I have an 86 F350 dual tanks,replaced rear tank everything new, gauge works for a short time when full of gas but does not read correctly,when tank is down to about 2/3 or 1/2 gauge reads empty,same thing for front tank.Gauge does not drop completely like when the ignition is turned off just to the empty mark.

 

Boy I have to go back for this one. If it involved only one tank that's one thing , but since it is involving both tanks I would suspect something from the fuel tank switch to the fuel gauge. The older vehicles had a voltage regulator mounted behind the dash that provided a reduced voltage for most of the dash gauges. Others had it incorporated into the temp. gauge. Do all the other gauges read correctly? If they are OK a thorough check of the wiring from the fuel tank switch to the gauge is in order.

 

All the other gauges read correctly,but when the fuel gauge wire is grounded the fuel gauge goes to full which is normal,but unable to find fire in that wire even low voltage when testing,this also puzzles me because I don't know where the fire starts at and I didn't know about the voltage regulator which may be the problem if I knew where to find it. Thanks

 

You might start at the switch that goes from one tank to the other. One wire should be the voltage feed and the other should be the return from each tank gauge sending unit to the dash gauge. The colors are usually consistent.

 

Thanks hanky,I'll try that.