Fuel Pump Problems: BMW K100RS

Reader Contribution by Keith Fellenstein
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Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Keith’s notes

In the May/June 2013 issue, a reader asked a question about his BMW K100RS. He was having problems with his fuel pump not staying on after initial engine cranking. Without much experience with that model, I offered the only solution I could come up with after looking at the wiring diagram, which was a possible faulty ignition switch. A reader with more experience than me recently replied to this article and his advice is probably closer to right than mine. Here’s the original question and the updated answer.

Q: I have a 1985 BMW K100RS, which uses L-Jet Bosch fuel injection. It had been running pretty well, though with an occasional miss. I filled it with gas the other day and now it won’t start. The fuel pump in the tank is not running. I took the fuel pump relay out and it bench-tested good. I can jumper the relay socket terminals and run the pump, so the wiring is good. I can hot-wire the left side of fuse no. 6 and the pump will run, proving that the pump is getting both power and ground, and the connector through the tank is good. If I hit the starter button while the pump is hot-wired, the bike will run and rev, proving the ignition controller is good and the injection computer, also. What’s left? I seem to have proven that all the components in the fuel supply side are good, and also that the bike will run when the fuel pump is hot-wired. Something’s missing and I don’t know what. There has to be some little thing I’m overlooking?

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