Voltage to Drive a Fuel Pump

[soundu]

Originally Posted by Automender12345

If you hook up a 12 volt light to the fuel pump connector I bet you will see it light for a second and then go out. You are not measuring any current at the 6.8 volts. If the PCM uses an voltage comparer to determine if the pump gets voltage it may be using a reference voltage of 6.7 volts. This voltage but current restricted is injected and used to determine it the input is higher than that. If so you may be picking up on that voltage. It's been years since I worked on these type of circuits and the pcm may not use them but I believe that is what your seening. The light will prove the theory. If the light stays lite then you really have a problem since the PCM should be shutting all voltage and current after one second thru the relay operation. Maybe there is a techee out there who knows how the circuit would detect the pump supply voltage.

I may be wrong but if you put it all back together with a new pump it will work.

Per the schematics, the PCM DOES monitor the fuel pump voltage.
BUT, most all contemporary circuits would use a buffer to isolate
the voltage before any comparison is done. Unless, the PCM is
defective, it should NOT be putting out voltage on this line.
If it was a phantom voltage, any kind of load on the line would make
it disappear

If the fuel pump is connected and he is getting 6.8v across it
(meaning there IS a load on the line), that voltage is coming from
somewhere).

If that voltage is coming from the relay socket, there had to be a high
Resistance contact in the line from the socket to the pump. There is no
other way for the voltage to drop across the line.

He established that if he hot wired the pump, it runs. The fuel pump relay
should be feeding the fuel pump 12volts when energized. There should be a
straight wire connection between the fuel pump relay socket and the fuel
pump. Anything in-between that is adding a resistance to the line should
not be there (that includes a bad inertia switch or high resistance contact)

you can do a test to take the relay out of the equation. Get a spade
terminal that will fit into the relay socket. make a pig tail with a 10 amp
fuse in line. plug that into the battery's +12v terminal. Then plug the spade
into the relay sockets line feeding the fuel pump. You know for a fact
there HAS to be 12V now feeding the fuel pump from the front of the car.
If your NOT reading 12 volts at the fuel pump hot terminal with a known
good ground in the back of the car, the line from the relay socket has an
issue somewhere in it.

IF the pump was back in the tank with gas in it, you could also hot wire
it the same way for a short amount of time and see if the car starts and runs.

[Automender12345]

The real question would the pump start when connected to a fifteen foot wire of the same guage as in the car. The long wire will limit the in rush starting current to the pump. The starting current is more than twice that of running current. If the normal voltage drop while running is usually .5 volts, at startup the voltage drop is much greater. The fuses are designed to handle inrush current.

[behlinla]

I also believe the 6.8V is a phantom sensing voltage from the PCM. This doesn't necessarily mean the PCM is bad. If you were to connect a test light my guess is you would see the voltage go to zero and the test light would not light. The fact that the 6.8V is staying after the key is off means that you have an open or high resistance circuit. The bad connection is somewhere between the relay terminal and the pump ground. It could also very well be the pump itself if the brushes are worn out and not making contact at certain points in the pump's rotation.

You said you checked the orange wire and it had 12V before and after the key was on, but just to be clear did you confirm the orange wire held 12V DURING the test with the relay plugged in and the pump connected?

Motor current draw test would tell you if the motor was drawing too much current from an internal short or the like.

[Automender12345]

I have read thru the posts and qman says the voltage dropped to zero when he plugged the pump in.

"I squished a piece of wire into the green/yellow wire slot for the relay, plugged the relay into it and connected that to the multimeter. With pump plugged in zero point zero once more. When I turn on the key the voltage jumps up over 12V and then goes down to the 6.8V that I've been rambling on for these few days. It didn't seem to be 2 seconds though. I did the one thousand one, one thousand two and it dropped after one. Did it a bunch of times."

When he turned the key it when to 12 then 6.8. There isn't a cross voltage from an other wire because it it goes to zero when he plugged the pump in.

[behlinla]

Yes, qman could you tell us if the 6.8V after the key is on is with the pump plugged in or unplugged? My understanding was he has the pump plugged in and was reporting 0V before the key was turned and 6.8V after the test period with the key still on.

[soundu]

Original quote from post #1

Originally Posted by qman

98 Taurus with 175 Kmi and original fuel pump.

Voltage at the pump harness is 6.8V. With pump plugged in this will spin the pump about one in 6 tries.

If I put 12V on the pump it spins every time.

Checked with a friend of a friend of a friend, who friend #2 says is a mechanic, and he says 6.5-7V is optimal voltage for driving a pump. Can anyone confirm this for me?

If the voltage is correct I'm gonna replace the pump (probably will anyway). If not I have more work to do.

Ok, back tracking

"Voltage at the pump harness is 6.8V. With pump plugged in this will spin the pump about one in 6 tries.

If I put 12V on the pump it spins every time."

If the pump spins every time with a direct connection and not when going
through the cars wiring, there has to be some issue with the cars wiring
or interconnect components.

Let him put the new pump in. Hopefully, it works.

If not, its wiring tracing time.

On thing that may not have been tried.
check ground from the ground terminal of the pumps wire connector
to a bare metal ground on the unibody
A bad ground can also add resistance to the circuit and cause voltage drop.

When the pump was hot wired to check it, was it with its existing ground,
or a temporary ground and hot?

[Automender12345]

qman has checked all his grounds and cleaned them all up. At least if he gets the car started he can do a voltage drop test over the whole circuit and then if that in in fact a 5.3 volt drop to get to 13.8 with the car running he can back trace the wiring to find the drop. He should also disconnect all the wires from the inertia switch and do a resistance check on it. THe new pump should not be run out of the gas very long maybe just a few seconds. The gas flows thru the motor and cools it.

[Automender12345]

Qman give us an update on what you are finding out with you fuel pump.

[qman]

Thanks everyone. I must apologize that I kinda blew off this subject after my last post stating that I was waiting for the mail order pump to show up. Well, it showed up yesterday. Plugged it in this morning, turned on the key and bingo . . . it spun every time. Put everything back together and the car is once more back on the road.

BTW, I purchased the $18 ($24 with shipping) pump from rockauto. It is a Precise product. Spending big dough on the entire module is a waste of ca$h. It is nothing more than a plastic container, some hoses, and the float. At least that is my opinion at this time.

It was way too mofok'n'g cold to spend time today trying to check out the electrical things. Especially since it fired right up with the new pump.

[Automender12345]

Great news, sometimes you can start chasing all kinds of issues that don't exist. I would still be interesting to see the total running voltage drop on the pump circuit. Hope the pump lasts as long as the car. I don't like doing rework even if it is a guaranteed part. I had to change out a radiator from Advance Auto Parts twice because of leaking transmission cooler connectors. Finally just when to Ford and swapped out the fittings.

Remember we all learn we we find out the final outcome.