Fox Mustang fuel pump circuit issue

[mustangmike6996]

Trying to help another member on here.

87 hatch GT with a mass air swap (harness and PCM)

After the swap the fuel pump would not cycle with the key. PCM was swapped to verify that the new one was not the issue (PCM didn't fix it)

I found the high current side of the fuel pump relay had 7 volts on the positive side. When you jump 12 V to the relay it will fire the pump. So we supplied keyed 12v to the switched side and the car ran and started. After the car was shutoff we noticed the pump was still running and the brake lights were stuck (pedal was stuck slightly depressed) When we moved the pedal off of the brake switch the lights turned off and so did the fuel pump. After finding this, you can depress the pedal and the pump will also run (seems like a short to ground, possibly a pinched wire during the install)

Prior to pulling the dash again and removing the harness, does anyone have any input or ideas? I was not there for the initial install and have been randomly troubleshooting it here and there but it needs to get fixed soon.

[mustangmike6996]

Reviving an old thread....

I ended up diagnosing the fuel pump issue to an incorrect engine harness. For the Foxbody people here, the MAF conversion with the A9P/A9L EEC-IV- this is a common mod and it is supposed to be somewhat univeral. There are some exceptions to the rule (certain year cars have the fuel pump relay under the seat or in the engine bay. 1991 cars have a unique firewall harness that is incompatible.) Everything else is pretty much plug and play.

After many hours of tracing wires, checking voltage drops and everything in between, I found the ECM fuel pump ground control circuit was being routed to the ESC (electronic speed control/cruise control). At that point I was able to locate a factory 1987 harness. I added a MAF extension harness, repinned some ECM connectors and test-fit the harness.... Turned the key on and the fuel pump did the couple second prime, the car was able to start as it should. That was a big win, so there is some closure to this thread.