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Fuel Pump Relay... MY TRUCK WON'T START!!


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My 4.0L 1988 Eliminator has been sitting for a little while and I went to start and move it, much like I have in the past, and it fired a few times and died as if it was out of gas. Looking into it, I didn't hear the fuel pump engage so I figured I'd go from there. I traced the wires back to the fuel pump relay and everything looks good (no breaks or hard spots). If I energize post number 87 on the relay (it's an orange wire) then the fuel pump starts pumping and I could start the truck that way... but why?!?! :wall: Very frustrated right now... any ideas for what to look for or what to test next?

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Pin 87 is the NO (Normally Open) load contact of the relay and goes directly to the fuel pump. So when you apply 12V to it, your fuel pump runs. What you have to find is why the relay isn't energizing thru the coil contacts (pins 85 and 86) when you turn on your IGN key. Either the 12V to the ungrounded coil pin isn't getting there, or the relay itself might be bad.

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When I turn it over it just turns and turns and turns. Also, I have tried to swap relays with no result.

 

Okay, sounds like no 12V is being applied to the FP relay when you turn the IGN on. Get your meter on either coil pin 85 or 86 (whichever pin is not grounded), turn on the IGN, and see if the 12V signal is applied and that the relay latches. You can hear and feel the relay latch w. your hand if the IGN trigger signal is applied.

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hornbrod, what about the resister, could this be part of his problem?

 

The ballast resistor isn't relayed into the FP circuit until after the engine starts. But I suppose it might be a good idea to jumper it out since there's no telling what a previous owner might have done.

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My 4.0L 1988 Eliminator has been sitting for a little while

 

How long is a "little" while????

 

More that a month or two???

 

Could just be old gas. If you drain out the old gas, and add some fresh gas, that might be your problem right there.

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:agree: An old tow truck driver trick I've learned is to take a hammer and beat (somewhat gently) on the bottom on the fuel tank... Although I can't seem to remember why it works off hand, IIRC it clears the sump :hmm: ... But I have seen it work with success :thumbsup:

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hornbrod, what about the resister, could this be part of his problem?

 

The ballast resistor isn't relayed into the FP circuit until after the engine starts. But I suppose it might be a good idea to jumper it out since there's no telling what a previous owner might have done.

 

 

ballast resistor is part of the FP system at all times with ignition on. remove one connector from the resistor and turn the key to IGN and the pump will not prime.

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I had no constant power going to the relay (red wire). I followed it back a ways and found it had a break... I found droppings and a fried mouse nearby!

 

Everything works again and I couldn't be happier!

 

Good troubleshooting neohic. :thumbsup: Dayem mice.........

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