Fuel pump does not stay energized while cranking

[Bigtater]

Hi all, short time lurker and first time poster. Thanks in advance for any responses

I recently acquired a 1990 K75 that I am trying to resolve an electrical issue with. The previous owner said the bike started to stall intermittently, but eventually would restart after a short time. Eventually the bike would crank but would not run. The PO replaced the fuel pump, but a new fuel pump didn't resolve his issue. This is the state I received the bike in.

I began troubleshooting by verifying the new fuel pump's operation. I supplied B+ and B- to the pump and it buzzed to life. The fuel pump fuse has voltage on both sides. I tested the tank fuel pump connecter and got the following results:

Key off test:
gr/wht wire- B+
yellow wire- B- (with some variance)
Brown- B-
White- B-

Key on test:
Gr/wht- B+
yellow- B- (with some variance)
Brown- B-
White- 1.0 volt

when cranking:
gr/wht: 0 volts (12 volts drops to 0)
Brown- B-

When back probing the green/white wire, while cranking, the pump buzzed to life. I was able to duplicate this by wiggling the probe. I then attempted to start the bike, and it purred to life. However, it would then slowly die, sort of like it was running out of fuel. I started it again and probed for voltage. As soon as the bike starts running the voltage drops to 0. As soon as it dies, the 12 volts return, and I can hear the pump buzzing. At this point I bypassed the connector and tested for 12 volts again while the bike ran. The voltage dropped to 0 when running and returned when it stalled.

So here are my questions:

Is the voltage drop on the green/white wire while running correct? or should I see 12 volts?

Am I correct in understanding the fuel pump should only receive voltage when the ECU sees a signal from the crank position sensor?

Am I going the wrong direction here?

[daveson]

"Am I going in the wrong direction here?'"

No, I think you're going in a logical sequence, and have eliminated the plug as the problem. Keep working back with your probe until you get to where the Voltage doesn't drop out. The problem will then be between there and the previous step. One possibility is that the fuse holder is a loose fit, then it could have 12 Volts but not good enough under load so it drops out. Put a new fuse in, clean the fuse holder, and crimp it somehow if it's loose to make it tight again. I think of the fuses as deliberately the weakest link in the system.

It's a good idea to consider the quick, easy stuff first, I've told myself a hundred times, but still I sometimes forget.

[Bigtater]

Thanks for the reply daveson.

I did figure out the issue. It was a two part issue.

First, I re-did the fuel pump connector. At this point, the fuel pump would run constantly. It would run whether the key was on or off, but would lose voltage when attempting to start the bike.

After reviewing the wiring diagrams some more I found I determined to pull the fuel pump relay and preform resistance checks. I found the load side of the relay had continuity constantly. I swapped the relay with a spare relay and boom! Problem solved! The bike now has voltage on the green/white wire while cranking and running. 

[daveson]

Bonus. It's good to see an occasional thread that has a quick fix.