I'm having some trouble working out a bug in the Fuel Pump circuit - I recently discovered that the oil pressure switch does not cut the fuel pump as it is supposed to at low pressure (or when disconnected).

I can start the car and, while the engine is running, disconnect the oil pressure switch and the pump remains running.

If I disconnect this wire while the engine is running, the oil pressure switch will keep the pump running (this is correct), and if I disconnect the oil pressure switch at this point, then the pump stops (again, this is correct).

I don't want to get into the details of my troubleshooting, but I'll just say that I measured the voltage at pin D7, and it is high when the car is running, which I believe makes the oil pressure switch pointless. My understanding is that this output is supposed to de-energize once the engine is running, leaving the fuel pump circuit 'latched in', and will only stop the pump upon loss of oil pressure (or when the switch has been disconnected).

Can anyone help my understanding of why this 'Fuel Pump Relay Drive' output (pin D7, '727 ecm) is remaining energized when the engine is running?

Any help on this is greatly appreciated!

 

Common mis-understanding. The oil pressure switch acts as a backup for the fuel pump relay. The ECM energizes the fuel pump relay as long as it sees the engine running (via pulses from the distributor). The oil pressure switch is in parallel with the fuel pump relay and provides voltage once oil pressure exceeds about 3 psi. This is not a low oil pressure fail safe to turn off the fuel pump but is a fail safe against a bad fuel pump relay. HTH!


P.S. A20 is the ECM monitor for voltage at the fuel pump. This is used to alter injector pulse width according to set values in the program which helps to compensate for lower fuel pump output due to the lower voltage.

 

Thanks for the reply. That definitely changes my understanding of how it is supposed to work.

So there is no fail safe to protect the engine on loss of oil pressure... I guess if GM designed it that way, then I can accept that.