On my 78 f250, it originally had dual tanks but it only has rear tank under it now. I am trying to get the fuel gauge to work, which has me puzzled. Someone replaced the mechanical fuel pump with and old electric pump. Also there are three wires back by my sending unit, one hanging down loose, other two are electrial taped to the sending unit. Then there is the two wires that run the pump. I have tried two different guages and fuses look ok. Any help is appriciated.

 

On my '76 the sending units for both tanks require 2 wires. One is clearly attached to ground, and the other picks up the level and resistance off of the indicator in the tank. The gauge level reading reflects the variable resistance of the sending unit. Make sure you have a good ground. Make sure you have the tank selector in the "Main" position. Also, if you take the plug off the sending unit and put a jumper wire on it and turn the key to "on", it should peg the gauge. This will test the gauge and wiring. Do not leave it like this for long, just do the jumper and then switch the key on to see the gauge go up and then tirn it off. I had one tank that registered on the gauge and one that didn't. I did the same tests and found a bad sending unit. I rolled the dice and bought a new float and when I pulled the failing sending unit out, the float was full of gas. Thankfully mine was on the aux tank and accessible with the tank in place. It will be a challenge to do the testing on the rear tank bit it's doable. It beats pulling the tank for no reason. Now I have both tanks working. The rear tank is the "main" and mid ship tank is the "aux". The fuel pump has nothing to do with the sending unit. Atleast it shouldn't.

 

May I suggest reading this thread I have been responding to in the 80-86 forum.

It relates directly to you problem as they are the same system.

https://www.ford-trucks.com/forums/s...d.php?t=514082

 

I checked the ground and its a good ground. the other wire that goes to the sending unit does not have power to it with key on or off. And the gas guarge never moved with the selector on either tank. Also the fuel switch has been bypassed with the electric pump. It don't matter what tank the selector is on, it only pumps from rear tank. Is there a diagram or a way I could run a new wire from gauge to sending unit for the gauge to work?

 

The 2 circuits are integral to each other.

The person who put in a electric has caused the reading problem apparently by using that circuit to power the pump.

They cannot be used in unison so you would have to re-dedicate the circuit to the normal fuel delivery/monitoring system.

Sounds like you need to redo some wiring.