Fuel Pump Help Needed

[Aaron]

I think I have a bad fuel pump on my '90 GT, but to be sure, I need help from the Stangnet experts

Fuel pump does not prime when key is turned on.

Tried tricking fuel pump into running. Jumped the connector in the upper RH corner of ECC test connector to ground. Could hear relay turn on when jumped, but the fuel pump did not turn on. Pink/black wire from relay to pump is running at 12v while test connector is jumped.

12v is present on both sides of inertia switch

Pink/black wire going through floor of hatch to tank is running at 12v also.

Do I have a bad pump?

 

[jrichker]

You have done your homework and arrived at what I would consider the correct answer.

Pump wiring: Anytime the ignition switch is in the Run position and the test
point is jumpered to ground, there should be at least 12 volts present on the
black/pink wire. With power off, check the pump ground: you should see less
than 1 ohm between the black wire and chassis ground.

At this point, I would replace the pump.

 

[Aaron]

Thanks for your help.
I see 2 black wires going through the floor, along with the pink/black wire and a yellow wire. I assume one of those black wires is the ground wire, but I don't know which one is the right wire, and I don't know what the yellow wire is for.

I tested both black wires and I measured 5-6 ohms between each black wire and a good ground.

 

[jrichker]

The yellow wire is the fuel tank sender to the fuel quantity gage. The two black wires are grounds. One ground is for the fuel tank sender and the other is the fuel pump. The ground for the fuel pump may be larger gauge wire that the fuel tank sender ground wire.

Make sure that the power is off the circuit before making any resistance checks. If the circuit is powered up, your resistance measurements will be inaccurate.

You should see less than 1 Ohm between the black wire(s) and ground. To get some idea of what a good reading is, short the two meter leads together and observe the reading. It should only be slightly higher when you measure the black wire to ground resistance.

 

[Aaron]

Make sure that the power is off the circuit before making any resistance checks. If the circuit is powered up, your resistance measurements will be inaccurate.

You should see less than 1 Ohm between the black wire(s) and ground. To get some idea of what a good reading is, short the two meter leads together and observe the reading. It should only be slightly higher when you measure the black wire to ground resistance.

I left the key out of the ignition and tried it again. I'm still getting the same reading of 5-6 ohms between each black wire and a good ground. Granted, I'm no electrical expert. When I shorted the 2 leads together, I got .2 ohms.

 

[jrichker]

I left the key out of the ignition and tried it again. I'm still getting the same reading of 5-6 ohms between each black wire and a good ground. Granted, I'm no electrical expert. When I shorted the 2 leads together, I got .2 ohms.

The 5-6 Ohms is too high since you got .2 Ohms shorting the meter leads together. You should see 1 Ohm or less. It seems like you should plan on cleaning the place where the grounds bolt the wire to the body. There should be bare metal, clean and shiny. After you haved cleaned the grounds and bolted them back down to the body, cover the area with dielectric grease. You may want to clean the connector that joints the ground wire to the body harness. The non-flammable brake parts cleaner is an excellent choice for that job.

 

[Aaron]

Do you know where in the car the wires ground to? They go into a wiring harness and I'm not sure where they go...yet.

 

[jrichker]

The two grounds in question bolt to a spot between the gas tank and the underside of the trunk area. The only way I know of to get to it is to drop the tank.

 

[Aaron]

Well I ended up pulling the pump, and sure enough, it was bad. Dropped in the new one and I've got a car that runs again!

The timing on this was especially strange. I have had the car on jackstands for a few weeks as I rebuilt the entire suspension and steering system. Before I started working on it, everything was fine. During "construction", the fuel pump died. That's why I wasn't so sure of my assumptions.

Thanks again jrichker. As always - you're the man!