BMW K100RS Worries

| 4/15/2013 4:15:13 PM

Tags: Tech Corner, BMW, Classic German Motorcycles,


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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 

7/31/2014 11:54:04 AM

I have a 1991 K100rs. I have had cut outs and the feeling of running on two and three cylinders. It was my ignition switch. Had to change it twice. Once from age the second time moisture. Once changed it has been running fine. I to thought it was the fuel pump. Duke

11/21/2013 6:22:09 PM

Is your advice applicable to BMW motorcycles too?

11/19/2013 4:48:10 PM

A set of contacts in the air flow meter keep the field coil of the fuel pump relay energized after the release of the starter button, possible causes of this circuit not closing are air leak "down stream" of air flow meter, door stuck in air flow meter, bad contacts in air flow meter. The owners manual wiring diagram is fully comprehensive, just find a real BMW car mechanic or similar to look at it, would only require one hour of diagnostic time at my shop. P.S. its not the ignition switch.

gerald estes iii
7/11/2013 11:04:01 PM

its german, 28 years old with presumably many miles on it - some wire or contact metal somewhere has rusted away or worn through its insulating sleeve and is shorting out. ...and its a job security thing because when a 2013 models start to have similar ailments 30 years on, im pretty certain patience wont be a prerequisite - but lots of ammunition sales, landfill area and a money printing machine will probably be required to fix one. i had a similar problem w / the last of model years straight 6 gmc p/u - tail lights and dash board lights kept shorting mechanic wants to be the one to peel a bunch of grungy and brittle wires out of the protector sleeves to find a fault. i started with the dash since the thing had more than a couple 100,000 miles on it and YES the first wave of thin sheet printed circuit boards had worn away its alignment holes just enough to allow it to eventually slip and blow a few circuits at a time no matter how tightened it was sandwiched in there. rtv might have been the cheap fix, you cant imagine how much they wanted for an oem replacement...the trade in towards a new one was $75 dollars.

john skala
7/11/2013 6:59:59 PM

There is a very good group on Yahoo that has a tremendous amount of BMW K Bike information and knowledge. go to and search for the KBMW group. Check the archives or post a question there and you will be amazed at the depth of knowledge there. Unlike the later models, 1985 K bikes did not use a tank float.

tony carlos
7/11/2013 12:40:53 PM

Hard to believe a faulty float would kill the engine. To my knowledge, the float just activates that fuel gauge and the low-fuel warning light. Going from an OLD memory, I recall some issues with the side stand. A faulty stand-down switch might be killing your pump. Or do the 85s still have the mechanical retractor in the side stand instead of a switch?

philip moss
7/11/2013 12:21:10 PM

My K100LT did the same thing after filling tank. The mechanic a Boca Cycles in boca raton, FL replaced the tank float, and it never gave me another problem.

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