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Lights not working, 1985 K100RS


NeonGuy

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Headlight, tail light, instrument lights are coming and going on my recently acquired '85 K100RS. The bike is in very good condition, and runs great. I was told it might need the instrument cluster removed and the electrical connectors cleaned. The seller said indications for this service would be fast cycling of the right turn signal flasher, and this began on day two of my ownership. Intermittent lighting issue then began on day four. This is otherwise an awesome machine, with unbelievable power, acceleration, great handling at speed, and I like it very much. What say you, BMW Brothers?

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Fuses are not blown, fuse panel is clean, wires all look good. Battery is charged. Maybe there's a lighting relay to check? I don't see one on the wiring diagram in my service manual. Please indicate if you know there is a relay and where I might locate it. Thanks for your input, guys!

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Believe or not it could be the brushes in your starter. Try tapping on your starter to see if the lights will come on. Are you having any trouble with the starter not working at all sometimes? The starter works in conjunction with the load shed relay to control your lights during startup. If you have a starter brush problem it will sometimes affect the load shed relay. The load shed relay is under the tank. Also you can short the starter terminal to ground, if the lights turn on then you know you have a starter brush problem. Do this with the key on but DO NOT press the starter button.

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Dennis Andress
Believe or not it could be the brushes in your starter.

 

The common cause. The headlight relay gets its ground through the starter motor winding. That ground goes away when the starter is engaged, causing the headlight to turn off. The brush set used to be unbelievably cheap for a BMW part. If the brush set is no longer available you might try a one from a later model, or getting a used starter from Beemer Boneyard.

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Believe or not it could be the brushes in your starter.

 

The common cause. The headlight relay gets its ground through the starter motor winding. That ground goes away when the starter is engaged, causing the headlight to turn off. The brush set used to be unbelievably cheap for a BMW part. If the brush set is no longer available you might try a one from a later model, or getting a used starter from Beemer Boneyard.

 

The headlight does not get its ground through the starter motor winding. The load shed relay does and that is what makes the headlight go out with a starter problem. The load shed relay has a set of normally open contacts on it that will open with a starter problem and also when the starter is energized. These contacts control several things on the bike that are not needed for the engine to run. The purpose for doing this is so that more amperage can be delivered to the starter motor and engine electronics for starting purposes.

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Here is a diagram of the starter and load shed relay circuit for an early K-bike. As you can see the positive side of the battery is switched by the load shed relay before it goes to the headlight. You will also notice that the ground side of the load shed relay coil goes through the starter motor.

 

starter.jpg

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Thank you, sir, for your expert knowledge and for sharing it with me and our other Brick riders here on this marvelous BMW ST Forum. I now have other events to report, but they are minor. The speedometer has become intermittent, the right turn signal is flashing very fast intermittently, and SURPRISE! The headlight/instrument light issue has disappeared, apparently resolved spontaneously. I rec'd advice on these matters from the seller, who told me that if the right turn signal began to act up, it meant that the instrument cluster had to be removed, opened up, and the wiring connections cleaned and serviced. I'm going to do that ASAP. I'll get back to you as soon as this has been completed to let you all know the results. I was told the instrument cluster has been replaced, and it looks much newer than the rest of the bike

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QUESTION: Now that it's evident I have to remove and service the instrument cluster on my 1985 K100RS, just how is this accomplished? I was told the fairing had to come off first, but looking at the cluster, it appears it might be possible to pull it off without removing the fairing. But if I do have to do that first, what are your recommendations for fairing removal? And once I have it off, precisely what is the proper procedure for disassembling the instrument cluster for electrical connections service? What's the best cleaning technique for the small pins and sockets? What physical cleaning procedure should be followed? Is scratching with a file or tool correct? Or maybe brushing of the contacts recommended, with a tiny brass brush? ? What spray contact cleaner or other product works well? Should I use dielectric grease upon reassembly? What say you? And THANK YOU in advance for your answers! God bless.

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The cluster can be removed w/o removing the fairing. There is a small phillips head screw that holds the wiring loom plugs cover, about in the middle of the cover, A small mirror will reveal it's location. The looms (there are (2) of them, side by side) that once the cover is removed, these can be wiggled out. Easiest to pull up and out a bit at the bottom of the wiring loom on each side.

 

After they are unplugged,(from memory), there are four mm allen head bolts (can't remember the size, but kinda smallish) that hold the cluster to it's mounting bracket. Use the mirror again, or fingers to feel for them. A flashlight pointed at the mirror will help.

 

When doing this, easiest to turn the bars to the left (if you prefer to work on it from the clutch side).

 

It could be that the looms are loose, or need to be unplugged/cleaned and re-plugged in, yet the plastic cover does a pretty good job of keeping it dry and dirt free.

 

Once the cluster is off, there are screws on the rear that once removed will allow the rear cover to come off. There is a rubber band type gasket that surrounds the cover, so make sure it's positioned properly when reassembling.

 

They had a problem of "fogging" up in wet/moist conditions, so I always taped over the gasket seam with some electricians tape after reassembly.

 

The bulbs are easily replaced. I've had the cluster off and apart numerous times.

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Dennis Andress

The instrument cluster on my `85 K RS had a problem with internal electrical connections. I would take it apart, disconnecting every little circuit board so I could clean every contact, and it would work for awhile. I recall reading once where somebody vented the cluster by drilling two small holes through the cover and covering them with Goretex.

 

There's lots of advice here: http://www.ibmwr.org/ktech.shtml

 

Dennis

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