- VIDEO: How The Neutral Switch & Clutch Switches Work in the Starter Circuit
- Possible Causes To Investigate
- Checking the Instrument Cluster
- Checking Neutral Switch and Wiring
- Replacing the 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 starter relay and neutral switch circuit.
I found the following resources helpful in this work.
- Bob Fleischer: Demystifying & Troubleshooting Airhead Boxer Electrical Systems
- Bob Fleischer: 5 Speed Transmission Neutral Switch Replacement
VIDEO: How The Neutral Switch & Clutch Switches Work 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. I also show the symptoms of what two different failure modes of a diode in this circuit look like.
VIDEO: 1983 BMW R100RS How Neutral & Clutch Switches Work In Starter Circuit
Possible Causes To Investigate
There are a number of possible causes for problems with the neutral bulb, clutch switch and start circuit:
- Neutral bulb burned out
- Dirty circuit board contacts inside the instrument pod
- Loose or dirty connections between instrument pod pins and electrical cable
- Broken wires in the neutral circuit
- Neutral switch failed
- Neutral switch leads dirty, broken, or the wires are loose, disconnected
- 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Then I use a large screw driver to lever the center spacer up and out of the bosses on top of the oil pan.
Now it’s easy to remove the two wires from the old neutral switch.
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.
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.
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.
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.
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.
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.
Before I go any further, I test that the new switch is working correctly as shown in this short video.
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.
There is a fat washer that goes against the outside of the 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.
Then I put the nuts on the engine mount rod and torque them to 55 Ft-Lbs.
2019-02-23 Replaced Flickr video with YouTube videos on my channel.
2020-05-10 Replace video to correct a mistake in the simple wiring diagram.
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
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.
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
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.
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
It should work. That said, in the states, a multi-meter isn’t too expensive and is very handy 😉
Thanks for the update Brook,they are not to expensive here either,am i betraying my Scottish heritage? : D .
Well I have Scotch ancestors in my family tree as well and have inherited the “frugal” gene too. 🙂
Hi Brook,is there one main power wire to the instrument console? i started out with gen,and oil lights working,for no known reason they stopped working,took the console apart replaced two bulbs(not the gen and oil),now no lights at all,could a relay or ignition switch be a source of the problems,indicators,horns,brake lights,all work,have been checking the wires,have not found a broken one yet…this is sure one hell of a hobby.Ian
You don’t mention the year/model of your bike, but perhaps it is also an 83 RS. I would check the fuses for a blown one.
The instrument cluster receives power from the large rubber block that plugs into the bottom of the housing. See this diagram for a /6
NOTE: There are changes in these boards due to BMW changing the lights in the instrument cluster so the pins and what the do are specific to this model bike. But this diagram should help you see identify what pin is the power pin for your bike.
IIRC There is one pin that is the power pin. I would clean those female sockets with contact cleaner and see if this fixes the problem.
Next, I would remove the instruments, and open up the housing exposing the plastic coated flexible circuit board. Look for broken copper foil which is common. If there are several breaks, you may be far better off replacing this circuit board with one available from KatDash.
On my site you will find a write-up on repairing the circuit board for the 83 RS
I hope this helps.
Thanks for that Brook,it is a 76 R100S will keep digging.
Well i took that console apart..again..cannot find thr problem,i had the backlights and high beam indicator working before,on a different circuit i think…then i cleaned the contacts at the starter relay….and now those lights have disappeared,my question was,there is a double wire,from the starter relay that carries power,could this cause any of my problems? thanks.IAN
Perhaps. The red wire is “hot” and not fused. IIRC, the instruments are fused.
Tor the 83 R100RS there is a green-black wire on terminal 86 of the starter relay that powers the instruments and one that goes to the ignition control unit. Check that and make sure it’s clean and seated. Perhaps you bumped it when poking around the starter relay
You might post a note to the micapeak airhead forum where many far smarter people than I hang out.
I hope this helps.
Thanks for the diagram Brook very helpful,rewired the starter relay terminals,and the gen light came on…one down Lol!,and the starter switch on the handlebar started to work with the clutch lever pulled in…and a spark…will write to the group,when i figure out how to frame the question..again many thanks.IAN
Good on ya. It’s always a boost when the mysteries start getting resolved.
Och aye! save a penny and the pounds take care of themselves : D thanks for your help.IAN
Hi Brook,never discovered the actual reason why my neutral light on my 76 R100S did not work,have Katdash,so no bulb,checked and rechecked the wiring,new switch ect,put on the list not so important things to do……today it just came on….wt! Lol!
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?
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.
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.
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…
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.
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….
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.
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?
Great writeup, your site has been a great reference for working on my airhead. How is the plast-aid holding up?
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.
Thank you again for a precious review.
While doing this, how was the wieght distribution on the jack-vs-center stand?
Was it stable enough?
The jack was used so I could push up on the engine slightly so it was easy to remove the rear engine mounting stud. The center of gravity of the bike is just a bit in front of the center stand mounting bolts so the bike was stable when I placed the jack as shown. I put a wood board on top of the jack to distribute the load so the weight of the bike would not cause the jack to bend or deform the oil pan.
Hope that helps.
Thanks. My question was actually if is it OK to lift the bike with the oil pan being the ONLY area of contact holding the whole bike’s weight (ofcourse it should be done right and distribute the weight over a plate or wooden bars and what’s so on)
I do that all the time without any problem. 🙂
Great write up – thanks Brook. The problem on my ’79 R80/7 is that the neutral light is always on.
Is the neutral switch always on? It could be mechanically stuck. That would keep the neutral switch lit even when the bike is in gear.
Another possibility is that the wire from the neutral bulb that goes to section 85b on the terminal board in the headlight shell has frayed insulation and is grounding to something metallic.
I hope this helps.
Hi Brook. One other question please. I am completing a top end reseal on my ‘94 R80RT. Is it best that I use Threebond 1207B to seal around the cylinder to engine case before I reinstall my cylinders as you show in your video “1983 BMW R100RS Install Engine Top End”? I only ask because changes occur in subsequent years where procedural steps differ. Thanks in advance.
Yes, use the sealant as I show here and in the video you mentioned:
–> 11 BMW 1983 R100RS Install Engine Top End
–> VIDEO: 1983 BMW R100RS Install Engine Top End
The ’94 engine also has Nikasil cylinders so the application of the sealant is the same as for the 1983 engine.