Questions and Answers

F-150 Fuel Gauge Problem Q. Mr. Ciulla, I've been trying to help my father out with what appears to be an electrical fault in the fuel gauge circuitry of his 1988 Ford F-150 with a 4.9 liter straight six. Since he has owned the truck, the gas gauge has always acted funky.

Now by funky, I mean that it would read "full" (pinned on full) for about 100 or so miles after filling the tank, then slowly drop like a stone until it was reading "empty" at around 200 or so miles. Dad was able to get around that by just using the odometer and filling the tank up around 250 or so miles (figuring he probably had another 50 or so miles left of gas in the tank).

A month or so ago the single fuel tank developed a leak (then tank is located above the spare tire under the truck bed), so we went out and bought a replacement tank, new fuel sender assembly, new fuel pump, and new fuel tank straps, figuring it was smart to replace the whole mess while we were under there. I was hoping that this would cure the fuel gauge problem as well, but alas, no such luck.

Now the fuel gauge is buried above "full" (basically pinned on the high end).. and doesn't move no matter the fuel level is. This indicates to me that something in the sensor circuitry is most likely either shorted to ground, or to +12V.. but I'm not sure which... or if this is something indicative of a faulty dash gauge/ control component?

I do know that there is four wires in a harness running to the fuel sender assembly in the tank: red, black, yellow, and brown. The red and the black are for the "in-tank" fuel pump, and I presume the yellow and brown are for the sending unit / fuel gauge circuit. Can you tell me which is the "hot" and which is the "ground" for that circuit, and if there are any tests I can do to nail down the problem before I resort to dropping the tank again to check the fuel sender unit (sans tank) for proper operation? Is there something on the fuel gauge side (such as a voltage regulator) that might be faulty? If so, where is it, and how would I check it?

Barring any "miracle solution" you might come up with, my intentions are to drop the fuel tank again, pull the fuel sender out of the gas tank, make sure the float still moves freely (it did when we put it in), and then just plug the sender / pump assembly straight into the wiring harness (sans tank) AFTER disconnecting the fuel pump (so we don't pump air into the fuel line). Once the sender assembly is plugged in, gently move the fuel sender float arm up and down and see what happens with the fuel gauge.

Any suggestions, solutions, or insight you might have on this would be greatly appreciated...

Sincerely,
Steven

A. If the dark blue/yellow, or yellow/light blue could be either, at the pump/sender assembly is grounded, it will peg the fuel gauge. Power goes through the gauge to and through the sending unit and then to ground. If the wire is broken, the gauge will read EMPTY and if it is shorted, it will read FULL.

If you unplug the pump/sender and the gauge still reads FULL, you have a shorted wire or a bad dash board fuel gauge.