First some background. Not really new to BMW's, had an r1200c with many issues, but a great ride, also multiple japanese sport bikes and now I have a Duc 848.
My father picked up a 91 (I think) k100lt. It's red, awesome bike. I came to greenville to visit him, took it for a ride, it was unbelievable for being 20 years old. You barely had to touch the starter to get it going. I mean literally tap it, and boom, fired up. A big difference from my 2 year old ducati with 3000 miles on it that takes 5 seconds of cranking to start.... oh well. It's Italian. Moving on.
Got back from the ride, he asked me about the his fuel gauge not working, I said I'd take a look. Opened up the upper dash area below the windscreen and found a disconnected yellow wire. Traced out some continuity with a multimeter from the 4 pin connection behind the side panel just to be sure. Then tested the gauge by sticking a 9v across it and making sure it moved, reconnected the yellow wire to the proper terminal, put the bike back together and..... it doesn't start. Starter motor turns, will check spark in the morning. What I have found is that the fuel pump doesn't appear to have power. Please bear with me here, I spent all day yanking the tank off and trying to troubleshoot this so I may wander a bit in my explanation.
Connector behind side panel has 4 wires and from what I can tell, they are as follows:
Brown - Earth
White - constant 12V after kill switch is put in run position, appears to supply voltage to slide pot that is the fuel gauge attached to the float
Green with a black stripe - 12V when i depress the starter switch (fuel pump?)
Yellow - appears to be the wire that supplies fuel sending unit's signal to the gauge.
Correct me if I'm wrong there, but that's what I came up with.
Next, when I metered the pump in the tank, I have no 12V to it both with the engine kill switch in the run position and with the starter button depressed. My understanding from looking over your forum is that the pump doesn't run until after the starter button engages the FI relay. That's different than I'm used to, but it's logical.
So now for where I'm stumped. I have 12V across the green and brown wires coming from the bike. I'm not even worried about yellow and white unless you guys tell me I should be.
I have continuity from the green pin of the male FSU connector all the way to the screw on jack that connects to the pump.
Likewise I have continuity from the brown pin on the male FSU connector that becomes a black wire going to a screw terminal on the fuel pump.
My thought is that if I plug the 4 pin back into the bike, I should see 12V across the 2 screw terminals, but I don't. I don't get any voltage there.... It's very frustrating.
FYI I touched a 9v battery to the pump to be sure it spun and it did.
Any info would be greatly appreciated. I'm contemplating running down to the dealer and picking up an FSU and plugging it in to see what would happen, but would really like to figure this out so I don't blow $100 on something that could be fixed easier. Thanks again, pardon the long post.
