I'm having an interesting time with my 1991 F-250 fuel selector switch. Both fuel pumps seem to work (I hear them start when I turn on the key). When the front tank is switched on, the gauge reads empty, regardless of contents. When the rear tank is switched on the gauge reads full regardless of contents. When the key is turned off, the gauge moves up to a position which seems to accurately indicate the contents. I looked in the fuse box and found where the "auxiliary fuel tank selector" fuse should have been and it was not there. Upon closer inspection, I saw that one of the little metal clips (which clasps and pinches one side of the fuse) was missing. Therefore I cannot replace a fuse which isn't there. I just recently bought the truck. The previous owner told me that the rear-tank was fine, but when switched on it always read full. The Front tank/switch/gauge worked for me for the first week and then it quit and now only reads empty. Any thoughts? I understand that there is a relay somewhere that may need cleaning or replacing, but I can't find it and it's not in my Chilton manual. Any help would be greatly appreciated. Thanks!

 

To start with that fuse place you found is for diesel trucks only.
The float in the front tank may have sunk and the one in the rear tank sender may have lost the ground path through it.

There are no relays for the fuel gauge system.
You have one relay for the fuel system and it is for the pump motors only.

Other than bad senders the only other possibility is that the selector switch on the dash may have an open to the rear tank or the wiring to the rear tanks gauge.

 

Subford,
Thanks. Just to clarify...it sounds like the place to start is with the wiring from the switch and follow it to both the gauge and all the way back to the tanks looking for a broken ground wire. Repairing a ground should fix the rear tank issue, but the front tank issue may require a new float. Another question-when you say "bad senders" are you referring to a break in the electrical circuit or is there an actual "sender" which is a separate device?

 

The sender is a separate device and mounted on the hanger inside the fuel tanks.

I did not say you had a bad ground wire. Your ground wire is OK or the pumps would not run. They use the same ground point and the same wire from the tank to the ground point.
I said you have an open in the ground circuit through the sender to ground from the selector switch for the rear tank. There is a variable resistor in the sender exposed to the fuel and stuff can build on the resistor element and lose contact with the wiper that is attached to the float. When this happens you have an open in the ground circuit and the gauge will read passed full.

The fuel gauge sender:








Fuel sender parts showing two kinds of floats:


Resistor Element:



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Subford,
Thanks very much for the photos and the thorough answer! Am I correct in assuming that I'll need to drain and drop the tanks in order to get at the sending units?

 

You can do that OR you can pull the bed off to get to them.

You can also unplug the plug at the top of the tank before you do either and turn on the key and check to see that the gauge is now pegged for that tank passed full and then short the gauge pin to ground and check to see if that gauge is now pegged below empty.

If it does as described above you know the wiring and gauge are working OK and the problem is in the tank sender or hanger.

 

Also note the ground path for the sender is byway of the round plate at the top of the hanger and this far left attachment of the ground for the sender from the ground wire at the plug.
This point can open the path to ground when it gets corroded.



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i have a related problem with my 94 250, except the floats and sensors seem fine only when i switch to rear fuel tank, the truck shuts off..even though i know there is gas in there and the meter shows the same.

 

+1 on pulling the bed off to do this, esp if you are going to work on both tanks. Dropping is a pain and you don't need to drain the gas to work from up top.

 

Bad pump in rear tank or selector switch on the dash.