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
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
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
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.