Hi all,

My instrument cluster fuel gauge works fine for the rear tank, but not when the front tank is selected. It just shows "empty" on that tank, even when it is full and I'm using that tank for gas. To be clear, fuel delivery from the tank works, it just registers empty when it is not. This is on a '86 F150 2WD, carbureted I6 engine.

My Ford manual says that if you have this problem, the first thing to check is the ground wire for the fuel sender on the tank that isn't registering proper fuel volume. I can see in the diagram here that there is supposed to be a ground wire associated with each sender. Near the bottom, the diagram indicates that each fuel sender has a ground.


I looked around for this wire under the truck but could not find it. Does anyone know where this ground wire is specifically located? The diagram suggests it exists but gives no details on where physically on the truck it is grounded. But I'll bet some of you guys know the answer.

 

It's bolted to the sending unit, and the goes into the harness. After that I am not sure. I bet if you add a ground wire to the sending unit, it still doesn't work. These trucks are famous for the resistance unit in the sending unit wearing out. You could also have a float problem and it has sunk.

 

I have never seen a separate ground wire, everything I've worked on grounds through the case to the body (in this case, gas tank).

Just like there is no separate ground wire for the alternator, it grounds through the case to the engine. At least on any I've seen.

 

OK, thanks guys. I guess I just got confused by the diagram above and these Ford "one gauge doesn't work" directions here which give as step #1 "check sender ground connection".


So based on your advice, I gather the ground wire must be part of the wiring connector that plugs into the fuel sender. I wonder how Ford intended that to be checked? They left out the previous step needed: Remove the gas tank.

Based on what Franklin2 said - am I right to assume my problem (fuel guage doesn't work for one tank only) will probably require fixing/replacing the fuel sender that lives inside the tank?

 

No, you're still confused... the *body* of the sending unit *is* the ground connection. The sending unit connects to the gas tank and is held in place by the lock ring; the tank is bolted to the body, the body is electrically connected to the engine, the engine connects to the battery negative cable.

The sending unit is just like the starter relays on the passenger-side fenders - they ground through the case to the metal fender material they're screwed to, there is no separate ground wire.

Same for the radios, they don't have separate ground wires, either - they ground through the case to the truck body (dash).

The factory wiring connector contains only the wire that leads to the gauge; the sending unit provides for a variable-resistance ground connection (at least on the earlier models it's like this).

(Later-model sending units include power wiring for in-tank pumps.)

You could follow Franklin's suggestion and, rather than removing the tank just to check the ground connection, merely run a wire from a clean area of the body of the sending unit to a clean & unpainted area on the frame or body and see if the behavior changes.

Yeah.

To test the gauge & wiring - remove the wire from the sending unit (the factory wire, not one you may have added per the above) and, with the key in RUN or ACCY, ground it to the frame and the gauge should swing all the way full. Un-ground it and the gauge should read all the way empty.

If that test passes, the wire and gauge are good, the problem is in the sending unit - either the float has filled with gasoline and has sunk, or the mechanism has worn through the very very very fine wire that makes up the rheostat inside. <-- I'm betting on the latter

 

Come to think of it, Franklin may be correct, at least in certain situations - specifically, if you have an in-tank pump - and maybe even on some of the later trucks that don't have the pump, they may have a separate ground wire in the harness that positively connects the sending unit to a ground.

I'd have to look at the wiring diagram to be certain, I'm going from memory here and it's been a few years since I was in my gas tanks; I don't think my 1981 has a separate ground wire at all.

In any case, to test the grounding of that device, run a separate ground wire from the sender to the frame and see if the behavior changes.

 

Ok - got it now, THANK you very much for the clarification. Final question - and I apologize in advance for my ignorance - but do I have to drop the tank to perform these tests? My preliminary investigation under the truck left me with the impression that the sending unit body and factory wire cannot be accessed without dropping the tank. Does that sound correct?

 

It's certainly easier when the tanks are dropped BUT you can probably get away with just dropping it maybe 3 inches or so, enough room to wiggle your hand up into there.

A mirror on a stick and a flashlight will help.