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Fuel Pump bad ground?


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I have a 1988 Comanche 4.0 automatic 4WD and just swapped out the fuel pump with strainer. I test drove the truck for a day, then it sat for two. When I tried to start it two days later it would not.

After some troubleshooting I wound up removing the pump and noticed that the hot wire to the pump (on the inside of the tank) had gotten so hot that it melted itself out of the insulation and came in contact with the fuel guage wire. I'm not convinced that the fuel gage wire melted all the way through - it was still working. So I put 12V to the pump and it still works.

 

What I'm not clear on is why the wire got so hot. Do I have a ground problem? I will replace the two wires, but want to make sure that when I re-assemble everything that I will be OK.

 

The only thing I can think of is that I have a poor ground and that is why the wire got so hot. Can anyone help me to better understand what happened here?

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The ground for the fuel pump and sender is also the same ground for the tail lights, it's located behind the left (drivers side) tail light, pull the tail light, you will see a wire and screw into the sheet metal. Pull the screw out, scrap some paint, and install a larger screw with a good dope of dielectric grease.

 

For the wire to burn off the insulation, it really had to get hot, if there is any way you can check the resistance on the fuel pump, try to check that.

 

Did by chance you use the Carter # P74155 as a replacement pump??? That is if you had the Bosch pump as OEM.

 

Also, check under the hood, left front corner, behind the air box and see if you have this Ballast Resistor-

 

 

That was added in '88 or '89+ to use full voltage at start up, and then cut the voltage down for the pump to run "cooler" with less voltage input.

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Good advice from the Wildman. If all that fails, measure the fuel pump amp draw with a mulitmeter by disconnecting one lead from the ballast resistor and connecting your multimeter in series on the 0-10 amp scale between the wire you disconnected and the empty ballast resistor terminal. Start it up and read the pump current draw. It should read no more than 8 amps. If it does your pump is probably shorted internally and needs replacement.

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Thanks for the quick reply wildman

 

The resistance across the two terminals is 5.5 ohm

the ballast resistor checks out at 1.1 ohm

the part number on the replacement fuel pump matches that of the one that was in the tanks - E7006

Is has a mfgr date of 391 on it, so I guess it may have been replaced as part of a recall.

 

When I was checking the resistance I did notice that the terminal had moved CLOSE to the mounting bracket. Maybe it shorted?

I'd have to figure out how to better secure the pump.....

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hornbrod, I could plug it in and short the D1-5 and D1-6 diagnostic connector pins to start the pump and check the current as you described. Is it OK to run the pump dry to check current?

 

I would not; as you know it's a submersible pump and uses the fuel for cooling and lubrication. It should be checked with the ammeter in series under load running in it's normal circuit configuration. That way you know if the entire circuit is okay. :cheers:

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I'll agree with not running the pump dry.

 

I've used a bucket of water to check pumps, lot safer that an open bucket of gas, just don't drop the whole pump into the bucket of water, keep the wires and terminals out of the water.

 

The wire that is used for the pump is not like normal off the shelf auto wire, but I'm sure that a good auto parts house would have a wire thats make for the in tank fuel pump.

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