61 BMW 1983 R100RS Diagnose and Repair Neutral Switch

A problem with this bike when I bought it is the neutral light doesn’t light when the transmission is in neutral and it will only start by pulling in the clutch lever. Something is not correct in the neutral switch circuit.

Resources

I found the following resources helpful in this work.

How The Neutral Switch Works in the Starter Circuit

Using this information and a wiring diagram for the bike, I put together this video. I show how I use the wiring diagram to draw a simple starter and neutral switch circuit diagram. I use the simple diagram to explain how the neutral switch, neutral bulb and clutch switch work in the starter circuit.

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Possible Causes To Investigate

There are a number of possible causes for why this problem is happening:

  1. Neutral bulb burned out
  2. Dirty circuit board contacts inside the instrument pod
  3. Loose or dirty connections between instrument pod pins and electrical cable
  4. Broken wires in the neutral circuit
  5. Neutral switch failed
  6. Neutral switch leads dirty, broken, or the wires are loose, disconnected
  7. Neutral circuit diode failed

Checking the Instrument Cluster

Since the odometer is not working, I start at the instrument cluster. I remove the cluster so I can have the odometer repaired and I can check the neutral light bulb, circuit board and connector.

Instrument Pod

Instrument Pod

Instrument Pod Electrical Connector

Instrument Pod Electrical Connector

Instrument Pod-Outer Cover Removed

Instrument Pod-Outer Cover Removed

Instrument Pod-Printed Circuit Cover Removed

Instrument Pod-Printed Circuit Cover Removed

Instrument Pod-Printed Circuit Exposed

Instrument Pod-Printed Circuit Exposed

Instrument Pod-Close up of Electronic Tachometer Wires

Instrument Pod-Close up of Electronic Tachometer Wires

Instrument Pod Assemble-Order is Bottom to Top

Instrument Pod Assemble-Order is Bottom to Top

I removed the wiring circuit board from the back of the cluster and tested the neutral bulb (4th bulb from the top) with an ohm meter. There was continuity (zero ohms) so the bulb is good.

Neutral Switch Bulb in Bulb Holder

Neutral Switch Bulb in Bulb Holder

While the bulb is out, I use electric contact cleaner and 1500 grit wet/dry paper to gently and carefully clean the contacts on the bulb holder and the small copper tabs that fold over the plastic backing into the hole for the bulb holder. Go slow, easy and be careful since the copper foil is very thin and you don’t want to break it.

Printed Circuit Contacts for Neutral Switch

Printed Circuit Contacts for Neutral Switch

Next I test the leads on the back of the circuit board that go to the neutral bulb (2nd pair of pins from the bottom) to see if there is continuity between the two pins that go to the neutral light. To do this I install the bulb holder into the cleaned circuit contacts. The circuit contacts had continuity (zero ohms) so they are not broken.

Neutral Bulb Pins on Printed Circuit

Neutral Bulb Pins on Printed Circuit

I cleaned the male pins on the circuit board and the female sockets in the wiring connector with electric contact cleaner and plugged the connector into the circuit board pins multiple times but the neutral light still did not light.

Neutral Light Not Lit

Neutral Light Not Lit

That crosses off items 1, 2 and 3 from the list of possible causes.

Checking Neutral Switch and Wiring

When I looked at the neutral switch on the underside of the transmission, it was covered in dirt and transmission gear lube. That likely means the neutral switch is leaking, a common failure for these switches.

Neutral Switch with Grunge

Neutral Switch with Grunge

Neutral Switch with Grunge

Neutral Switch with Grunge

Here is a short video showing how I diagnosed that the neutral switch failed using a simple test. This also tested the wiring from the headlight shell terminal block to the neutral switch/oil pressure connector shown in the video.

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To clarify the wires going to the connector, the neutral switch wire is brown with black stripe and the oil pressure wire is brown with green stripe.

Neutral Switch (Brown with Black, Front) & Oil Pressure (Brown with Green, Rear) Wires

Neutral Switch (Brown with Black, Front) & Oil Pressure (Brown with Green, Rear) Wires

Replacing the Neutral Switch

Before I replace the switch, I clean up the mess at the bottom of the transmission housing and around the top and sides of the oil pan. It’s easier to work when everything is clean and there is less chance of getting dirt and grime into the transmission and the neutral switch threads.

Degreased Bottom of Transmission and Engine Oil  Pan

Degreased Bottom of Transmission and Engine Oil Pan

I put a large oil change pan under the engine and use engine cleaner in a spray bottle. I use a toothbrush to loosen the caked on crud and use a second spray bottle with water to wash it off. Most of the grimy liquid ends up in the pan. I add this to my recycle oil containers for disposal.

Drain the Transmission

I just put new gear lube into the transmission, so I clean out my oil change pan and finish this up with some brake cleaner so the pan is nice and clean. I drain the gear lube into the clean pan so I can add it back into the transmission when  I’m done. I remove the transmission drain plug, and then the fill plug so the gear lube will drain faster.

Remove Transmission Drain Plug

Remove Transmission Drain Plug

Remove Transmission Fill Plug

Remove Transmission Fill Plug

Draining Transmission Gear Lube Into Clean  Container

Draining Transmission Gear Lube Into Clean Container

Getting Access to the Neutral Switch

The switch is surrounded by the rear engine mounting rod that goes through a hollow spacer on the top of the oil pan. I use a floor jack with a wood block on top to help support the engine so it will be easier to remove the rear engine mount rod.

Floor Jack to Support Engine

Floor Jack to Support Engine

I loosen the nuts on the front engine mount rod, but I don’t remove that rod. I remove the rear mounting rod and use a long, narrow screw drive to drive it to the left side.  Mine slides out very easily. I don’t want to scratch the inside of the spacers since that will make it hard to install the rod.

Pulling Rear Engine Mount Rod Out On  Left Side

Pulling Rear Engine Mount Rod Out On Left Side

Then I use a large screw driver to lever the center spacer up and out of the bosses on top of the oil pan.

Prying Out Center Engine Mount Spacer

Prying Out Center Engine Mount Spacer

Now it’s easy to remove the two wires from the old neutral switch.

Remove Wires From Old Switch

Remove Wires From Old Switch

Old Switch Ready to Remove

Old Switch Ready to Remove

I use a 19mm open end wrench to remove the old switch. I clean the area around the hole with brake cleaner and a clean blue shop towel so there is no grit or crud that can get into the treads of the new switch when I install it.

Clean Switch Hole Before Mounting New Switch

Clean Neutral Switch Hole Before Mounting New Switch

Preparing New Neutral Switch for Installation

I bought a BMW stock neutral switch (Part No: 61 31 1 243 097) that has the aluminum base and a new washer/spacer.

New Neutral Switch and New Washer/Spacer

New Neutral Switch and New Washer/Spacer

Neutral Switch Function

There are two styles of neutral switch; one where the switch is open (off) when the plunger is out and the other where the switch is closed (on) when the plunger is out.The one for the 1983 R100RS is closed, meaning, when the switch button is not depressed, the switch is closed (zero ohms).  This short video shows how the switch  works and that the old switch doesn’t work.

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The rumor is these switches are prone to leaking. Tom Cutter recommends using a green “wicking” type of Locktite (LocTite 290) and putting this around the edge of the block phenolic insulator and the aluminum housing, letting it soak in and wiping the excess away. I chose to try an experiment with something different, Plast-aid.

New Neutral Switch with Plast-Aid Applied to Prevent Leaks

New Neutral Switch with Plast-Aid Applied to Prevent Leaks

It is an acrylic polymer that chemically bonds to many plastics and creates a chemical bond to metals as is the case with epoxy. I mixed up a little and put it  around the edge of the aluminum case and phenolic insulator and around the base of the electrical tabs where they come through the phenolic insulator.  We shall see if this prevents future leaks.

I install the switch with the 2mm thick washer, spacer and tighten it with a 19mm open end wrench.

New Neutral Switch with New Washer And Plast-aid Sealing Aluminum Case to Phenolic

New Neutral Switch with New Washer And Plast-aid Sealing Aluminum Case to Phenolic

I sung it up and add just a bit more so I don’t bend the aluminum case and crack the Plast-aid or weaken  the seal between the phenolic and aluminum case.

I attach the wires to the terminals.

New Neutral Switch Wired Up

New Neutral Switch Wired Up

Before I go any further, I test that the new switch is working correctly as shown in this short video.

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Success 🙂

Neutral Light Works Now :-)

Neutral Light Works 🙂

I clean the middle hollow spacer in the parts washer and lightly sand the end of the spacer to remove any burrs or gouges that will interfere with sliding it back into the bosses in the oil pan. I put it the freezer for an hour while I got lunch and then gently tapped it into place with a rubber mallet.

I insert the threaded engine mount rod from the left side and through the small space that fits between the frame and the oil pan, then through the middle spacer and then another small spacer that fits between the right side of the oil pan and the frame until it just shows on the other side of the frame.

To get the muffle bracket on the end of the engine mount rod easily, I loosen the right side muffler mount screws so I can move the right side exhaust pipe to help position the muffler bracket onto the rod.

Loosen Muffler Mount Bolts to Position Exhaust Bracket Easier

Loosen Muffler Mount Bolts to Position Exhaust Bracket Easier

There is a fat washer that goes against the outside of the frame.

Fat Washer Goes Against Frame

Fat Washer Goes Against Frame

I hold it and the muffler bracket in place and with my left hand tap the rear engine mount rod through them. This may take a couple of tries to get everything to line up. I don’t want the muffler mount bracket to mangle the treads on the engine mount rod.

Positioning Exhaust Bracket Onto Engine Mount Rod

Positioning Exhaust Bracket Onto Engine Mount Rod

Then I put the nuts on the engine mount rod and torque them to 55 Ft-Lbs.

18 thoughts on “61 BMW 1983 R100RS Diagnose and Repair Neutral Switch

  1. Brook : Once again you have provided an invaluable service to the airhead community. The step by step process is easy to follow and very clear. Since the neutral switches are a “weak point” your post will be useful to many of us. Keep us posted on how that plastaid works! Years ago the crimps on the back of my /6 ignition switch were separating with subsequent intermittent engine cut outs. Pressing in the crimps and applying some JB Weld solved the problem. So these types of homemade fixes can be durable

    • Dale,

      Yes, I’m curious if Plast-aid will hold up over time. It’s very easy to use and doesn’t take long to setup, so that’s a virtue.

      Best.
      Brook.

  2. This a great help as i am about to embark on a similar process,that is removing the console,my neutral light,has failed to light,and the start button does not work,but then neither does pulling in the clutch help,i have a question,is it not possible to check the switch while it is still fitted on the gearbox?by testing the continuity between the spades.IAN

    • Hi Ian,

      Yes, you can see if the switch is working while it is in the bike. That said, it’s cramped in there, but with a set of alligator clips on some leads, you should be able to test with an ohm meter.

      Best.
      Brook.

      • Thanks for that Brook,would a simple circuit tester suffice?,one that lights a bulb for example…..yes you guessed it i don’t have an ohm meter. : D

    • Ian,

      Well I have Scotch ancestors in my family tree as well and have inherited the “frugal” gene too. 🙂

      Best.
      Brook.

  3. Thanks very much for this easy to follow instruction. I may have to use it soon as my neutral light stopped working on my R75/6. However the starter still works so I assumed it wasn’t the switch. I’ve tested the bulb and it’s fine. The post on the connector block doesn’t register any juice on the multimeter so I’m guessing it’s something on the switch side. If the switch is faulty could the bike still start?

    • Hi Franko,

      It sounds like you can start the bike when the transmission is in neutral. So, the neutral switch is working okay. You could also pull the clutch lever which is a second path for getting power to the starter relay to start the bike.

      Two other causes of no neutral light occur to me.
      (1) the large rubber connector that plugs into the bottom of the instrument may not be making good contact with the pins in the instrument cluster. Try wiggling that plug and pushing it on and off the pins to see if the neutral light works. If it does, the connectors are dirty and need to be cleaned.
      (2) Inside the instrument cluster is a circuit board. The copper foil traces make the electrical connections to the bulbs. The foil can break, particularly where the foil on the tab folds over into the bulb socket. This is not uncommon.

      Here is a picture of a tab where the copper foil has come off completely.
      https://flic.kr/p/MDgyrK

      You can look at the other pictures in this album to see how I repaired it using a copper foil patch I cut out of the foil used for stain glass. I haven’t written up this repair yet but hope to do so eventually.

      Best.
      Brook.

  4. Thanks Brook.

    It’s good to know that it’s likely not the switch since the bike starts in neutral just fine.

    I did test the instrument pin sockets and only registered power from two of them, 1 and 12. Do you know of a diagram that identifies which pin goes to what? If not I guess I could remove the circuit covers and trace the foil paths.

    However I assumed that, since I didn’t register any power at the connector block behind the headlamp, the fault was probably not on the instrument panel side and was likely on the switch side.

    I’ll double check this is the case…

    Thanks again.

    • Hi Franko,

      I sent you a drawing of the instrument cluster pinouts via Email. I hope this helps.

      There are two power input pins to the instruments for the lights. All the other pins are energized when a switch somewhere else is closed, such as the neutral light. I’m not sure what “the connector block behind the headlamp” refers to, so I can’t visualize what you tested for power.

      Best.
      Brook.

  5. Finally gotten a chance to look at this again. I’ve gone ahead and replaced the neutral switch even though I didn’t think that was it bc the previous one was leaking pretty badly. For some reason I’m not getting voltage to the switch or to the wires leading to the bulb. There is a diode in the wiring diagram and I think it’s behind the connector board in the headlight shell. However to get to this I think I’ll have to disconnect all the wires which I’m really really not looking forward to….

    • Hi Franko,

      If it is the little diode on the back of the board, you could replace it with one on the front of the board across the same two terminals. I think Bob Fleischer suggests this as a simple way to repair this if it’s gone bad.

      See this article: http://bmwmotorcycletech.info/startingprobs.htm, from which I quote below:

      “Regarding that diode:
      The diode located under the connection board in the headlight shell is the one that fails now and then, still, somewhat rare. Much more rare is a failure of the diode located elsewhere’s. The diode on all Airhead models that use it, must, in some conditions, pass the starter relay coil current; and absorb any high-voltage ‘kickback’ from that starter relay coil. I recommend a diode rated at 400 or 600 volts; and at 3 amperes. 3 ampere diodes have considerably more reliability in this usage, than 1, 2, or even 2-1/2 amp diodes, due to the internal construction of the diode. Be SURE to install the new diode so that the band-marked end (gray or silver stripe) is in the original direction. I have seen these diodes installed backwards. For the diode when mounted on the underside of headlight bucket wiring board, the banded-end (gray or silver stripe) of the diode is connected to terminal LKK.”

      I hope this helps.

      Best.
      Brook.

  6. I found the issue and it was indeed the diode board. But I replaced the switch anyway to see if that would help with the oil leakage in that area. The problem is the switch I ordered from Motorworks, when installed with the spacer/washer, is not oriented with the contacts facing outwards. It is instead almost front-to-back so the rear facing one clashes with the engine mount spacer!
    Has anyone encountered this before?

  7. Hi Brook,

    Great writeup, your site has been a great reference for working on my airhead. How is the plast-aid holding up?

    • Hi Rick,

      So far, so good. I’m trying green wicking loctite on another bike to see how it works. This is another recommended method of avoiding oil leaks from a new oil pressure and/or neutral switch.

      Best.
      Brook.

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