Restoring proper neutral light function - /6

I claim no authorship for what follows, having found the repair on the ADVrider forum.

The neutral light 'start in gear' function on my 1975 R90/6 had ceased to operate maybe a decade ago. The wire connecting the switch at the clutch handlebar lever to the Molzner circuit board in the headlamp shell had broken at the clutch lever end. This switch works with the diode on the back of the circuit board to allow starting in gear when the clutch lever is pulled in and the neutral light stays off if the bike is in any gear except neutral.

Not, you understand, that I ever stall, but should someone stall the bike at a traffic light, a non-functioning switch dictates that the bike be first placed in neutral before the starter button is thumbed. Of course this means that the crazed soccer mommy in the 4 ton SUV behind the rider goes ballistic because of the 2 second delay in her life, and you really do not want that happening. Arguing with 8,000 lbs. of Chinese steel on a 500 lb. motorcycle is not a winning strategy.

So when it came time for a major service on the R90/6 I determined to restore proper function of the neutral light switch in the clutch lever assembly. The replacement, wired switch goes where the red arrow points - the small retaining screw is visible immediately above the arrow:


That switch retaining Philips screw is very hard to access. As my handlebar grip was due for replacement I cut it off, disconnected the clutch cable after releasing it at the gearbox end, and slipped the whole assembly off the handlebar for installation of the new switch, part # 61 31 1 243 403. The other end of the wire is terminated with a connector which does not work with the /6 wiring chassis, so it is snipped off, the wires passed into the headlamp shell making sure to install a rubber grommet in the related hole in the shell, the ends bared and two female crimp connectors installed and fitted to the circuit board thus: Brown to terminal 85, brown with yellow stripe to terminal 85B - green arrows in the illustration below:


Off for a ride and all was not well. The neutral light would illuminate every time the clutch lever was pulled in, which renders its function useless.

Some digging disclosed the brilliant fix on the ADVrider forum which has a lot of long distance Airheads participating.

The cause is a blown diode on the back of the circuit board. The diode has gone closed circuit and transmits current both ways. Correct function is one way only. The snag is that to access and replace the diode on the rear of the circuit board 5 million wires must be disconnected, the two retaining nuts removed and the diode replaced. Then try to replace all the wires in their right places. Not fun.

The ADVrider fix takes advantage of the fact that the blown diode is closed circuit and simply interpolates a new diode between the appropriate wire (female) and circuit board (male) on the front of the board. No removal of the circuit board is necessary.

I went completely insane and blew $4 on a 1N4007 1 amp diode from Amazon.https://www.amazon.com/gp/product/B00SWK68C4/ref=ppx_yo_dt_b_asin_title_o01__o00_s00?ie=UTF8&psc=1.

Then I ensconced the diode in some 1/4" ID clear vinyl tubing for insulation, crimping on the male connector to the end of the diode with the white stripe and the female connector to the other end. You must get the polarity right for this repair to work. As I long ago installed the KatDash LED instrument lighting harness the very low current draw of the LED neutral bulb allows a low current rating diode to be used. If you are still using the OEM incandescent bulb board then I would suggest using a 3 amp diode for safety, as the incandescent bulb draws much more current.

Here is the new diode assembly.


Before installation i placed a small sticker stating "Start in gear" on the vinyl tubing. This is a legacy motorcycle and its next trustee owner should know what is what.

Now it simply remains to pull one of the brown with black stripe wires from the circuit board at location LKK and interpose the new diode between the removed female wired connector and the male connector on the Molzner circuit board/distribution panel.

Here's the pulled YKK connector:


And here's the installed new diode:


Button up the headlight shell, confining the Rube Goldberg spaghetti wiring to darkness, and you are done.

Now when I pull in the clutch lever in any gear other than neutral the green neutral light remains extinguished and the electric starter functions as it should. And crazy mommy in her SUV is happy as it means Little Johnny will arrive at soccer/karate/music/basketball/football practice 2 seconds earlier.

Thanks. I am getting ready to do something like this on my R90S. In my case, the diode is open rather than shorted.

swall said:

Thanks. I am getting ready to do something like this on my R90S. In my case, the diode is open rather than shorted.

Click to expand...

With a closed circuit diode, as in my case, it's as if the diode was not there. Thus the LKK black/brown stripe connector is simply interposed, the diode installed in series.

In your case, open diode, you would wire the diode in differently. Here's the relevant extract of the wiring diagram for my 1975 R90/6:


Thus is appears that you want to attach one end of the diode to 85B on the distribution board and the other to LKK. The white stripe on the diode which indicates the cathode (-ve) would be connected to LKK.


Be sure to check the circuit diagram first for your year and model. Maybe post here when done for others to follow?

Yes. I have 1 amp plug-in diodes from Littelfuse and the diode holder, which comes with 16 ga. lead wires.

I checked the MaxBMW fiche and it states that the incandescent neutral indicator bulb is 1.2 watts, meaning 0.1 amps current draw, so a 1 amp diode s/b OK even with incandescent bulbs, correcting my earlier caution in the original posting.

Neutral/Clutch Switch

I have the KatDash installed on my early '74 R90s. Not sure if I've had this problem for awhile, but after replacing the complete stock (and brittle wiring harness about nine years ago, every time I pull the clutch lever, the neutral light comes on. (Stock BMW terminal connection board in the headlight).

I have the clutch switch connected between Blue/Yellow and Brown (frame ground). According to my schematic, Blue/Yellow would be the Anode side of the diode mounted in the terminal connection board, and Brown/Black would be the Cathode.

I picked up a 1N4007 1 amp diode and installed it first with the Cathode to the brown wire, leaving the Blue/Yellow wire in place. Place bike in 1st gear, turn key on and neutral light is off as expected. Pull in the clutch and Neutral light is still off, but starter button doesn't activate.

Rotate the diode around so the Anode is on the brown wire side and now with the bike in gear, pulling the clutch lever turns on the neutral light and of course, the starter button now works.

The problem is the same though as before. Every time I pull in the lever, the neutral light is going to light.

Appreciate any guidance.

RickR90s