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Unread 10-21-2006   #1 (permalink)
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Question 7 volts to fuel pump?

i have a 1991 mustang LX 5.0 hatch... i got the tank dropped out of the car to replace the fuel pump... hooked it up to test... and nothing! checked the fueses - they were all good. then i whipped out the voltmeter and checked the battery for a baseline - 12 volts there. checked a random fuse for power. got 12 volts at the fusebox. then i checked the connection that goes from the car to the fuel sending unit and got 7 volts. where in the world did my five volts go? is it mabye my fuel pump relay? is that under the passenger seat? and (dont make fun) could i just wire this up to a switch on the dash? i've spent hours and hours and cant figure out why im only getting 7 volts to the pump. if i could only get that magical 12 volts then im positive the car will start. someone please help before i give up hope!
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Unread 10-21-2006   #2 (permalink)
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Do you have a diagram for the fuel pump circuit?

If you are on the correct wire, and your checking available voltage to the pump, make sure you are doing it with the fuel pump in its running position, or the reading won't be accurate. In other words when you check voltage there, kick the key over (when it primes the fuel system) and then check your reading.
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Unread 10-21-2006   #3 (permalink)
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The title of this topic says "fuel pump", then in the message body you said you tested "the fuel sending unit" and got 7v, and then you said you're getting only 7v at the "pump". So are you checking voltage at the sending unit or at the pump?
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Unread 10-22-2006   #4 (permalink)
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Are you checking the voltage on the pink-black wire (my stang is a 88 but wire should be the same) That wire comes right from the fuel pump relay. To make it easier to check turn the key to run and ground the single wire plug at the computer test conector. This will close the fuel pump relay and supply a constant 12V to the pump. Check voltage at conector first with pump unpluged and using a good ground. if you don't have 12V there remove the relay and install a jumper wire from the pink-black wire to the orange -lite blue wire, if you get 12V then replace the relay. If you still don't start checking for a bad wire.
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Unread 11-17-2006   #5 (permalink)
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well thanks for all the input... apparently the car we got had been sitting outside for some time with the window part way down (this was apparent because the carpet was still damp under the seat) and we found that we simply needed a new relay... put that in and the car fired right up... idled and runs fantastic... i think i got my 1000 dollars worth
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