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Discussion Starter #1
So I have this issue with my 89 5.0. Everything was working perfectly, then I went out to start it and it wouldn't start. I noticed that the fuel pump was not priming when I turned on the key, so no problem I just need to check the fuel pump.

I disconnected the battery and lowered the tank. Pulled out the fuel pump and tested it while it was out of the tank...nothing. I automatically assumed it was the pump so I went and bought another one.

When I returned with the new one I thought I'd try the old one again just in case, all of a sudden now it's working.

So I put it back in the tank and tested it again in the tank and the car started up. I then raised the tank back up and bolted it up and then attempted to start the car again...now the pump no longer primed.

So I lowered the tank again and pulled out the pump to see what was happening. At this point I'm thinking maybe it is some faulty wiring around the connectors at the pump. I used a test light and tested the power connector that connects to the pump...there is power there.

As soon as I connect the ground wire to the pump, the power wire no longer has power. But if I remove the ground wire then there is power again. So, it seems that the ground wire was causing the issue.

I then ran a separate ground wire to my frame and connected it to the pump with the power wire also connected...same thing happens. There is power at the red wire until the ground is connected then the power at the red is lost.

Can anyone tell me what is causing this problem and how to fix it?
 

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Since ac and dc properties are alike, I would assume that you have a weak connection on the power side to your pump. Once the ground is connected, your power is virtually disappearing becasce you're going from checking it with no load to checking with a load. I'm an electrician by trade, so this is the only thing I can compare it to as AC power does the same thing. This is especially apparent when you have an underground wire burning up. With no load, you may have 120v on that leg, but when you put a load on it, it may be very low or zero. You need to check the power all the way back to the source at the fusible link. Don't forget there is a relay under the driver's seat. And that is basically a switch that gets flipped everytime the key is turned on and the contacts inside it could be suffereing from electical deposits of arcing or a weak magnetic field holding them together.
 

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Discussion Starter #3
That makes sense. I'll put a tester to it today to see how many volts there are at the pump when both wires are connected. It should be 12v right?
 

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Update - I went out this morning and tested the voltage and there was 12v at the pump with both the power wire and ground connected.

I turned on the ignition and the pump turned on...weird I thought since I messed with it a bunch last night and could not get it to work. So then I put the pump back in the tank and bolted the tank back in the car and started the car.

It ran for about 5 minutes, just long enough for me to start heading towards my house - as I was working on it in the middle of a public street, then it instantly died and now the pump is not working again.

I am completely stumped as to what is wrong with this thing. What could be causing this issue?

The main confusion that I'm having is why was it not working at all last night, I let it sit overnight, then all of a sudden it works again when I tested it first thing this morning.

Hopefully someone has experienced something like this before and could give me some advice. It's appreciated.
 

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Uhm your pump is bad. Put the new one in and fix it. Dont know why you didnt the first time. Its called an intermittent fault because the motor is worn out, but under certain circumstances, it works for a short period.
 

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Did you check for voltage at the connector above the tank after it died on you? You don't have to have the pump out of the tank to check for power.
 

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Update - So as FastStang suggested I checked the voltage at the plug on top of the tank, there was power. So I pulled the pump out of the tank and checked the power at the bottom of that plug, there was power.

Then I checked power at the connectors where they connect to the pump, no power.

So then I left my ammeter positive touching the power wire at the pump connector then touched my ammeter negative to the bottom side of the plug connector and there was power.

This indicated to me that the ground wire had a short, so I inspected it a little closer and noticed a very small slit where there was a little bit of exposed wire.

I removed the ground wire and replaced it with a new wire and new connectors and voila, we now have a 100% working pump.

Uhm your pump is bad. Put the new one in and fix it. Dont know why you didnt the first time. Its called an intermittent fault because the motor is worn out, but under certain circumstances, it works for a short period.
So, uhm my pump is not bad and I will go ahead and return the new one and receive my $100 refund.

Do you understand now why I didn't replace it the first time? Sometimes jumping to conclusions without testing everything will lead to more problems and more money.

I figured someone with as high a post count as you would understand that concept, but thanks for your input anyway.
 

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With the info you gave, that was the obvious conclusion. Glad you got it fixed. If I could have deciphered the terms you used, it would have made it easier. And you used an ammeter to check voltage, or am I reading it wrong?
 

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And what was the ground wire shorting to exactly?
 
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