- 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.