R90/6 Electrical Fault: Mysteriously cuts power, mysteriously comes back on.

[banzaibob]

The last 2 weeks there has been a specifically vexing problem with my 1976 R90/6. About two weeks ago while driving in suburban traffic, it started missing and backfiring, typical of an electrical short. I put it away until I had time to look at it. I got around to getting it up on my lift for an oil change a couple of days ago and gave it a modest shakedown. Battery terminals are solid, ground to the back of the engine, solid. I took off the headlight to see if there was anything obvious. I shook the wiring, fuseblock , etc. No shutoff. I even removed the front engine cover and checked the diode board for anything loose....nothing. Yesterday I took it for a 3 hour cruise. It was damp and a little rainy. During the wettest portion of the ride it started missing and sputtering just like one would expect from a loose wire. I rode through it and eventually weather conditions dried out and I had no more problems.

Today I took it down to the gas station and filled it up for a little ride today. After filling it up, I turned on the key and nothing. No lights, no ignition, no nothing. I removed the headlight and poke at what I could see (bright afternoon sunlight made it difficult) still nothing. I eventually called my wife and she brought the van, etc., etc. Tonight I unload it and start to manuvear it to line up with my lift. I put the centerstand down and roll it on to it and "click", on come the lights. Fires right up.

I put it on my lift and this time I am giving the connections and junction a lot more scrutiny with the hopes of making it flicker....nothing. I looked at the factory wiring diagram to see if there is a main relay or circuit breaker (that would be too easy). Nope. Tomorrow I hope to go through and re-tighten and clean what contacts I have.

Question 1) The main positive goes from the starter to the diode board, that connection appears tight. However has anyone known a bad diode board to do something like this? Question 2) Is there something like a main circuit breaker or relay that I am missing? Key switch? There are no bad circuits anywhere; turn signals work, horn sounds, brake lights are fine, etc. Any great ideas????

 

[pglaves]

Under the tank, on the backbone, left side, is the starter relay which serves as sort of a Grand Central Station for electrons. The connections on the bottom of this relay are notorious for faults such as you are experiencing. I would clean all of those connections - especially the big red wire.

And no - the diode board is unlikely to provoke these symptoms.

 

[banzaibob]

I'm going through everything with contact cleaner, scotchbrite and dielectric grease either tomorrow or Sunday. Does all of the current for the entire bike go through that one relay? (Or in this case, can it?) Anybody ever had one apart? Are they serviceable?

 

[Cal]

Check your ignition switch (key switch).

 

[12907]

+1 on what PGlaves said. Had the exact symptoms your describing and it was the relay. In my case the connector had gotten loose. Hope this helps.

 

[barron_williams]

Chronicles of my adventure in this area

http://forums.bmwmoa.org/showthread.php?48594-Perplexed-R90-6-Still-No-Electric-Power&highlight=

If you have a solid ground connection and don't have dash lamps lit when you turn on the ignition switch, and nothing when you press the starter button, start with the contacts on the starter relay as mentioned above, then keep following the red wire.

My major lesson learned was that you can't trouble shoot with a digital volt meter. The meter will fool you into thinking the connection is ok, because it will show 12 volts on a marginal connection that can't carry a load. You have to test with something like a lamp bulb that puts a load on the circuit.

Barron

 

[SCBuckeye]

My R100/7 had a similar problem about 1-1/2 years ago. I would be going along, and for a split second the power and lights would go out, but because I was in gear and moving about 60 MPH, it started itself right back up.

However over a few weeks, it got worse and a couple of times while I was stopping at a light or stop sign, it stopped the engine, but since I wasn't moving, the engine had to be restarted. I would switch off the key, and re-start. Once when it was off. and the dash lights went out, I reached under the front left of the tank and simply wiggled my fingers in that group of relays there. Voila!! Dash lights came back on!

later that week, I took tank off, took apart all of those relays, one at a time, cleaned and reassembled them. Then I took a piece of real thin plastic (like from a milk container) and put up a shield so that that group of relays could not get water on them while I rode.

Never had the problem again!

Perhaps this may help you. Do wiggling of wires and connections - maybe there is one dirty or loose.

 

[banzaibob]

I was at my local BMW dealership this morning picking up parts for another bike. I decided to ask if they had a starter relay in stock. Knowing the prices they usually charge, I figured that if they had one they would want $120 for it. To my surprise they only wanted $55 so I ponied up and bought one. I brought it home and proceeded to put it on the bike. I removed the old one and started disconnecting wires. The cover fell off in my hand. The inside did not look specifically corroded but I intend to take a closer look this afternoon.

If anything appears to be the culprit, this is it. Will post the results.