wescat

1997 PA Ultra, 277000 miles with intermittant fuel supply. Has fresh fuel filter. I have factory manuals, so I've done a bit of troubleshooting. The #8 pin off of the "blue" CM connector sends + battery signal to the fuel pump relay when the key is energized. When I unseat the relay with the key on, this pin has 9.78V at the relay. When I then seat the relay, ~0.0V. I have tested the wire lead from the PCM connector to the relay bus pin socket, it is good, will not open under a load. So, I provided a 12V supply to the FP relay - relay our of the buss, using jumper wires, so as not to backfeed the PCM. The relay kicks in, sends 12V to the FP control module in the trunk, but still the pump does not run. I checked for 12V at the FP connector, it is powered. The book is a bit vague, so here are a few Q's: 1) there are 2 power outs from the FP control module, hi and low, but only one lead on the FP: are these 2 wires joined in the harness, and isolated for backfeed via a diode? 2) It appears that the FP grounds through the FP control module, is that correct? Through its case, or a wire? Which wire? 3) What sensor inputs tell the PCM to energize the fuel pump? I assume that could be the cause of all my trouble. Thanks for any help.

HotZ28

The FP relay ground is activated by the PCM when the ignition switch is turned to either run, or start. Power (12v) to the relay comes from fuse #6 with ignition turned to run, or start. The grey wire feeds the FP control module and the module may have a black ground wire, or it may just ground to the chassis. My schematic shows two separate wires going to the FP motor, grey & pink. The FP motor is grounded to the chassis. When the switch is turned to run, the PCM provides a 2-sec prime and then drops the ground to the relay. If start the engine, the PCM provides a continuous ground to the relay. No one particular sensor is used, the PCM automatically knows when the engine starts!

__________________

wescat

Thanks for the reply. Let me clarify:

We are talking about the function of two different devices: the fuel pump relay and the fuel pump control module.

The PCM does not supply ground or control "ground" to the fuel pump relay, it supplies 12V + to energize the relay. Ground is continuously supplied to the relay at its #2 post via the BLK/WHT wire. So says my manual, so says my multimeter when troubleshooting the circuits. As I stated, the 12V + is not being properly supplied by the PCM. That is why I asked what inputs does the PCM sense to supply this output to the fuel pump relay. The PCM is a computer, it "knows" nothing. It supplies finite outputs given finite inputs, within specified parameters. Either the PCM is bad, or it is not recieving the correct input from its sensors. Exactly which sensors does the PCM need input from to supply proper 12V + output feed to the fuel pump relay? If I can discern that, I can test them for proper function.

The fuel pump relay does not output to the fuel pump control module via a grey wire, it outputs to the fuel pump control module via a red wire.

Could you give me a detailed explanation of how the fuel pump control module functions, including all modes (this is a 2 speed pump)? That would help a lot.