Engine 1990 LX 5.0L EFI Mustang Fuel Management Problems - HELP!

Ok looking at the two diagrams above, it appears that the FP Relay is wired and receives power from the Fuel Inertia Switch (Red/Blk) AND from Wire 22 (Tan/Grn) on the EEC. From the FP Relay, the fuel pump receives power.

Here are the voltage readings I'm seeing at the connected Fuel Pump Relay:

• Wire #37 (VPWR) - Switched Power = 0v w/Ignition OFF and 12.5v w/Ignition ON
• Wire #1 (KAPWR) - Constant Power = 12.8v w/Ignition OFF and 12.35 w/Ignition ON
• Wire #22 (FP) = 0v w/Ignition OFF and 12.5v w/Ignition ON
• Wire #19 (FPM = 0v w/Ignition OFF and 12.5v w/Ignition ON for a second or two, then the relay clicks and voltage measured .22v

At the wiring harness plug at the rear bumper there are four wires that feed both the fuel pump and the fuel sending unit (new). Testing this unplugged fuel tank wiring harness, I see 8v in one of the wires and .22v on another wire, and 0v in the other two. I assume that the 8v is for the fuel gauge sending unit and the other two wires w/0v are grounds. The one wire w/.22v is the same measure I was getting at the relay after the relay "Clicked". My pump is not priming or building any fuel pressure - Seems like the pump isn't getting power! What voltage should I see at the pump harness?

BTW - This is a great post and content for anyone that is having fuel pump / pressure issues:


I followed this procedure and found that my new pump is being supplied 12.5v and is not running. I need to drop the tank again and troubleshoot the new pump I just installed. Yes - I should have bench tested my brand new pump to make sure it was working - It likely would have saved me this re-work. What can I say, I assumed the new pump would work - I'm not a professional...

AeroCoupe said:

Wire #1 - Constant 12V+ from battery to EEC relay and fuel pump relay.
Wire #19 - When key is on and ECU commands the pump to run this wire provides 12V+ to the fuel pump and to the ECU. Its provides 12V+ to the ECU so the ECU knows the pump has 12V+ and should be running.
Wire #22 - This wire is a ground from the ECU to the fuel pump relay. This how the ECU commands the fuel pump to run. The same wire also goes to the self-test connector in the engine bay on the driver side near the brake master cylinder. This is so that when you plug in the code reader it can test the relay. You can also put a ground to this wire and force the pump to run with the key in the "run" position.
Wire #37/57 - This wire is 12V+ from the ECU to the fuel pump inertia switch and then to the fuel pump relay coil. When the ECU provides a ground (Wire #22) to the other side of the coil then Wire #19 from the fuel pump relay provides 12V+ to the fuel pump and a run signal to the ECU.

With all of that you should not be seeing 12V+ on Wire #22 as this is the wire that grounds the coil in the fuel pump relay which comes from the ECU.

Click to expand...

Hey AeroCoupe - Really appreciate your post and the specific detail you've provided - Very helpful.

I actually ran the ground test on the "self-test" connector and can see that when I make this connection I hear the fuel pump relay fire (click) and have 12.5v at the fuel tank harness at the rear of the tank. When I plug in the fuel pump / sending unit connector to the tank harness I also hear clicking in the tank when I ground the "self-test" connector but the pump does not run. My next step is to drop the tank again and pull the fuel hanger again to test the new pump.

All the test I ran yesterday were with the fuel pump relay plugged-in. When I tested Wire #19 (FPM = 0v w/Ignition OFF and 12.5v w/Ignition ON for a second or two, then the relay clicks and voltage measured .22v. Does this sound like it is functioning correctly? I understand that normally the fuel pump is supposed to fire briefly to pressurize the system and then turn off. The amount of time the pump fires / runs to charge the fuel system is typically about a second, maybe two - I read this is a set amount of time by the computer. Once the engine starts to turn over the computer senses this, the computer completes the ground for the pump to continuously to run. Please let me know if you have any insights or thoughts - Ciao!