Airhead Instrument Cluster Repair

I've had more problems with the instrument cluster lights on my 1974 BMW R90/6 sidecar rig.  This model has several idiot lights including Brake, Neutral, Charge,  Oil, turn signal activation and high beam.

Symptoms:  Turn signal indicator would only work with the ignition switch in the 'run' position (that is, not in the 'run+lights' position).  High beam indicator action was reversed – illuminated on low beam.  Neither the speedo or tach illumination bulbs worked.  All the other indicator lamps worked normally.

As the rig was still technically “legal”, I continued to use it – until the turn signal indicator completely failed – even in the 'run' position.  Time for a little TLC for the 41+ year-old bike. 

I will note here that BMW's '74 /6 models were unique in a lot of ways – and your Airhead might be quite different.  As always, YMMV.


First, I worked on the no-turn-signal-indicator-at-all problem.  With the cluster off the bike and disassembled on the bench, I found that the ground-side tab (nearest pin 7) for that bulb had finally fallen off. 
Side note:
I think this is a common failure mode for these clusters.  My bike has signs that the cluster held water at some time – lots of corrosion at the bottom.  Since the turn-signal indicator lamp is the lowest, it was doomed to eventual failure.  You might assume that the best repair would be to replace the entire bulb holder - and you would be right.  Sadly this part is NLA new from any source I could find.   These BMWs are getting old, and some parts are now getting scarce.


In my junk box, I found a piece of sheet copper and cut it to a tab plus a bit to attach to the remaining copper strip.  I then very carefully cleaned the original copper strip to bright shiny metal and tinned both the strip and the new tab.  Here's an image:

The forceps hold the new 'tab'.  The screwdriver is to hold the plastic away from the copper strip while soldering.  The razor blade was the handiest thing nearby that would protect the plastic holder from melting – a popsicle stick might have done just fine.  Tinning both surfaces made it relatively easy to make the connection:

I then connected just the bulb holder to the bike and the turn-signal indicator flashed properly when the ignition switch was in the 'run' position – one problem solved.

Nest, the other problem – no turn-signal indicator with the ignition switch in the 'run+lights' position.  By this time I had convinced myself that it was a missing ground problem – always a suspect when electrical circuits begin acting really funky...  Also, studying the wiring diagram told me that the bulbs I was having problems with only had one thing in common – a ground.

First, I carefully traced the repaired circuitry on the bulb holder and convinced myself that it was now OK.  Then, just to be sure, I substituted a known-good turn-signal relay from another bike.  The problem stayed with the '74.

After a careful check of the wiring diagram, I looked for continuity from pin 7 on the connector to the brown wire at the brake fluid reservoir (ground).  No connection.  I then re-connected the bulb holder, leaving the pins exposed and very, very carefully connected a clip-lead from pin 7 to ground.

The turn-signal indicator (along with the instrument illumination lights and the high-beam indicator) worked!  Properly!  Aha!

I spent quite a bit of time trying to locate the break in the ground wire without any luck at all.  As this plug and cable assembly is part of the main wiring harness and is usually listed as NLA (or as a several hundred dollar NOS item) I decided to give the instrument cluster a new ground.



Here's how I did it.  It might not be the best way, but it is a way...
 
First I removed the cluster mounting plate and soldered a piece of copper braid (with roughly the cross-section of the original ground wire) to the back.  I also removed a nicely sized female connector pin from an old Molex connector from the junk box.  I found a lot of pins that would do – the Molex connector was just on top...  I clipped the connector to size and attached it, with some sleeving, to the copper braid as shown:

I then put the jumper in place as shown below.  This solution appealed to me (as opposed to soldering a wire to pin 7) as it is easily reversed.

I assembled the instrument cluster and installed it on the bike.

The cluster is shock mounted.  This means that although I had the ground to the 'outside' of the cluster, the metal mount that supports it was 'floating' electrically due to rubber bushings.  So I ran a new ground wire from one of the three instrument cluster attachment bolts to a handy/logical chassis bolt as shown:

I then tested my work:

It blinks!  Yea!

I humbly acknowledge the technical contributions to my project (some direct, some indirect) by several on the AirList, notably Tom Cutter, Matt Parkhouse and Brooks Reams.  My sincere thanks for sharing your knowledge and experience. 

I hope this helps someone else to keep 'em on the road.

cln – Nick
WB5BKL