Fuel pump not working

Tiny
STUART BROWNSTEIN
  • MEMBER
  • 2003 DODGE RAM
  • 5.7L
  • V8
  • RWD
  • AUTOMATIC
  • 275,000 MILES
The other day I removed some wires under the rear of the truck bed to remove the four prong flat trailer light wires. There are two black wires that are coming out from one of the wire looms on the left side of the frame that I took apart and now my truck will not start. My question is this, is it possible that one or both of those black wires have something to do with the in-tank fuel pump causing it not to work?
Thursday, September 14th, 2017 AT 12:35 PM

3 Replies

Tiny
CARADIODOC
  • MECHANIC
  • 33,869 POSTS
The ground wire for the fuel pump is a black wire, but it is grounded to the body way up in front in the engine compartment. Is it possible that wire is cut and you are seeing the two ends that are supposed to be connected? Measure the resistance between ground and those two wires. If one goes to ground, you'll measure a real low resistance; in the order of five to ten ohms.

If the second wire is the ground coming from the fuel pump, you will find twelve volts on it for one second when you turn on the ignition switch, and again during engine cranking. If you find both of those things, connect them together, solder the joint, and seal it with heat-shrink tubing with hot-melt glue inside to seal out moisture. Do not use electrical tape because that will unravel into a gooey mess on a hot day.
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Thursday, September 14th, 2017 AT 8:25 PM
Tiny
STUART BROWNSTEIN
  • MEMBER
  • 4 POSTS
I want to thank you very much for your answer Mr. CARADIODOC it was really starting to get to me, but when you said the black wires were positive and negative I started thinking why would someone connect the four prong flat trailer light wires to it. So I removed the wires and reconnected the black wire together, turned the key and heard the fuel pump motor recycling, turned the key and my truck fired right up. Problem solved.
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Friday, September 15th, 2017 AT 7:05 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,869 POSTS
Yippee! One in a row!

I went home concerned that my explanation was not adequate about those wires. One of them is not a positive wire. That is a single wire that goes from the pump to ground. It is the pump's ground wire. We have not done anything with its positive wire. The secret I eluded to was that when there is a break anywhere in the circuit, like you had, when twelve volts is switched onto it, you will see that full twelve volts all the way through the positive wire, (like normal), through the pump motor, and through the negative wire all the way up to the break, then zero volts everywhere after the break. That is why I said to test for twelve volts on those wires, and it should be there for one second when turning on the ignition switch, and again during cranking. That would simply prove you found the ground wire, and it was not grounded.

This is a real simple circuit in that it has just two wires and two connector terminals, so my testing might seem overly-involved. It is when there are three, four, or five connectors in the harnesses before you finally get to the motor or light that this becomes a valuable test to narrow down the location of the break. Regardless, I can go home now knowing you are back on the road.
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Friday, September 15th, 2017 AT 7:33 PM

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