I have utilized crazy frogs very detailed information on this forum to trouble shoot a fuel pump problem. These are the things that I know after approx. 40 hours of trouble shooting. The pump is good, hard wired it runs fine. I have 12 volts at the green/white wire to the pump when I press the starter button, remove pump check voltage at pump 12 when start button is pressed. I have confirmed good ground and all parameters according to crazy frogs guide for the fuel pump trouble shooting. Again, 12v from fuse block to the starter relay all the way to the pump. I have replaced the tank connector with the 4 pin connector removed from the relay box. It would seem that any time the starter is in the circuit the fuel pump does not work even though the meter tells me the pump is receiving 12 volts. My suspicion is that the starter is some how disrupting the ground connection for the pump. Anyone have any thoughts on this??? I would greatly appreciate some input. Thanks, Craig

This sounds similar to one of the problems in an old thread, recently revived, called "troubleshooting a 1985 k100 that won't start, stay running" the fuel injection relay was the cause in that case. It was as if the relay was weak, still working with smaller loads, but unable to cope with the added load of the fuel pump. The thread gives more details.

Check that the relay pins and sockets are clean (with electrical contact cleaner) tight and properly engaged. If that checks out, one thing to try would be a jumper wire between both 87 pins of the fuel injection relay socket, then replacing the fuel injection relay (5 pins) with the horn relay (4 pins) If the pump and starter then work at the same time, you want to get another fuel injection relay. If that's what you need you can probably get one at the local junkyard if they have BMW's there. It's light green with five pins, part number 6136 8 373 700.

If you already have another fuel pump relay, don't bother with the jumper wire and horn relay, just swap fuel pump relays to see if that solves the problem.

I had the opportunity to further trouble shoot the problem, suffice to say I screwed up on the diagnosis big time. I installed a new starter, I keep a spare and that of course had no effect. So I scanned the  the schematic again and I discovered that I had been checking the wrong relay connections. On my bike the load shed relay and the fuel pump relay are one and the same. I performed all the checks again and discovered that indeed the relay looks like the culprit. I have ordered a new relay and will receive it this week, another weekend in the shed, but I have learned a lot about the electrics of the bike. Hopefully the relay will solve the problem as the symptoms are as you have described. Thank you for taking the time to reply to my question, I appreciate it, Craig

cfmckay wrote:I had the opportunity to further trouble shoot the problem, suffice to say I screwed up on the diagnosis big time. I installed a new starter, I keep a spare and that of course had no effect. So I scanned the  the schematic again and I discovered that I had been checking the wrong relay connections. On my bike the load shed relay and the fuel pump relay are one and the same. I performed all the checks again and discovered that indeed the relay looks like the culprit. I have ordered a new relay and will receive it this week, another weekend in the shed, but I have learned a lot about the electrics of the bike. Hopefully the relay will solve the problem as the symptoms are as you have described. Thank you for taking the time to reply to my question, I appreciate it, Craig

On some of the Ks you can swap the fuel pump relay and the horn relay. . . . .

I'm thinking it would be cool to know what year and model bike you're talking about here, but unfortunately, I'm too lazy to search the forum to find where it might have been posted if it ever was.

Hi, that was my 85 K100rt in an earlier post with similar problems.
I had a few problems all at once.
Main two problems,
I had a shorting starter button causing power to cut out at low revs.
And also had a faulty new fuel pressure regulator.
I was looking in all the wrong places for a fault. But learned a lot doing it.

Solved the fuel pump issue, it was in fact a defective fuel pump relay. Thank you for the help, you know who you are and I do not appreciate snide remarks from other posters. If you had looked at my avatar you would clearly see a BMW K100RS with a sidecar.

You might notice that the inmates list the model(s) they own and in some cases include VIN's.  BMW changed numerous details in these machines during the production run.  These changes can affect troubleshooting, especially with regard to relays and electrical systems.

When you come here asking for help it makes it a little easier for the inmates if you share a little information about your particular machine.  Did you really expect the inmates to look at your avatar to identify what model and year you are having a problem with?  As luck would have it, the bike in your photo that is hidden behind a sidecar and another bike could be an early K100RS, a later K100RS 2v, a K100RS 16V, or even a K1100RS.  All of these bikes have differences that could have a bearing on the problem you were having.  It is a bit presumptuous to come here asking for help and not bothering to share information about your machine.

Hi, I’ve just the same relay problem, went straight to it after this thread. I’m going to do the relay mod on the lighting system now after reading about that. Would be worth your while checking that thread out. Cheers
Porto

cfmckay wrote:Solved the fuel pump issue, it was in fact a defective fuel pump relay. Thank you for the help, you know who you are and I do not appreciate snide remarks from other posters. If you had looked at my avatar you would clearly see a BMW K100RS with a sidecar.

True, but you can't tell from the photo if its a 8v or 16v bike and they differ. I have an 83 and an 84K and lots of difference even in those.

My one is a 85 , 4v.
I one e had a shorted starter. The wires would touch as I would take off with vibration. This was cutting power to the all but needed electrics. Took me ages to find that one. Cheers

A happy ending to the problem. Ks look alike sound alike ride alike,BUT are not alike, The VIN No is the DNA of all Ks without it we are working in the dark, people get upset, egos get bruised, when all we are doing is trying to HELP. SO dear members new or old PLEASE PLEASE include your VIN No   There endeth the first lesson