Technical Q and A for classic motorcycle maintenance and repair.
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 your subject.
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? — Charlie/via email
A: I’ve been staring at your BMW’s wiring diagram for so long I see lines when I look away! From your troubleshooting, it follows that all the circuitry is working when you apply power directly. I think that leaves just one thing to consider: the ignition switch. What happens if you hot-wire the circuit at the switch itself? If everything still works at that point, your ignition switch is faulty. I guess an alternative would be that the relay you tested could show continuity and still not be able to pass enough current to run the pump. Sorry I can’t be more helpful; this one is a baffler. Readers? Has anyone experienced a similar problem? MC