On my 1991 F-150 XLT Lariat that I had recently purchased for only $700 a buddy and I were working on it because you know if you buy a truck for only $700 something has to be wrong. Well the engine (Not original, '93 5.0 302) wasn't recieving any fuel pressure at all so we assumed that it was the fuel pump. My buddy ran a mulitmeter on the wires running to the pump and it was only recieving about 6.2 volts. Some research later we found some guys that had the same problem and they said that it was just the wiring. We tested it out buy running 2 wires straight from the battery to the pump and it worked like a charm. Does anyone know what would need to be replaced? the whole wire? just a relay? the connectors? anything will help. thank you.

Little list of the other things wrong with the beast
- previous owner took out the heater core so there are just some open hoses in my engine bay
- power steering pump is bad, gonna need a new one
- 2 of the 4 tires are showing wire
- transmission is leaking like crazy. the truck sat for about 5 months and the tranny is now drier that the Mojave, hoping to god I dont need a new one and if so the previous owner said he'd give me one to make it 4WD but I personally believe that it would make the whole thing a much bigger pain in the ***
- i dont even think i have to say it but the battery is shot
- my exhaust pipe ends around the end of the cab (no muffler or cat) but my god she sounds so nice

 

On my 89 Ranger when I swapped in a V8 I found that the fuel pump power runs through the ECM. So I had to run a new hot wire to the pump from the ignition through the impact switch then the fuel pump to power it (carbed it so bypassed the ECM)

 

The EEC activates the fuel pump through a relay, usually installed under the driver's seat. Hopefully the people who installed that engine kept it, as the EEC can not drive the fuel pump directly. If you have power, you can key-on without starting, and you should hear the relay click on to run the fuel pump for 2 seconds, then click off.

 

When we turned the key without the wires running directly to the pump itself, we could hear a click but not the pump run but the pump works. Since the pump was only getting around 6.2 volts do you think it is a bad relay?

 

Possibly a bad relay or bad wiring from the relay to the pump. You will have to trace the wire from the relay to the inertia switch, and from there to the pump. The color of that wire varies by year and model, so you need to find the right one. Two wires seem to be constant: Yellow from an always hot source going to the relay contact opposite of the pump wire, Red from switched source going to one side of the relay coil. If you try to apply power to the coil to energize the relay, make sure you apply + to the spade that the red wire attaches to. The coil usually has a reverse biased diode connected across it to suppress back-EMF from the coil, and if you apply voltage in the wrong direction, the diode will conduct and blow up. Then you'd need a new relay anyway.