fuel pump relay issues!?
03-02-2009, 10:44 PM
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fuel pump relay issues!?
went out to start my newly purchased truck the other day(1991 f-150 4x4 300-6 cyl.)and the batt. was dead so i put the charger on it and the fuel pump came on when the key was in my pocket so i did some research and replaced the fuel pump relay and the truck started and ran all day.went out the next morn. for work and it was dead again ,put the charger on it again and the fuel pump came on again without the truck on.any ideas bad batt.,dead short SOMEBODY PLEASE HELP!
03-02-2009, 10:55 PM
#2
Some sort of short in there. Relay was a good idea. After you shut it down, does the fuel pump continue to run? Try jiggling the ign switch to see if the pump quits. I think I would suspect the switch next.
03-03-2009, 06:07 PM
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ok got a new batt. and as soon as i hook it up the fuel pump primes like i have the key on it was running all the time now it is not however i assume something it making it come on periodicly draining my batt. i'm thinking the ignition switch anymore ideas and thank you for the much needed help i'm about to pull my hair out
Last edited by JR.; 03-03-2009 at 06:10 PM.
03-03-2009, 06:50 PM
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03-03-2009, 07:32 PM
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Very good question Dominick, its going to be interesting where this one goes. I like my own thought, the computor (ECM) relay energizes the fuel pump for starts, once the oil pressure is established, the fuel pump relay keeps the pump going. I'm thinking faulty or short in the oil pressure circuit. But then again that circuit should be dead when the ECM relay shuts down. Interesting!
03-03-2009, 08:44 PM
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ok you guys are confusing me 
when you say ignition switch you mean the one under the dash not the key switch right?
when you say ignition switch you mean the one under the dash not the key switch right?
Last edited by JR.; 03-03-2009 at 08:46 PM.
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03-03-2009, 10:38 PM
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Since you say you just got this truck you can also check the diagnostic connector under the hood. The fuel pump can be energized from that by grounding one of the terminals, and it may have been rigged up so it would run before you got it.
Yes, they mean the electrical ignition switch, not the key lock cylinder.
03-03-2009, 11:07 PM
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So if computer relay is stuck in the on pos then the computer would keep cycling the pump.
When you hook up the new bat does the pump keep running?
Or just that few sec prime?
03-03-2009, 11:18 PM
#10
The computer relay is energized first. The relay turns the computer on and gives the positive supply to the fuel pump relay "turn on" circuit. The computer gives a negative signal to the fuel pump relay "turn on" circuit completing the circuit and turning the pump on. If the engine isn't running then the computer takes away the negative signal turning the pump off, which is why I said it might keep the fuel pump on because it shouldn't really. On the diagnostic port you can ground the fuel pump "turn on" circuit bypassing the ground signal the computer gives, which is why he should check that too. Either one of these by itself shouldn't keep the pump on, but they're worth checking.
