I have a 2000 Mercury Grand Marquis with 170,000 miles, 4.6L V8. Yesterday the car cranked, but didn't start. I noticed that I didn't hear the humming of the fuel pump when I turn the key to ON. I pulled the following codes: P0230 (fuel pump primary circuit) and P0231(fuel pump secondary circuit low).
The battery has enough charge, I checked the relay for fuel pump (swapped with a known good one) and checked the fuses for PCM and fuel pump. The inertia switch is not tripped either. The pump just doesn't want to turn on.
I checked the voltage at the relay and it reads 12V when the key is turned. Then I pulled out the cable to the inertia switch and I'm only reading 6.6V. From what I understand, that is common among Fords to get low voltage at the inertia switch, and it's apparently the PCM checking the health status of the pump. But at the same time, should I not be reading 12V there when cranking?
Because of the low voltage reading, I am kind of doubtful that it's the fuel pump itself. Also, I had no prior signs of a fuel pump failure.
If anybody has encountered this problem, please let me know how you solved it. Thanks
It's a long run of wire from the battery to the fuel pump. Plenty of opportunity for wiring/connector faults, corrosion, etc.
Probe for voltage starting at the front and work your way to the back.
Took your advice and started probing for voltage changes across the system. I can rule out any issues with the PCM, fuel pump relay, and fuses.
When I took the voltage of the 87 pin on the fuel pump relay (output to the inertia switch) the voltage jumped from zero to 11.8V for less than a second and back down again. Same with pin 30 (input side from the battery). The voltage spike lasted for a shorter period than what is normal for the fuel pump to prime.
So to sum up:
1) harness chafing between the relay and fuel pump
2) ground fault between relay and the fuel pump
3) fault with the pump itself
I hope I'm on the right track. Any thoughts?
It took a whole day but I figured out the problem. After checking for voltage output and resistance in various locations, I decided to connect a spare 12V battery directly to the inertia switch output, connected the ground, and the pump started to hum right away. I then knew that it was neither the pump or the ground that was the problem.
I then did the same to the output pole (#87) of the fuel pump relay and again, the pump works.
It turns out that the culprit was the cable between the fuse for the fuel pump and the relay. I'll just stretch a new wire directly from the fuse to the relay instead of going through the spaghetti of wire harnesses in the engine bay. Thanks again for the advice, it turned out to be a $3 fix instead of going through a hassle of buying and installing a new pump only for the problem to persist.