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Old 03-02-2009, 10:44 PM   #1
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Default 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!
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Old 03-02-2009, 10:55 PM   #2
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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.
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Old 03-03-2009, 12:53 AM   #3
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Check the computer relay, right next to the fuel pump relay. If it sticks the fuel pump might stick on too.
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Old 03-03-2009, 06:07 PM   #4
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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

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Old 03-03-2009, 06:50 PM   #5
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Originally Posted by Just call me Sean View Post
Check the computer relay, right next to the fuel pump relay. If it sticks the fuel pump might stick on too.
Ok if the computer relay was stuck on wouldn't the truck stay running
even if you shut it off and took the key out?
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Old 03-03-2009, 07:32 PM   #6
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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!
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Old 03-03-2009, 08:44 PM   #7
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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?

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Old 03-03-2009, 10:38 PM   #8
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Ok if the computer relay was stuck on wouldn't the truck stay running
even if you shut it off and took the key out?
No, the coil is turned off with the key separately from the computer. So even if the computer stayed on the coil would shut off and kill the truck.

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.
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Old 03-03-2009, 11:07 PM   #9
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No, the coil is turned off with the key separately from the computer. So even if the computer stayed on the coil would shut off and kill the truck.
Ok after looking at the book both coil and Dist are shut off when key is off sorry didn't see that.
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?
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Old 03-03-2009, 11:18 PM   #10
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So if computer relay is stuck in the on pos then the computer would keep cycling the pump.
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.
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