I have an 88 f150 il6 5 speed 4x4 dual tanks. My gas gauge the other day just skyrocketed to full. I replaced the instrument panel with one out of a 90 ford. The gas gauge was working in that one and as soon as everything got hooked up it sky rocketed again. For a while I could push on it and it would read properly. I replaced one of the pigtails that plugs into the instrument panel the one on the left side if your sitting in the truck looking at the panel. I haven't gotten around to replacing the other one but I do have it. Also my battery charge gauge has never worked yet the battery charges and holds charge. Any help on this would be greatly appreciated!

 

check the wiring to the tank selector. If it's not hooked up, or hooked up correctly the gauge will sky rocket to full

 

Sending unit has probably shorted out inside the gas tank, as for the volt meter I would test the wire from the alternator and work your way to the gauge, on a 88 should be a light green and red wire and check for current.

 

Nobody has a working gas gauge unless they changed the fuel level sending units. The alternator plug may be bad.

 

I replaced the gas tank and sending unit a few months ago on the front tank and everything for fine for quiet some time. Don't know if that helps at all. and the plug that actually plugs into the alternator? and how would I go about testing it?

 

I also had to clean up my tank selector switch to make the gauge more accurate.