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Old 05-30-2010, 15:16   #1 (permalink)
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Fuel pump voltage issue

I got this 1998 Taurus that I decided had a bad fuel pump. So I bought a new pump and pulled the old out. I hooked up the new pump and turned the key on and nothing. So I checked the voltage and found that I only had 6.65V. I swapped the relays and checked the fuses, but I would think I would get 0v if that was the problem. Other than a bad connection on the plug at the tank, where else should I look for a voltage drop?
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Old 05-30-2010, 16:17   #2 (permalink)
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Re: Fuel pump voltage issue

Do you know how the system works ? The computer gives you the first 3 seconds of
voltage to run the pump and prime the injectors. If your looking for voltage after that time you will find nothing.
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Old 05-30-2010, 17:18   #3 (permalink)
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Re: Fuel pump voltage issue

So, how should I go about diagnosing it? Should I see 12V for 3 seconds and then nothing?
Right now all I am seeing is 6.65V all the time.
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Old 05-30-2010, 17:33   #4 (permalink)
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Re: Fuel pump voltage issue

To do that properly you need a scope. Even two people would help. You could be getting a phantom voltage. Is there amperage to the voltage ? Why did you think the
pump was bad in the first place ? What made you change it ? It's almost the same as changing the engine because it quit when it ran out of gas. On todays cars there is a LOT of research and testing to do before any part is changed. Are you using a digital
volt meter ? What amperage are you getting at 6.65 volts ? Why are you getting 6.65 volts and where is it coming from ?
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Old 05-30-2010, 17:54   #5 (permalink)
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Re: Fuel pump voltage issue

I decided it was the pump after getting the car and it did not run. I shot starting fluid in and it ran briefly. I listened back at the tank and there was no sound. I also pulled the tank and put power to the pump and it did not work. However I did hook up the new pump and it did not work.
I am not sure how to diagnose it. I do have another person, but no scope. I am very open to advice. I am fixing this up to give to a family in need.
Thank you.
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Old 05-31-2010, 08:13   #6 (permalink)
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Re: Fuel pump voltage issue

The easiest way is to supply a clean 12V power and ground right at the pump connector. Connect a pressure gage up in the engine compartment and see what pressure you get. If that works, then you can rule out the pump and start checking for bad wiring/relay, etc. If the pump doesn't work with separate power and clean ground, well, then you can be assured that your pump is truly bad
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Old 06-02-2010, 18:49   #7 (permalink)
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Re: Fuel pump voltage issue

Well I now have a strange issue. I pulled and bypassed the fuel pump relay. I now get a full 12 volts to the pump and the new pump runs. With the relay in it I only got 6.5V, I swapped the relays around and still only got 6.5V.
Could the relays all (3) be bad from sitting over a year?
It is strange that I got the same voltage with 2 relays and then full voltage with a jumper.
Any ideas?
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Old 06-02-2010, 20:51   #8 (permalink)
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Re: Fuel pump voltage issue

Mabe the battery is weak and to run both is too much for it.
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