Fuel gauge problems
 

The fuel gauge in my '78 F150 4x4 only works when it is VERY warm outside- 80F or higher. It sometimes works if the truck gets driven a fairly long distance at one time, say 50+ miles, but it still must be fairly warm outside, so the problem is definitely temp related. When not working it rests below "E".

Anyone ever seen this or have some input on correcting it?

I would assume pretty sure its a " resistance " fuel gauge.. so check your wires to make sure there's excellent contacts and none are cracked or broken ( just hanging on) also you can pull the sending unit out an check its hmm i think ohm resistance .. ( someone correct me if i'm wrong ). as the float rises it changes the resistance up or down, an that's when you see the meter move in regards to it. Short of that depending on how old it is you may just want to change the sending unit and re wire it. I haven't done it ( did on in my '70 cougar) in my F series but i'm sure is the same or just as easy.. hope this helps.
Any other opinions out there?

Unplug the frame wiring harness where it plugs into the short fuel sending unit harness and, with the ignition on, ground the wire from the frame harness. If the gauge reads full then the wiring and gauge are probably okay and the problem is the sending unit or possibly the sending unit wiring harness.
Unplug the harness from the sending unit and check the two terminals to be sure they are clean and making good contact. Also check the ground terminal where it's connected to the frame.
If all is okay then it's probably the sending unit.

I had this same problem with a replacement sending unit and I found that the float was actually punctured. It was only a matter of days before it filled with gas entirely, and sank to the bottom of the tank permanently (reading empty). It's also possible the rheostat is bad, and the advice given by both Bill and Mike will help troubleshoot that possibility.

Did you get it fixed?

sender OK, and the wiring ohms out OK from dash to the rear. I think it's going to be somewhere in the IP; not good for my bad back..... I'll make a hole in the shop somewhere and tear into it- I prefer to work IP's in the A/C. Fuel gauge working today because it's near 90 outside, it didn't work last night at 60deg F.

thanks for the ideas so far
Jim

Something else to check while you are under the dash is the instrument panel voltage regulator. It's a small box that is attached to the back of the instrument panel gauge cluster by one screw and has a connector that looks like a 9v battery connector.

thanks mike- didn't know about that. Is there any way to test its integrity?
I have several spare panels, so if it's the gauge itself or a problem within the panel or its connectors, I should be OK. I don't know if I have a spare of those, however.
Jim

This is taken from the Ford Shop Manual:
Disconect the fuel gauge lead from the terminal at the sender unit and connect the lead of a 12 V test light or the positive lead of a volt meter (20 volt scale) to the gauge lead that was disconected from the sender unit. Connect the other test lead to a suitable ground. With the ignition switch in the ON position, a flashing light or fluctuating voltage will indicate that the instrument voltage regulator is good and that the gauge circuit is not interrupted. If a pulsating voltage is shown but the gauge is not accurate, perform the calibration test.
If the light stays on or the voltage reading is steady, replace the I.V.R.
If no voltage is indicated by the voltmeter or test light, check the I.V.R. for proper ground and check for an open circuit in the IVR, the gauge windings, the printed circuit.
DO NOT ground or spark either terminal of the IVR. This will burn out the dash wiring harness or the IVR or both.