BMW K100RS Worries

Reader Contribution by Keith Fellenstein
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BMW worries

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

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