I have a problem with my ’91 K100RS 4V. I’m in Maine and getting to a BMW
mechanic is difficult so I’m hoping for a lucky break and someone has an idea
what’s wrong. I’ve tried to include everything that happened in the proper
order (I think I remember it all).





So, I turned on the key and all the lights came on as usual.
I then turned the kill switch to run and on came the fuel pump as usual. When I
pushed the start button, there was a click and all the lights went off but the
engine didn’t start. After flipping switches and turning the key, I realized
that by putting the bike in gear and moving it with the clutch out something
happens that allow the lights to come on and I can start the motor. BUT, when
the engine starts ALL the lights go out again. When you rev the engine up to
3-4K the lights flicker back to life as long as the RPM stays up. The other
thing that happens is that the speedometer shows speed that increases with the
RPM/Lights WHILE THE BIKE IS SITTING STILL? And, of course the odometer is
moving with the speedo.





I can’t think of what could cause this. The engine runs
great once it starts. The battery is fully charged, the alternator is charging
well. I don’t understand what connection there could be between the rocking of
the bike in gear and the electrical system.





I hope someone has some helpful input that might direct me
to a solution before the snow here melts.





All the best to everyone during the holidays!

As usual, a bad ground creates a lot of crazy problems. I would bet that you have a problem with the brushes in the starter. When rocking the back wheel when in gear temporarily fixes the problem gives me a good clue of the problem.
As`crazy as it sounds, a lot of the ground is passing throught the inductor of the alternator.
Please click here to look at the schematic to understand the relation between the starter, the ground and the load shed relay.
The load shed relay controls your lights when pushing the starter button.
Please let us know if we pinpointed your problem.

Bert

Hi,
I am sure Bert has nailed it, all of the symptoms you describe sound like the issues one gets when the brushes in the starter are giving a poor connection to the starter commutator. When you move it in gear the engine moving seems to move the brushes into better contact for a while, I am told tapping the starter with a 1/2 in drive 10" long extension has the same effect.

In your sig line you don't list the mileage of your bike but I am guessing it is likely to be over 50K and the brushes wear. Fortunately getting the starter out is not too difficult and you can use the K1100 manual which is freely available in many spots on the net to overhaul the starter and replace the brushes, the two bikes are very similar in many respects. I have a 1990 K100RS so it is very similar to yours. I also have the official K100 4 Valve manual but it is in German so it is not hugely helpful unless you are very fluent in technical German, there is a good deal going with BMW microfiche at the moment where one can buy the manual on fiche for ~$15 and that is written in English! Downside is you need a fiche reader (which I also have) what we guys need is to get the fiches scanned into a very good electronic copy.

By the way excellent description of the symptoms.
Regards,
K-BIKE

Thanks for the tips, I am happy to have a starting place. I didn't want to be out in the cold taking apart anything more than I needed.

I'll start with the starter (no pun intended) as soon as I am back home after New Years. I'll post again as soon as I have a definitive result.

If I get a few extra minutes after the new year, I'll get a copy of the microfiche and transfer it into a .pdf for the community.

Thanks again,
Happy new years!

Or buy the BMW manual at Motorworks or MotoBins in the UK. They are still available (in English!).

Hi Rene,
I had a look at those a while back and just now, (my wife says I can never find anything), but they don't appear to me to have the K100 16 Valve manuals. They have the other 2 Valve and they have Clymer but I was after a genuine BMW 16 Valve workshop manual. As I say I did manage to get hold of a scanned copy of one in German but I have not heard or seen an English translation. If you have spotted what I have missed point me at it and I shall buy one as it is always helpful to have the genuine manual. The level of detail in the German one is great although the scanning quality of the photos etc. is not brilliant I must say (still one does not look a gift horse in the mouth).
Hoping that I have missed the obvious and the 16 Valve BMW manuals are right there.

Regards,
K-BIKE

Correction:

My statement "As`crazy as it sounds, a lot of the ground is passing throught the inductor of the alternator" should be : As`crazy as it sounds, a lot of the ground is passing throught the inductor of the starter.
Sorry about that.

Bert

Finally got a chance to pull the starter, clean out the housing, and replace the brushes. I also took a moment to wire brush all the ground connections involved.

I am happy to report that all now seems to be in order. The bike starts right up and has none of the issues with the lights I was experiencing before.

Now, if only it would warm up a bit so I can take it for a longer spin.

Thanks for all the good advice!

Michael

I am happy that you found your problem. It was tricky but very inexpensive to fix.
Enjoy your rides.
Because of the work that my wife asked me to do at home, I don't think my season will start before late spring.

Stupid question number time...

"a lot of the ground is passing through the inductor of the starter"

Is there a way to add another ground strap/cable to reduce the amount of earth passing through the starter?

When you have finished laughing, I would like an answer and be nice!

According to the Load Shed relay diagram, the addition of a ground strap is impossible. The only place one could be added is the connection between the starter and the relay. Unfortunately, that is also the positive feed for the starter and would consequently short when the starter is engaged.

Thanks Pencilhead,

I was just reading this post and this was what I would had answered....
You did a very good analysis of the starter circuit. Thumb's up!

It was a very good question and you had the perfect answer.

Crazy Frog wrote:I am happy that you found your problem. It was tricky but very inexpensive to fix.

so, how much was it, if i may ask?

On the Download page the first listing is Bob's BMW price list(2005). It's a PDF file and it's there to give you an idea of the cost of parts.
In 2005, the cost of a new set of brushes was $12.39

The actual cost for both the primary brush and the ground brush on the mounting plate is in the $22-$25 range depending on your dealer. The part labeled as "brush set" is not a set; it is a single brush with the ringpost attached.

PH