I have a 1991 Formula Firebird with the 5.0 liter and the LB9. The car will crank but will not start. I have installed a new fuel pump and sending unit but they both went bad. I have checked the wiring for the fuel pump. With my voltmeter I am only getting 6 volts of power at the harness on the rear floor boards near the fuel tank. My question is what voltage should I have at that harness? I have checked all my relays and inspected almost all the wires but I cannot get the car to start. The car will start if I put starting fluid in the throttle body. Also, I have a new distributor, plug wire, ignition coil and battery.

 

How many volts do you have coming out of the relay? The pump should be getting 12v. If you put 12v to pin G on the ALDL, you should be getting 12v coming out of the relay to the pump.

 

I am getting 6 volts out of the harness at the floor board near the tank. I checked the orange wire to the fuel pump relay and got 12 volts. I checked the relays and they are all working correctly. I had a mobile mechanic drop by and he checked the circuits for continuity and ground and they are good. The new sending unit worked for a while but it now does not show the correct amount of gas on the fuel gauge.

 

There is a connector in the rear passenger compartment (C313, tan/wht wire), and another at the firewall (C100). Something is creating resistance. C100, C207, C280, C315 can also be involved.

 

I ripped up the rug and followed the wires to the fuse block. I did not see any connectors in between the one near the gas tank and at the fuse block. Do I need to remove the dash to see some of the connectors?

 

What is the voltage on the tan/white wire at the fuel pump relay?

What happens when the fuel pump circuit is jumpered at the ALDL connector? Apply a 12V supply to the "G" terminal and then measure the voltage at the pump.

 

I tried to use the red “jump wire” at the relay. The jump wire just got hot and the pump would not run. Where are the other connectors you were speaking about? Thanks for your prior reply’s and help. This whole thing is just so frustrating. I hate electrical gremlins!

 

C100 is the firewall bulkhead connector, just left of the master cylinder opening. Terminal G4 in that array is for the fuel pump power. The other connectors C207 is for the ECM, and we are beyond that now. C280 and C315 bo not apply to your '91.

If the fuel pump is failing, it may be presenting enough load to cause a voltage drop at the C313 (rear) connector. The overheating jumper wire at the fuel pump relay might be an indication of that.

 

Thanks for the information. I will search for that connector and inspect.

 

Okay I am back on the job. I found a connector @ the firewall to the right of the master cylinder as you are looking at it from the engine compartment. So far I have removed a cover on the right side harness. Where the wires come into the plastic there is a lot of black almost like tar on the wires. I will keep trying to remove the plastic connector but it is very hard going now. Hopefully I am at the correct place? If this is the issue do they make this part in the aftermarket? Or do I need to make a trip to the junkyard?

 

That tar was packed into the exterior side of the connector to prevent moisure and contaminants from entering. It is important to repack that after repairs are completed. A good alternate to tar is conduit seal compound.

https://www.lowes.com/pd/IDEAL-1-lb-...t-Seal/3127723

It is best to warm up the compound before application so that it can flow into the spaces between wires and sockets to do its job.

The connector may or may not be available, but the pins and sleeves are still avaialble to replace any which are damaged. I believe a 1991 would have used the Series 56 pins and sleeves (possibly 59), but hopefully someone else can verify that.

 

I am going to trace back the wire to relay and test it for 12 volts just before the bulkhead connector. I believe there is a color change of the wire before the bulkhead. I believe it goes from orange to purple at the bulkhead. Is there a way to visually to know there is a problem with the bulkhead slime? Or is it all via the multi meter?

 

The meter is going to be a lot faster, and provide a lot more relevant data.

The previous report indicated that there was a constant 12V at the orange wire on the FP relay. We really need to know what is on the tan/white wire after the relay, but of course the FP relay needs to be energized to test that. .

 

Got it. I am going to peel back the rest of the shrink wrap and loom on the wires leaving the fuel pump relay and make sure I am getting the correct coloured wires. My shop manual says the gray wire should be energised @ the fuel tank but the only wire I get voltage on is the purple wire. I will update you ASAP. Thanks for everything.

 

I pulled the shrink wrap off the harness @ the fuel pump relay. I wanted to follow the wires from the relay to the bulkhead to the right of the master cylinder. The only problem is that it appears the wires from the relay go into a harness that goes behind the engine above the bell housing. I need to find the hot wire to the fuel pump to test it @ that bulkhead to prove the voltage drop is @ the bulkhead. I am at a loss of what to do next.