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Fuel pump relay test
I recently purchased a 1984 GL1200A that doesn't run, but just needs some TLC. When I attempted to start it after cleaning the carbs and replacing the salanoid, it cranked, but wasn't getting gas. So I checked and found that the fuel pump was not working when a grounded the black and white wires at the relay connection. I have ordered a new pump.

I have two books and only one tell how to check the relay. I have acquired two used relays off from bikes that were working and I get the same results on both of them as I do on mine. The instructions say to check the white wire and ground with a contenuity oame meter and do it with the kill switch on and the ignition switch on. If there is continunty the relay should be replaced. I get contenuity, or I should say the hand on the gage moves from left to right. Am I missing something? How does this relay work? Does it require action on the starter first? Is is just the ignition switch that activates? Or is it pressure in the pump? I am wondering if I should have disconnected the battery first.

Can anybody give me some insight and understanding?



#1 05-07-2010, 08:19 AM,
In basic terms, it runs off the wire from the coil, then to the tach. As you're turning the motor over, and if you can see the tach working , then you should see power going thru the relay and then to the pump.
For what is a man profited, if he shall gain the whole world, and lose his own soul? Matthew 16:26
For the love of money is a root of all kinds of evil. 1 Timothy 6:10
#2 05-07-2010, 10:56 AM,
would the fuel pump work ok if the relay was bypassed per se and powered directly by a 12v source?

i would think that this setup is actually a safety precaution in case engine isnt running and a fuel line is leaking
1987 Aspencade 129K
1986 SEI 93K
2014 Tri-Glide HD 17K

#3 05-07-2010, 11:31 AM,
The fuel pump relay isn't actually a relay in the strictest sense of the word. It's a transistor. Check this "basic" diagram of the fuel pump circuit.

[Image: fprelay.bmp.jpg]

The CDI unit controls both ground legs of the coils individually. It also supplies the ground legs that operate the tach and the fuel pump relay/transistor individually. Notice the tach can operate, but it does not necessarily mean the fuel pump relay is getting the correct signal.
The CDI pulses a ground for the FPRelay relative to engine rpm, since it's also firing the coil. The higher the rpm, the greater the frequencey of the ground.
This "pulsed" signal effectively cuts the "mean" or average voltage supplied to the fuel pump. The higher the rpm, the higher the voltage to the pump.
I haven't actually tested this theory, but I believe by back probing the voltage wire to the pump from the relay (A), you should see minimal voltage to the pump at idle, with an increase in voltage as the rpm's increase.
I'll have mine apart far enough this weekend to verify and post the results. If it is as I suspect, we may make it a "sticky"?

Although you can supply direct 12v to the pump and make it run, I don't believe it was designed to carry that amount of current indefinately.
You may also run into the situation where the pump overpowers the floats at lower fuel requirements.
#4 05-07-2010, 05:31 PM,
it is just nice to know that should the "relay" quit it would be possible to "test" the pump to see if its that or the relay while out in the "boonies"

at least it might get ya back to civilization
1987 Aspencade 129K
1986 SEI 93K
2014 Tri-Glide HD 17K

#5 05-07-2010, 05:47 PM,
It may very well be that I'm all wet. It may supply 12volts to the pump in bursts, and vary the frequencey of the pulses as the rpm's increase. :roll: In any event, I'll post again tomorrow after my tests.
#6 05-07-2010, 06:02 PM,
Thanks for all of the detailed information about the fuel pump and relay. I hope all I need to do is install the new pump and the system works fine. I am most interested in getting the results of the test that is mentioned and discussed.

Thanks again

Any additional information is welcome.

#7 05-07-2010, 09:20 PM,

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