61 BMW 1973 R75/5 Install Electrical System

It’s time to take a big step forward on the project and install the electrical system. I removed the wiring harness and all the peripheral electric components so I could have the frame powder coated.  I documented some of that disassembly in this write-up:

At this point, with the exception of the rear fender and the attached turn signal and brake light assembly, I have everything installed in the frame that the wiring harness connects to.

Parts List

Here is the list of new parts I installed.

Part # Description Qty
BCK-475473 Battery cable, (+), no grommet
[Euro MotoElectrics]
1
BCK-475473 Battery cable , (-)
[Euro MotoElectrics]
1
AT-12V20P Battery 12V/22AH – BMW R & K / WestCo AGM
[Euro MotoElectrics]
1
61 13 1 352 095 Engine top rubber grommet 1
61 31 1 351 646 Front Brake Switch, Rubber Cap 2
63 12 1 356 921 Headlight Mounting Bolt 2
16 11 1 230 030 Rubber Support, Tank Bracket
[aka, Rubber Wiring Loom]
1

Wiring Harness Repairs

The wiring harness is in need of a couple minor repairs. The outer sheath where the main harness separates into the rear frame harness, rear brake switch wires and the transmission neutral switch and engine oil pressure switch wires has cracked exposing the wiring underneath it.

Broken Wiring Harness Sheath

Broken Wiring Harness Sheath

I used electrical tape and wrapped over the sheath with two layers wrapped at 45 degrees to each other. I added small tie wraps on either end to keep the ends of the tape from pulling loose over time.

Fixing Broken Sheath on Main Wiring Harness

Fixing Broken Sheath on Main Wiring Harness

Starter Relay to Battery (+) Terminal Wire Repair

The connector was badly corroded after 40 years and needs to be replaced.

Corroded Lug on Starter Relay Wire to (+) Battery Terminal

Corroded Lug on Starter Relay Wire to (+) Battery Terminal

This is a short lead that connects to the (+) terminal on the battery. So I used my Dremel tool with a cutting disk to cut through the tabs that were bent over the wire and then carefully used a small screw driver blade to get the wires separated from the lug.I used a new lug and some shrink wrap tubing to make the repair.

Repair for Stater Relay Wire to (+) Battery Terminal

Repair for Stater Relay Wire to (+) Battery Terminal

I crimped and soldered the wire into the new lug and shrunk the tubing over the lug. I also used a small piece of shrink wrap over the cracked outer sheath where this wire separates from the main wiring harness. Here’s the completed repair. You can see the new lug on one end and the shrink wrap repair to the cracked sheath on the other.

Repaired Starter Relay Wire to (+) Battery Terminal

Repaired Starter Relay Wire to (+) Battery Terminal

Installing Electric Components on Frame

I already installed the rear brake light switch when I installed the swing arm and rear shocks. And, the front brake light switch was installed in the right control perch when I installed and adjusted the front brakes. The other parts that go on the frame are the starter relay, voltage regulator, horn and ignition coils.

Attach Starter Relay and Voltage Regulator

I attach the starter relay to the left side of the main frame tube and the voltage regulator to the right side using the Allen head bolts into the tapped holes in the brackets.

Mounting Starter Relay on Right Side Bracket

Mounting Starter Relay on Left Side Bracket

Mounting Voltage Regulator on Left Side Bracket

Mounting Voltage Regulator on Right Side Bracket

The voltage regulator on the right side connects to the plug coming from the engine wiring harness as shown above. The starter relay connects to wires from the engine wiring harness and from the headlight shell.  I keep a diagram of what wires go where when I removed the starter relay. I updated the diagram with the numbers of the terminals and show then in ( ) while [Front] and [Back] indicate which cable the wire comes from.

Starter Relay Wiring Notes

Starter Relay Wiring Notes

The wires from the engine wiring harness connect to the terminals on the back side of the starter relay (next to the main frame tube). Here is where they go on the relay with the backside against my thumb. I’ll connect he headlight shell wires after I install the main wiring harness.

Engine Wires Attached to Starter Relay

Engine Wires Attached to Starter Relay

Attach Ignition Coils

The coils also mount to brackets on the main frame tube using Allen head bolts and circular straps.

Ignition Coils and Mounting Brackets

Ignition Coils and Mounting Brackets

Before I mount the coils, I connect the terminals so the coils are in series. BMW uses 6 volt coils, one for each spark plug, and wires them in series so they draw 12 volts. Both plugs fire at the same time. I connect the (-) terminal on one coil to the (+) terminal on the other. If you look closely at the picture you can see the (+) and (-) markings on the black top cover of each coil.

Coils Wired in Series (+) to (-)

Coils Wired in Series (+) to (-)

I mount the straps loosely to the frame with one Allen bolt so I can easily slip the coils into the brackets and then tighten up the Allen bolts to secure them. I orient the coils so the black jumper wire between them is at the top with the empty two prong terminals at the bottom. There is a picture of this later on.

Coil Bracket Mounted on Frame

Coil Bracket Mounted on Frame

Attach Horn

This is the horn assembly. It has the horn, three mounting straps and hardware. The bolt, lock washer and the thin nut go together to mount the three straps in the horn bracket on the frame. The wave washer and thicker nut go together and mount the horn to the three straps

Horn Assembly

Horn Assembly

The three straps mount on the mounting bracket underneath the steering head.

Horn Bracket under Steering Head with Bolt Welded To It

Horn Bracket under Steering Head with Bolt Welded To It

The straps are bolted to the bracket, which has a nut welded to it, using the 13 mm bolt and the thin nut acting as a lock nut.

Three Mounting Straps Attached to Horn Bracket

Three Mounting Straps Attached to Horn Bracket

Three Straps With Thin Lock Nut on Threads of Bolt

Three Straps With Thin Lock Nut on Threads of Bolt

The horn I have has the chrome grill that is oriented so the connecting terminals on the back (which you can’t see in this picture) are at the 10:00 position as viewed from the front of the bike.

Horn Mounted

Horn Mounted

Installing the Headlight and Main Wiring Harness

I removed the headlight assembly with the main wiring harness during the tear down. I made some paint repairs to the headlight shell and didn’t remove the wiring harness from the shell.  One look inside the shell and I decided to leave well enough alone 🙂

Install New Rubber Wire Loom

The rubber wire loom goes on the smaller tube beneath the main frame tube and has a number of holes for routing cables and wires along the frame. The original one was rotted so I installed a new one. The large rectangular hole that’s open at the top fits around the main frame tube while the smaller hole that’s closed at the bottom fits around the engine wiring harness and holds it in place.

Rubber Wiring Loom

Rubber Wiring Loom

I slide it to the front up against the starter relay and then pushed one of the upper ears under the main frame tube.

Inserting Rubber Wiring Loom

Inserting Rubber Wiring Loom

Install Headlight Shell and Front Reflectors

Here is the front headlight mounting hardware. The rubber gaskets surround the chrome mounting bolts that go through the holes in the headlight ears and the bolt head pushes against the chrome caps.

Headlight Shell Mounting Assembly

Headlight Shell Mounting Assembly

The rubber gaskets have a raised section on one side that fits into the slot in the headlight ears and the round tube for the headlight bolt fits between the two gaskets.

Attaching Headlight Shell Gaskets

Attaching Headlight Shell Gaskets

The chrome bolt goes through the rubber tube and threads into a nut inside the headlight shell. I had my wife hold the shell for me as I juggled the three rubber gaskets and the chrome nut so I wouldn’t drop the headlight shell.

Mounted Headlight Shell

Mounted Headlight Shell

The front reflector mounts on the headlight bracket. It has a shaped rubber gasket with a locating rubber pin on the back that fits into the round hole next to the slot for the reflector bolt.

Back of Headlight Reflector Rubber Gasket Showing Rubber Locating Pin

Back of Headlight Reflector Rubber Gasket Showing Rubber Locating Pin

Headlight Ear Front Reflector Mounting Holes

Headlight Ear Front Reflector Mounting Holes

The reflector bolt has a large flat washer and locking nut to hold it to the headlight ear.

NOTE: If I did this again, I’d mount the reflectors before I mount the headlight as it is be easier to tighten the lock nut without the headlight shell in the way.

Protecting Unused Front Turn Signal Wires

I’m not using the front turn signal wires as I will be using the Windjammer fairing wiring harness to power the turn signals that are mounted on the Windjammer. I loop them on the Windjammer cable and secure with tie wraps to keep them save and out of the way.

Front Turn Signal Wires (Not Connected) Safely Tie Wrapped to Windjammer Cable

Unused Front Turn Signal Wires Safely Tie Wrapped to Windjammer Cable

Routing Control Perch Switch and Horn Branches

There are several cables coming out of the headlight shell. Two of them have the switch assemblies that mount in the control perches on the handle bars. There is a small pair of wires that go to the turn signals (which I’m not using and didn’t route through the headlight ears) and the large main wiring harness. Here is the routing of the left side switch assembly cable and installation in the left control perch.

Routing of Control Perch Switch Wiring from Headlight

Routing of Left Control Perch Switch Wiring from Headlight

Left Control Perch Switch Cable Routing

Left Control Perch Switch Cable Routing

Here is the routing of the horn branch next to the steering head. It goes under the small diameter brace tube over to the right side to attach to the terminals on the horn.

Horn Wire Branch Routing

Horn Wire Branch Routing

Routing the Wiring Harness Along the Frame

Taping Wires Together Before Routing

Taping Wires Together Before Routing

The main wiring harness includes a number of shorter branches and all of them have a mind of their own. To get them to do my bidding, I used electrical tape to bundle the shorter branches to the main one so I end with with a single cable that tapers down until it ends in the branch that goes to the rear turn signals and tail light.

Then I thread the cable from the front of the bike so it goes along the main frame tube between the brackets for the starter relay and voltage regulator.

Threading Main Wiring Harness From Headlight Along Frame Backbone

Threading Main Wiring Harness From Headlight Along Frame Backbone

The cable goes through the rectangular hole at the top of the rubber wiring loom next to the main frame tube and continues to where the battery box is.

Routing Main Wire Harness Between Brackets and Through Rubber Loom

Routing Main Wire Harness Between Brackets and Through Rubber Loom

There are two short branches near the steering head, one has two wires for the horn and the other has the wires that connect to the starter relay.

Starter Relay Headlight Wiring and Horn Wiring Branches

Starter Relay Headlight Wiring and Horn Wiring Branches

I connect the horn wires routing the cable to the right side of the main frame tube. I disconnect the starter relay and route the headlight cable behind the bracket and then connect the wires to the lugs on the relay and then reattach the relay to the bracket.

Routing Headlight Branch Behind Bracket to Starter Relay

Routing Headlight Branch Behind Bracket to Starter Relay

I attach the right control perch switch to the perch using the screw and then attach the switch to the housing using the smaller screws on the side of the housing.

Attaching Switch to Right Control Perch

Attaching Switch to Right Control Perch

Attaching Switch to Switch Housing

Attaching Switch to Switch Housing

I add the new brake light switch rubber boot to the short brake light branch and then mount the wires inside the terminals of the front brake light switch.

Front Brake Switch Wire Boot

Front Brake Switch Wire Boot

Right Control Perch Front Brake Switch Terminals

Right Control Perch Front Brake Switch Terminals

Attaching Front Brake Switch Wires

Attaching Front Brake Switch Wires

Right Control Perch Switch Cable Routing

Routing of Right Control Perch Switch Cable From Headlight

Routing Rear Frame Branch Cable

The main wiring harness divides into separate branch cables going to the rear brake light switch, the transmission neutral and oil pressure switches and the branch that routes through the rear sub-frame to the tail light and rear turn signals.

I use a piece of wire and thread it from the hole at the back of the rear sub-frame to the hole on the other side of the chrome grab rail.

Pull Wire for Rear Frame Wiring Branch

Pull Wire for Rear Frame Wiring

Pull Wire for Rear Frame Wiring

Pull Wire for Rear Frame Wiring Branch

I wrap the end of the wire next to the grab rail around the branch cable going to the tail light and turn signals and then cover this with electrical tape. Then I pull the branch cable through the rear sub-frame tube using the end of the wire at the back of the sub-frame while pushing it from the front near the grab rail. The last picture below shows the pull wire wrapped with electrical tape.

Attaching Pull Wire for Rear Frame Wiring

Attaching Pull Wire to Rear Frame Wiring Branch

Starting to Pull Rear Frame Wiring Branch Through Rear Frame Tube

Starting to Pull Rear Frame Wiring Branch Through Rear Frame Tube

Rear Frame Wiring Pulled Through

Rear Frame Wiring Branch Pulled Through Rear Frame Tube

I detach the pull wire and adjust the rear frame branch so the other end where it separates from the main wiring harness is positioned to allow the other branches to be routed along the back of the frame. The places the branch junction near the grap rail and the rear down tube of the frame as shown below.

Routing Rear Frame (Top) Rear Brake Switch, Neutral Switch, Oil Pressure Switch (Middle) and Starter Relay Wire (Ring Terminal)

Routing Rear Frame (Top) Rear Brake Switch, Neutral Switch, Oil Pressure Switch (Middle) and Starter Relay Wire (Ring Terminal)

Routing Rear Brake Light Branch

The rear brake light branches off the transmission neutral and oil pressure branch near the bottom of the battery box.

Routing Neutral Switch & Oil Pressure (Through Hole in Frame Gusset) and Rear Brake Light Wires (Under Battery Box)

Routing Rear Brake Light Wires (Under Battery Box)

I route the rear brake light switch under the battery box bracket next to the lower horizontal brace tube over to the rear brake light switch inside the right exhaust mount bracket. This picture shows the wire ties I used to secure it, but I didn’t add them until I had the wires connected to the rear brake light switch.

Rear Brake Light Switch Wire Routiing

Rear Brake Light Switch Branch Routing Under Battery Box Bracket

The rear brake light switch has a rubber cover with two small holes for the wires. I pushed them through the holes and then connected the wires to the screw terminals on the brake light switch.

Rear Brake Switch Rubber Cover

Rear Brake Switch Rubber Cover

Attaching Wires To Rear Brake Light Switch (Inside Right Side Exhaust Bracket)

Attaching Wires To Rear Brake Light Switch (Inside Right Side Exhaust Bracket)

Rear Brake Switch Wires

Rear Brake Switch Wires Attached to Switch

Then I made sure the wires were routed so there is some slack by the rear brake light switch and then added the tie wraps to secure this branch to the horizontal tube under the battery bracket as shown in an earlier picture.

Routing Transmission Neutral and Oil Pressure Branch

This branch routes through the gap between the main frame rear down tube and the gusset that reinforces the swing arm tube.

Routing Neutral Switch & Oil Pressure (Through Hole in Frame Gusset) and Rear Brake Light Wires (Under Battery Box)

Routing Neutral & Oil Pressure Branch Through Hole in Frame Gusset

This branch routes along the bottom of the lower main frame tube where it splits into the transmission neutral wire and the oil pressure sender wire. I route the oil pressure wire so it passes through the hole in the bottom of the transmission housing up through the brass button and through the set screw hole on the back of the transmission.

Transmission Neutral Switch Wire

Transmission Neutral Switch Wire Routing Underneath the Frame

The set screw has a very small slot in it requiring a thin blade screw driver to tighten it.

Tightening Transmission Neutral Switch Screw

Tightening Transmission Neutral Switch Screw

The oil pressure sender wire routes along the lower main frame tube and connects to the tab on the sender. I tie wrap the oil sender wire on the inside of the frame tube to protect it from being snagged by a foot when shifting.

Oil Pressure Switch Wire Routing

Oil Pressure Switch Wire Routing

Connect Starter Motor Solenoid Cable to (+) Battery Terminal

The new cable to the starter motor solenoid connects to the large screw terminal on the starter solenoid. The cable passes through a slot in the engine case and a rubber grommet to seal the cable to the case and the aluminum starter motor cover (not shown). I bought a separate rubber grommet (some cables already include it). The top of my grommet was solid rubber so I cut an X in it and then pushed the starter cable through as shown below.

Battery ( +) Starter Motor Cable with Gromet

Battery ( +) Starter Motor Cable with Grommet

I connect the cable to the screw terminal. The cable is thick and stiff and didn’t want to go on the terminal which has a boss that holds the cable in place. I ended up bending the tab about 30 degrees in my vice to make it easier to route. I also bent the cable by hand with a sharp turn where it has to go up through the engine case.

Connect Battery (+) Cable to Starter

Connect Battery (+) Cable to Starter

The I route the cable to the right side of the rubber wiring loom into the large hole at the top. The holes in the wiring loom are split and you can easily slide a cable into a hole by spreading one leg of the loom to open the split and slide the cable in. Then I loosen the coil brackets and pass the cable between them and angle it toward the left side of battery where the (+) terminal is bending it into an S-curve. I add a tie wrap to hold the cable against the side of the large backbone frame tube.

Starter Cable Routing Between Coils

Starter Solenoid Cable Routing in Rubber Wire  Loom & Between Coils

The (+) battery terminal attaches to both the starter motor solenoid cable and the smaller diameter starter relay wire that breaks out of the main wiring harness near the grab rail.

New Battery Installed with Starter Relay Wire on (+) Battery Terminal

New Battery Installed with Starter Relay Wire on (+) Battery Terminal

Starter Cable and Starter Relay Wire to Battery (+) Terminal

Starter Solenoid Cable and Starter Relay Wire Connect to Battery (+) Terminal

Attaching the (-) Ground Battery Cable

The ground cable has a 90 degree bend in one lug and that attaches to the (-) battery terminal.

Battery Ground (-) Cable

Battery Ground (-) Cable

Battery Ground (-) Connection at Battery Uses 90 Degree Lug

Battery Ground (-) Connection at Battery Uses 90 Degree Lug

The other end of the ground cable is a flat lug that connects to the transmission breather bolt that also holds the tachometer cable. I route the ground cable with a slight S-curve so it tucks between the vertical frame tube and the battery box.

CAUTION: Do not over tighten the transmission breather bolt or you will strip the threads.

Battery Ground (-) Connection to Transmission

Battery Ground (-) Cable Routing and Connection to Transmission Breather Bolt

Smoke Test

I pushed the key in the headlight shell down.  All the indicator lamps lit and no smoke was evident anywhere. 🙂

Smoke Test Success - It's ALIVE!!!

Smoke Test Success – It’s ALIVE!!!

I will test the turn signal and brake lights when I attach them.

DANGER: If your engine is disassembled as mine is DO NOT HIT THE STARTER BUTTON when the starter motor is attached to the flywheel.

I unbolted the starter motor so it was clear of the flywheel, grounded it and then pressed the starter button and the motor spun.

Then I disconnected the (+) and (-) cables from the battery so something inadvertent doesn’t happen as there are still exposed wires.

Here’s what the project looks like at this point.

What The Project Looks Like With Electrical Wiring Installed

What The Project Looks Like With Electrical Wiring Installed

18 thoughts on “61 BMW 1973 R75/5 Install Electrical System

  1. Pingback: 1973 BMW R75/5 Rebuild: Install Wiring Harness & Electrical System | Motorcycles & Other Musings

  2. Hello and thanks for the detailed pics and text about wire harness routing. They have been very helpful thus far.
    I am beginning to upgrade my 1970 SWB R75/5 wiring. The earlier owner had run additional/extra wires for some reason, probably to fix gremlins as they occurred. Although this has ‘worked’ for the 4 years I have ridden and learned how to wrench on it I now realize it may potentially dangerous and am ready to install a /3 harness.
    I don’t yet know what the difficulties will be, but I am looking forward to proper wiring (and fuses) for the next riding season.

    • Hi Mike,

      Thanks for coming by and reading some of the write-ups I’ve posted. The wiring is always mysterious to me until I take it apart and take pictures of what went where. Bob Fleischer (aka, “Snowbum”) has a lot of wiring diagrams on his site and those may be helpful to you.

      Best of luck on the “nervous system” upgrade on your R75/5.

      Best.
      Brook

  3. I want to add my thanks for this fantastic profile, including the volume of photos. I’m working through a similar wiring project right now on a ’71 75/5, and this information–particularly how it is presented–is exceptionally helpful.

    • Hi John,

      Thanks for the very kind words. I’m glad this material is helping you on your project.

      Best of success on your 71 R75/5 project.

      Best.
      Brook

  4. Just wanted to say thanks for this whole series of posts. I’ve got a 1970 R75/5 that came as a basket case and this series has been hugely helpful for things like cable routing and the exact positions of parts relative to each other.

    • Hi Chris.

      Thanks for dropping by and the kind words. I figured folks would appreciate some detailed photos showing cable and wiring routes as well as what goes where.

      Best of success in getting yoru R75/5 back on the road.

      Best.
      Brook.

  5. Thanks for some useful information and photos on your project ! Very helpful !
    I have a 1970 R60/5 and when out yesterday for a run found out that when removing the key to stop the bike (not to easy on the other side of the screen I may add) the warning lights all went off but the engine kept running. I stalled it then started it again OK but NO stop on removing the key. Today it all works OK again.
    What I would like to do as a useful alternative is wire in a stop switch button onto the bars as an alternative way of stopping it. Any ideas how to do this please ?

    • Hi John,

      I’m glad this material is useful to you.

      To answer your question, no, I don’t really have much advice on adding an engine “kill” switch to the bike. You can look at the wiring diagram for the R50/5 here:
      http://5united.org/articles/wiring/

      As a kill switch was added on the /6 series, you can see where that switch connects to the wiring on this diagram from Bob Fleischer’s site (aka, Snowbum).
      http://bmwmotorcycletech.info/schematic1975,1976.htm

      Based on what you are telling me, when you remove the key (aka, the “nail”), the switch isn’t opening to stop the engine. My quick glance at the /5 wiring diagram shows a green wire going to the low voltage side of the coils and another to the indicator lights. When the nail is removed, I assume this opens the connections to those green wires, except it isn’t opening the connection to the coil so the engine keeps running. If maybe that some cleaning with electrical contact cleaner would get things right again.

      And, if you want to get good advice on how to do this, you ought to join the micapeak airhead forum where you can post any technial questions you have and you will get very informed input.

      http://micapeak.com/mailman/listinfo/airheads

      I hope this is helpful.

      Best.
      Brook.

  6. What terminals do I use for the flasher relay on the base for a 70′ /5?

    (2) Green
    (1) Green-Yellow to switch

    Also is there a less expensive flasher relay I can use instead of the hella?

  7. Hi from England. Been following your excellent strip and rebuild notes which, in several cases, have saved me many hours and mistakes. Have one minor comment I.e. On my bike the starter relay is on the left of the frame and the regulator is on the right, your photos appear to show the same whereas your notation is opposite.

    Regards

    Dave

    • Hi Dave,

      Thank you for finding this mistake. I fixed the picture captions so they match the text.

      It’s gratifying to know this material helped you on your project. Best of success.

      Best.
      Brook.

  8. Hi it’s Dave from England,

    Just a comment after suffering ‘Sods Law’. I assembled the front forks following your excellent notes. All went really well until I attempted to fit the indicators on the brackets, which I had re-chromed, I then found that they are handed as the indicators would only fit onto the brackets facing backwards. I’ve checked all the diagrams on the various sites and there is no indication that they are handed and they just give the one part number. Hey ho all off again tomorrow and switch the brackets.

    Regards

    Dave

    • Hi Dave,

      Don’t you just love it when what you thought was straight forward turns out to be not the case. 🙂 ‘Tis the joy of overcoming the cussedness of inanimate objects that keeps us working on these bikes, isn’t it?

      Cheers.

      Best.
      Brook.

  9. Hi Brook

    Yes you’re right but still the best bikes ever made. Reversed the indicator brackets this morning now trying to work out the wiring in the headlamp. Is there a fools guide to re-wiring? I made notes and sketches but can’t follow my diagrams and the new harness, direct from BMW is on the face of it identical but the cables exit the loom slightly differently. Any advice would be most appreciated.

    Dave

    • Hi Dave,

      I’m not aware of any such guide. The /5 is a bit more confusing than the /6 as there is no board for organizing the wiring. I think an approach would be to thread the wiring harness along the bike so you can see where the ends go and identify the color codes used for each component (brake light, voltage regulator, etc.)

      The Haynes manual does show the color of the wires in the diagram (as in the line color matches the wire color) and that makes it easier to keep things straight.

      I would also use an ohm meter connected to each end of any wires you aren’t sure of. If the meter shows no resistance, then you know both ends are on the same wire.

      I’d start making connections for the clear and less complicated connections first to whittle down what’s more complicated. This eliminates the number of choices for what the wire connects to.

      In general, all wires with the same color code connect. If there are instances where that is not the case, the wiring diagram in the Haynes manual will show it.

      I hope this helps.

      Best.
      Brook.

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