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Discussion Starter #1
Was working on my rear brakes last weekend and had the rear wiring harness (fuel pump, transfercase, and transmission) hanging down. After I fixed my problem with the rear brakes, I took it for a quick spin out in the back yard to check out the brakes. Not thinking to well I left the wiring harness hanging down and the wire wrapped around the rear driveshaft and the Jeep died.

The fuel gauge wire (brown) was the only broken wire and spliced it back together. The Jeep still would not even turn over. Checked all the fuses but none were blown.

Then I discovered that the pin pulled out of the connector on the main wiring harness. This connector plugs into the rear wiring harness that goes to the fuel pump, transfer case, and transmission. I pushed the pin back in and the Jeep cranked over but it still will not start. I used starter fluid and the Jeep started up and died pretty quickly. This leads me to believe that the fuel pump is not work. I also noticed that my fuel gauge inside the Jeep read full and before I wrapped the wire around the driveshaft it read ¼ of a tank.

So I believe that there is a problem with the three wires that connect to the gas tank sending unit. I looked up the three wires for the fuel pump in my manual. 1st wire is ground, 2nd is 12v from the relay, and the third is the fuel gauge. The ground and 12v lines did not measure any voltage. I then measured the resistance of the ground wire to metal on my Jeep and is showed it was open. The fuel gauge wire measured 6-7 volts. It was getting dark so I called it a night. I am not sure if I had a good ground to measure the resistance of the ground wire so I am not 100% sure it is a floating ground.

I think two of the wires are still bad or just the ground wire is bad and the fuel relay uses this ground wire to work and that is why I am not measuring any voltage on the 12v line for the fuel pump.

Question:

1. Once the Jeep key is turned to the on position, should the fuel pumps 12 volt line be hot (12 volts) all the time?

2. The fuel gauge reads full and I measure 7 volts on the fuel gauge wire. I know the tank is only 1/4 full but after my Jeep died it reads full. Since I read 7 volts on the fuel gauge wire this leads me to believe that the ground is bad at the fuel pump and not completing the circuit. Is this what you would believe also?

3. If the ground is bad on the wires to the fuel pump will the fuel relay still work? I noticed that the starter relay gets its ground from this wiring harness also and it did not work until I fixed that ground.
 

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ftrain said:
1. Once the Jeep key is turned to the on position, should the fuel pumps 12 volt line be hot (12 volts) all the time?
With an in-tank fuel pump, you will only read 12 volts for the fuel pump for a brief second or two after the key is turned to the on position and then you will read nothing until the key is turned to the start position. The fuel pump runs these seconds at first to bring the fuel system up to pressure.

ftrain said:
2. The fuel gauge reads full and I measure 7 volts on the fuel gauge wire. I know the tank is only 1/4 full but after my Jeep died it reads full. Since I read 7 volts on the fuel gauge wire this leads me to believe that the ground is bad at the fuel pump and not completing the circuit. Is this what you would believe also?
The fuel level float changes the resistance on the wire. You should be measuring OHMs not volts on this wire. This should be 0 Ohms with an empty tank and 90 Ohms with a full tank. A voltage measurement on this circuit will not tell you if it is working properly or not.

ftrain said:
3. If the ground is bad on the wires to the fuel pump will the fuel relay still work? I noticed that the starter relay gets its ground from this wiring harness also and it did not work until I fixed that ground.
The fuel relay will still work if the ground at the tank is bad. The secondary circuit will still close the switch for the primary circuit, but without a proper ground at the tank, the fuel pump's circuit will not be complete and the pump will not work. You should be able to check the wire going to the fuel pump after the relay with a voltmeter and a good ground and see the voltage at the times I described above. The fuel pump is grounded at the tank, not at the same location as the starter. The tank must be fully and properly grounded to help reduce the rick of an arc in a tank full of fuel vapors.

Check the fuel pump wire as close to the relay as possible. Test between that wire and a known good ground. Then test between that wire and the ground at the tank. You more than likely have ripped a ground wire loose somewhere.
 

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Discussion Starter #4
Thanks for the help. That was the technical information I needed. Since the fuel tank is reading full and my fuel pump is not working I bet my ground is bad.

How does the computer know when to power the fuel pump? Can it tell the difference when the motor is running or stalled when the key is on the "ON" position?

I have Friday off so plan to go back into the woods and get the Jeep going. I will let you know how I made out.

Thanks again for your help.
 

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ftrain said:
How does the computer know when to power the fuel pump? Can it tell the difference when the motor is running or stalled when the key is on the "ON" position?
The computer powers the fuel pump when you first turn the key to on to bring the pressure up. It does not power it again until you turn the key to start and the engine starts to turn over. If the engine did not start and you turn the key back to the "ON" position, the fuel pump would not be running since the crankshaft position sensor tells the computer that the engine is not running.

Here is the sequence:
1. Turn key from OFF to ON --> Fuel pump is powered for a couple of seconds to bring pressure up
2. Turn key from ON to START --> Fuel pump is powered the entire time the key is in the start position
3. Release key and it turns from START to ON --> Fuel pump is only powered if CPS tells the computer that the engine is running. If CPS tells the computer that the engine is not running, fuel pump gets no power.

I hope that helps.
 

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Discussion Starter #6
That was some great information. It turned out that the ground pin in the main harness pulled out of the connector and also one of the three wires going to the fuel tank broke. After inserting the pin back into the connector and soldering and shrinkwraping the broken wire together the Jeep started right up. Been running with no problems.

Thank you
 

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I flip the switch on and the pump runs continuously and it's only supposed to be a few seconds, what could be wrong?
 

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This is a post resurrected from the grave!

A stuck or jumpered fuel pump relay, or someone hard wired the pump to Ignition + bypassing the relay. Just a guess
 
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