wld4x4 Posted February 10, 2004 Report Share Posted February 10, 2004 I have been posting about my fuel pump not running when it is cold until the car builds oil pressure when cranking. After checking the fuse and trying two new relays I still have the problem. Does anyone know why the system relys on the oil pressure switch in the first place? And why can't I run a wire from the switch directly to the pump? I do know that the pump works when it gets a power supply. Link to comment Share on other sites More sharing options...
KHE Posted February 10, 2004 Report Share Posted February 10, 2004 Do you have access to a wiring diagram? That would make troubleshooting the problem a LOT easier. It really sounds like a poor connection in the vicinity ofd the fuel pump relay. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
wld4x4 Posted February 10, 2004 Author Report Share Posted February 10, 2004 No I don't have a diagram. Do you know what color wire in the harness goes to the pump? Link to comment Share on other sites More sharing options...
KHE Posted February 10, 2004 Report Share Posted February 10, 2004 No I don't have a diagram. Do you know what color wire in the harness goes to the pump? I don't have a 1993 Deville service manual. Maybe someone on the board could scan the page and email it to you. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
bobnsue2 Posted February 10, 2004 Report Share Posted February 10, 2004 The oil pressure switch is in the circuit as a safety against fire. If you are in an accident, and the engine stops running, lack of oil pressure will shut off the fuel flow. Therer is a 2-5 second circuit that comes on when you turn the key on to get it started. Sounds like that is failing. Also there should be a short test jumper near the relay box that you can jumper 12v to so you can test the fuel pressure. But wiring to that jumper will keep the pump running when you turn the key off. Bob B Link to comment Share on other sites More sharing options...
wld4x4 Posted February 10, 2004 Author Report Share Posted February 10, 2004 what would happen if you ran a jumper wire to it that was from the switch? (power only when the switch was on) Link to comment Share on other sites More sharing options...
bobnsue2 Posted February 11, 2004 Report Share Posted February 11, 2004 Wiring to the test jumper from the switch should get it running, but without the safety of the oil pressure shutoff in case of accident. Or if you roll it on its roof and the engine falls out, the fuel line brakes and fuel keeps pumping out to feed the flames. BTW I had a hard time finding the jumper on my 91 STS, I don't know where yours might be. Mine sticks out in front of the relay box and looks like a wire that would plug in to something with a plastic plug on it. Bob B Link to comment Share on other sites More sharing options...
Jim in Phoenix Posted February 11, 2004 Report Share Posted February 11, 2004 The oil pressure switch is in the circuit as a safety against fire.If you are in an accident, and the engine stops running, lack of oil pressure will shut off the fuel flow. Wrong! the oil pressure switch is in the circuit to act as secondary backup source for power to start the fuel pump. When cranking, if oil pressure builds and the PCM has not turned the fuel pump on, oil pressure will. It actually bypasses the fuel pump relay. When cold, this delay is obvious, when warm or recently run, the oil pressure might build quick enough that you do not notice the PCM and /or the fuel pump relay has not functioned. The primary source would be the PCM telling the fuel pump relay to close for two seconds (key on) to prime the fuel circuit, then when it senses the engine is turning, close the fuel pump relay as long as the engine is running with the key on. You could wire the fuel pump to come on with the key switch but if you have a leaking injector, and the key is left on too long without starting the engine (passenger listening to radio?) you run the risk of hydraulicing a cylinder and bending a rod or worse. Is it worth the risk? The fuel pump relay is in the micro relay center in the engine compartment. Look for a pink medium sized wire (key on hot) switched to a grey wire. The coil wire would be a smallish dk green/wht from the PCM and the coil goes direclty to ground through the black wire. Are you sure you got the right relay? Jim in Phoenix Link to comment Share on other sites More sharing options...
wld4x4 Posted February 12, 2004 Author Report Share Posted February 12, 2004 DOES THE RELAY FEED THE PUMP FUSE, OR DOES THE PUMP FUSE FEED THE RELAY? Link to comment Share on other sites More sharing options...
rjhat Posted February 12, 2004 Report Share Posted February 12, 2004 Seems to me that the circuit powering the fuel pump through the oil sending unit would be there to cut fuel off if the oil pressure drops. This keeps you from running your engine with no oil. Many late model cars use this type of curcuit to protect the engine. Link to comment Share on other sites More sharing options...
Jim in Phoenix Posted February 13, 2004 Report Share Posted February 13, 2004 DOES THE RELAY FEED THE PUMP FUSE, OR DOES THE PUMP FUSE FEED THE RELAY? There is an always hot 20 amp fuse marked 1a that feeds the fuel pump through both paths, the fuel pump relay, and the oil pressure aux switch. The wire to the relay is pink and the wires from there and the oil pressure switch going to the fuel pump is/are gray. A single wire pigtail near the firewall can also be used to power up the fuel pump by running a jumper from a positive battery source. My 94 Bravada has the same circuit features as shown for the 4.9. Interestingly, if all is working as designed, the fuel pump continues to run after the key is turned off, until the oil pressure decays off. This kind of primes the fuel circuit for the next start because the injectors are turned off by then. Finally, Ford products have a gravity switch, designed to cut the fuel pump if the car crashes or gets inverted. I don't know that GM has any such automatic circuit interuption, but they do have a low oil level notification and of course low oil pressure light/notification. So as soon as you get upside down, turn your key off!! Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.