electrical problem.


    • Hi Brant,

      Wondering if you got to the bottom of your battery issue. I’m having the same draining issue with my 75/5.

  2. Simple question. I’m new to circuits. I have to remove the wires from the connector “block” for the turn signal switch on a 1990 R100GSPD so it can fit through the frame. I think I need to buy a little tool to release the metal connector from the block. Not sure what I need. I’ve heard Lisle is a company that makes such tools. Any advice. Thanks in advance.

  3. Brook,
    I’m going to hold two, one hour basic Electronic classes for some 5th grade students at the elementary school that I work at ( I received a small grant). I’m going to touch on AC and DC electricity/Safety and show them how to use a multi meter, soldering iron, and demonstrate an oscilloscope.

    Would you mind if I went over with them your basic electronics descriptions (fundamentals, electrons, conductors, voltage, current, and resistance) directly from this link? I will make sure to cite you and show them all of the sources where you found your information.

    It may help catch their interest if I let them know that I’m fixing a motorcycle with help from your electronics section.

    Please let me know, no worries either way.
    Art Elola

    • Hi Art,

      How cool. Please feel free to borrow whatever you think would be helpful. IMHOP, anything we do to encourage kids to “tinker” is a good thing for them and for the world at large.


  4. I’m not an electrical expert but I can follow a diagram…usually. My friend had a guy work on his 1974 R75/6, he managed to meltdown the entire harness from the taillight to the headlight. He shorted the wiring in the taillight.
    The wiring to the handle bars weren’t bothered.
    A new harness is not available from BMW but I located a good used 1975 R75/6. It came in good shape but the colors were not the same as the ‘74. I believe I have figured out the differences. The starter engages but I have yet to put the fuse in for the taillight, brake light, signals etc.
    Is there any reason or glaring mistake I could make at this point that the fuse won’t handle? I hate to think that the harness would let the smoke out again.
    The starter relay is my biggest worry, it is a 7 pin and I think the ‘75 is an 8 pin.
    If the wiring isn’t perfect on the relay will it cause problems? The 3 wires to the starter are good, that much I know.
    Any information is truly appreciated.

    • Hi Dan,

      That sounds like there was a “hot time in the old town tonight” with that short. 🙁

      Just to let you know, you can get a replacement harness for the 1974 bikes at Euro Motoelectrics at a reasonable price.
      –> https://www.euromotoelectrics.com/product-p/chassis-har457.htm

      I looked at the parts fiche and the starter relay part number does not change between 1974 and 1975. But, I would not be surprised that the 1974 relay may have had one less terminal (87) than the 1975 as the 1975 diagram shows two black wires going to it while the 1974 shows only one. In the end, the relay has terminals (30), (85), (86), (87) and (D+). So long as you route the correct 1974 wire colors to the correct starter relay terminal, the relay should function correctly,.

      That said, when I look at the wiring diagram for the 1974 and 1975 bikes, there are a number of changes in the wiring. In particular, there are different wire colors and locations for wires that connect to the starter relay. You said you figured out the differences so at some point you have to test things to see if you got it correct.

      In my experience, a dead short is what fries a wiring harness. Consequently, if the used harness has no spots with bare insulation that could touch the frame or other metal that creates a direct path to the battery ground, then you shouldn’t let the smoke out.

      “If the wiring isn’t perfect on the relay, will it cause problems?” Well, yes it will cause problems as it needs to include all the connections expected for the 1974 wires to the relay. I can’t help you with that issue as there is no way I can understand “…isn’t perfect”.

      I hope this helps.

      Brook Reams.

      • You are correct there are color differences but I don’t believe that is my issue now.
        I bit the bullet and energized the bike with the 7 pin. I had the starter with spark at the plugs along with the turn signals. No headlight, taillight, brake light, (front or rear brake actuation).
        I went back to the ‘75 schematic and noticed that the ignition switch is very different than the ‘74.
        Do you think that changing the switch for a ‘75 version would solve my problem?
        I’ll be calling Max’s Monday to verify that they are two different p/n’s and ordering one if they don’t have it. I have to go there Thursday anyway.
        If this doesn’t sound reasonable I probably will be chewing another bullet and getting that new harness from Euromoto.
        Any further guidance is appreciated.
        Thank you

        • Hi Dan,

          So, some success, but no cigar yet. 🙂

          The taillight and brake lights not working suggest that the GREEN-Black wire to the start relay terminal (86) from the wiring block in the headlight shell is not supplying current to terminal (86). The other GREEN-Black wires on terminal (86) power the brake light switches and the horn. You don’t mention if the horn works.

          The headlight not working suggests no power to the RED wire going to the headlight switch, which is terminal (30). That wire comes from the wiring block in the headlight shell. If it’s loose on the wiring block terminal or not connected to one of the spades on the (30) set of terminals that would keep it from working.

          Since you get power to the plugs, the ignition switch is getting power from the RED wire to its terminal (30) and is sending to to its terminal (15) because that’s how power gets to the turn signals and the ignition. So, changing out the ignition switch does not seem necessary.

          There is a separate GREEN-Black wire from the wiring block at terminal (15) that powers the front brake light switch. If that’s loose on the wiring block terminal, that would prevent it from working. The rear brake light switch and horn get their power from the starter relay terminal (86). So both the front and rear brake lights not working implies separate issues with power to those switches, not a single issue.

          That said, there is one place where the brake lights have a common set of connections, that is the GREEN-Red wires that attach to the (56) area of the wiring block inside the headlight shell. If the GREEN-Red wires aren’t connected there, the brake lights don’t work. And there is one other common connection of the GREEN-Red brake light wires that is inside the rear tail light housing. If both GREEN-Red wires aren’t connected together there, the brake lights don’t light.

          I hope this is helpful.


          • Brook, I hate to keep bothering you but where is the schematic showing the terminal numbers you are referencing?
            The Chiltons I have doesn’t have any numbering.
            Yes, I did have the horn but it isn’t the correct one for this bike. Someone put a single terminal Fiamm on it. That brings in another issue, I connected one of the wires to the horn. When the key was turned on the horn blew? I unplugged the wire and let it dangle…it hit the mounting nut when I had the key on and everything went dead! No lights on the gages, nothing. The fuses popped , I replaced them, still nothing. I then swapped out the 7 pin starter relay for an 8 pin and electrical life returned but nothing changed.
            When I pull the front brake lever the “brake failure” light illuminates.
            This is becoming a fun challenge but I’m close to giving up too. I’ll continue to review my actions, no terminals are loose, nothing is frayed and I’m getting the correct 2 terminal horn (round chrome one).
            Until it arrives I’ll review my work.
            Once again, I appreciate your guidance and time.
            Thank you

  5. Fantastic read and very helpful, I have a non starting issue with my 84 R80 the start button isn’t working (turning over the bike). Iv done some basic fault finding and know the starter motor is fine. I have also tested the button it self. I suspect it’s the starter relay (I have one on order) I have noticed some wear on one of the two black cables coming from 87 the one that goes to the starter seems fine but the other is worn.
    My problem is iv tried to chase it back to the headlight board but haven’t had much luck. Where should this second black wire be connected ???

    Many thanks for your help in advance


    • Hi Adam,

      On the 1984 R80, and other models, there are two BLACK wires on terminal (30). On terminal (87) on the starter has two RED wires, although they may have a black sheath over them making it more difficult to see the RED.

      For terminal (30), the thicker wire goes to the starter solenoid and the thinner wire goes to the headlight relay so the headlight turns off when the starter is working.

      For terminal (87) with the two RED wires, one RED wire goes to the clock and the other goes to junction that sends RED wires to terminal (30) of the headlight relay, terminal (30) on the connector block inside the headlight relay (for use in powering auxilliary items), and the last one goes to terminal (30) of the ignition switch.

      You ought to get a copy of the Haynes manual which has wiring diagrams. There are some errors, but the information will help you see what goes where.

      I hope this helps.


  6. BMWR100S 1977 Hi Brook I was enjoying the kinda rare sun here in the UK ,the bike was running great….then the voltmeter shot to the left, and smoke appeared from under the tank, on inspection the wire from the left hand ignition coil had melted along with the negative battery cable sheath, this was today so before i go looking do you have any thoughts or pointers? Regards Ian

    • Ian,

      I’m sorry to hear about the smoke escaping from your electrical system. 🙂

      I would carefully trace all the wires in the vicinity of where the short occurred for signs of damage or frayed insulation. Also, look for any locations where wires are able to chafe against bolts or hard surfaces and try to reroute so this can’t happen.

      It sounds like you got a dead short across the battery ground cable (-). Of course, I would replace the battery (-) cable. You can check the resistance of your coils to see if they failed with an open circuit using an ohm meter. Infinite ohms across the two small terminals means the coil is broken and needs to be replaced.

      At least you didn’t have a fire as that can really ruin your day.


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