Chicago Region BMW Motorcycle Owners Assoccrbmw

Hi all,

I've got a 1976 R90/6, and for the last several months, I've had an intermittent electrical problem. I'll turn the key and nothing will happen -- no headlight, no indicator lights, no relay clicks -- nothing. Light and cutoff switches have no effect.

If I then prod a bunch of stuff behind the headlight and try the key some more times, I'll get lights and everything will work fine. I've never had the power go out once the bike has started. Right now, however, it seems to have given up its intermittent nature in favor of reliable failure.

I've done a little probing under the headlight cover, and I'm a bit confused. With the ignition switch off, I read 12v between the ignition red and ground (brown) wires. With the ignition switch on, I read 0v.

Other things I think I'm seeing make no sense at all to me -- once I've disconnected the battery and test resistance, I'd swear the connection between the red wire and ground is open when the switch is off, and has no resistance when the switch is on. But that sounds like it'd be shorting the battery to ground, and the lack of blown fuses and fire makes that seem unlikely. It could be I'm measuring something wrong.

I don't have a really clear plan of how to test and troubleshoot further. I've read http://bmwmotorcycletech.info/electricalhints.htm and will try the starter relay jumper connections -- but I'd expect to not see 12v at the ignition switch if that were the problem.

Any suggestions?

Many thanks,
-Nate

Hi Nate. Many of the Chicago Region's best electrical experts are en route to the BMW MOA Rally in Tennessee. I'm sure some will chime in when they return. Meantime, have you posted your question to the Airheads list (www.airheads.org)?

I don't have my wiring diagrams in front of me, and it's been a couple minutes since I had a mid-70s 900, so I can't help much. But it's likely that the answer will be a relatively simple one...

Tim C.
Park Forest, IL
en route to Johnson City, TN

Thanks -- good idea on the airheads list.

I've figured out that the problem is somewhere between the positive side of the battery and ignition -- jumping the positive lead straight to the ignition switch causes the lighting relay to trip and lets me start the bike.

So... I've got some connections to check.

Thanks!
-n