Fuel pump operation behavior

[Diesel-Runner]

Just picked up a 96 LX450 and I noticed that when the key is turning to the run position the fuel pump is on even though the engine hasn't been started and will continue as long as the key remains in the run position. From my operational understanding from the FSM the pump should only activate once the ECM detects a crankshaft signal. Truck runs fine with no CELs or other issues. Just curious if anyone has seen this? My next step is to trace the wiring to see if something is bypassed. Appreciate your time.

 

[ppc]

Check the diagnostic connector on the firewall to make sure that FP pin has not been jumpered to the B+ pin or any other 12V source.

 

[Diesel-Runner]

Check the diagnostic connector on the firewall to make sure that FP pin has not been jumpered to the B+ pin or any other 12V source.

Just checked that, no jumpers.

 

[ppc]

The Circuit Opening Relay is what normally provides the power source to the fuel pump relay. That may have a problem or has been jumpered somehow. I would also monitor the voltage on the FP pin on the diagnostic connector. Normal operation would be 12V on initial startup, drop down to ~8/9 volts then switch back to 12V on higher loads on the engine.

 

[Diesel-Runner]

I definitely have a bypass going on, fuel pump still runs with the efi relay and fuel pump relay disconnected. I hate hack jobs. Going to start at the fuel pump and work forward and see whats up. Thanks for your help Phil.

 

[Diesel-Runner]

Ended up finding the root cause after a whole lot of ringing out wiring. At one of the connectors at the driver kick panel one connector had 2 wires swapped. I have no clue how or why this was but everything is now working and wired IAW the FSM.

 

[vsatking]

Good god man! What a rabbit hole! I am chasing a no power to fuel pump issue now. I am a little out of my depth but I'll keep at it. Thanks for posting your results and fix.