I am having a serious fuel problem. I have replaced my fuel pump twice now and it still has not fixed the problem. I have replaced the fuel pump relay and fuse.

The problem is, my fuel pump will not engage when my car is cold, or it is cold outside. When it is hot outside or warm, my fuel pump engages and it starts right up. I was wondering if my oil pressure switch could be causing this. Any help would be appreciated. It is on a 91 trans am 305 TPI

 

As long as the fuel pump relay is good, your fuel pump will run even if the oil pressure switch is unplugged. The oil pressure switch closes to send power to the fuel pump when there is oil pressure…if it does not close, the ECM will use the fuel pump relay to run the pump anyway. Your problem is not with the oil pressure switch.

Diagnose it like this.
Either run 12v to the red wire (cavity C) in the fuel pump relay, or pull the fuel pump relay and jump the terminals in cavities A and C. If the fuel pump runs, your wiring is fine and something is messed up in the ECM. If it does not run, you have a blown fuse or broken wire.

 

If the oil pressure sending unit goes bad it will blow a 20 amp fuse near the battery that goes to the fuel pump(when it does this you will get no ses light with the key forward), when it goes bad you can disconnect the sending unit and the oil pressure gauge will peg out and the car will run if you have replaced the blown fuse, if you jump the terminal for the fuel pump the pump will run even if the 20 amp fuse is blown but the car wont run.

 

If you’re referring to the 20 amp fuse that supplies power to the ECM, Fuel Pump Relay, and Oil Pressure Switch…if that fuse is blown and you jump the terminals (cavities C and D) on the oil pressure switch/sending unit, the fuel pump will definitely not run.

The 89-92 oil pressure switch/sending unit is wired as follows:
Cavity A: 18ga Tan wire goes to oil pressure gauge
Cavity C: 16ga Gray wire goes to the fuel pump, fuel pump relay, and ECM
Cavity D: 16ga Orange wire goes to fuel pump relay, ECM, and 20 amp fuse that’s hot at all times

If you jump the Gray and Orange wires, the fuel pump will only run if the 20 amp fuse is not blown. Current flows through these two wires when there is oil pressure in the engine and the switch closes. It’s difficult to jump those wires because of the location of the oil pressure switch. You can alternatively jump those wires by unplugging the fuel pump relay and jumping the Gray and Orange wires there. Same circuit.

 

I meant the aldl terminal under the dash, maybe my.002 has no significance here just thought id post incase any thing i have learned through expierience can help.

 

Hey man, I'm not trying to give you a hard time...I'm just trying to make everything clear. I build new TPI wiring harnesses and often build them specifically for the 3rd gen f-bodies.

I'm a little confused where the ALDL comes in with the fuel pump? Can you explain?

The ALDL has the following (off the top of my head):
Cavity
A: BLK/WHT (To Ground)
B: WHT/BLK (To ECM)
F: TAN/BLK (TCC Control and ECM)
M: ORG (Data Stream from ECM)

There’s another in there for the airbag and possibly another for emissions. I don’t often build harnesses for emissions and never have to include the wire for the airbag.

 

The g terminal on the aldl port will energize the fuel pump to test when jumped with positive current. And i didnt think you where trying to give me a hard time, its all good. I was just posting some rambling that probably wont help,so i just thought i would clarify that i understood thats all it was.

 

That wire (cavity G) is cavity C in the fuel pump relay connector. In some vehicles that wire is about 6 inches long and has a connector you can connect to instead of running all the way to the ALDL connector.