Truck Repair: chevy fuel pump, oil pressure switch, oil pressure sender

Question
Van   I have a 1988 k2500 chevy truck, 350 v8. I bought a fuel pump and guage unit, (old guage didn't work and pump was at least 10 yrs old so I figured I put in the whole thing. Much time passed before I got around to putting it in. Now I see the truck has 3 wire plug and new unit has 2 wires. Is there somewhere or someway to determine which wire does what. I am stuck. there is purple and grey and black from the truck. I think purple and grey from unit. I can't seem to get any voltage reading from truck wires. Help.!   Thanks                  Jim

Answer
Hi Jim,
OK...the plug on the truck...the black is ground, and should show GOOD continuity to ground. Make sure it is connescted clean and tight to the frame somewhere, probably near the tank, or about under the drivers seat area. The pump, as well as the sender will depend on it being good.

The Gray is power to the pump. it will be battery voltage, a full 12 volts, or whatever you show at the battery. BUT..it will only be hot at certain times. They are:

1. When the ignition switch is turned to run, (not cranking or anything, but just turn to run), the wire should have power for two to three seconds, and then go back dead.

2. When the key is turned to start, and the engine is either being cranked over, or is running, that wire will be powered by the fuel pump relay for that entire time...battery voltage.

3. As long as the engine is running, and oil pressure is above 4 PSI, that gray wire will recieve power,(battery voltage) through the oil pressure switch,(not to be confused with the oil pressure sender), weather the fuel pump relay is working or not.

That takes care of the pump circuit.
The other wire on the truck, purple, is from the fuel gauge in the instrument cluster, and the voltage on that wire will be small, and harder to read, and might even blink. If you ground it while the ignition switch is on, the gauge should go to one end, and with it disconnected from ground, the gauge should go to the opposite end. I don't remember which way shows empty or full, but if it doesn't change the gauge, the fuse or gauge is the problem.

As to why your tank unit only has two wires, I am confused. The truck harness should plug directly to the unit. But it will need all three wires.
I'd be real hesitant to just start connecting battery voltage to the wires. I wouldn't want to send full voltage to the gauge sender.

On that tank unit, was there a separate black ground wire?

Van