I am having a problem with my 96 F350. I just bought the truck and filled up both tanks. I ran the first one out and switched to the rear tank. The needle shows past full. After driving into town it didn't seem to move at all so I switched back to the front tank. I went and filled them both up and it seems like the rear tank wasn't being used at all. I now have both tanks full and the selector on the dash set to the rear tank. I'm going to drive it a little ways and see if it ever goes down and then check the front gauge to see if that's going down.

If the fuel gauge is working, but only the front tank is sending, what could be the problem and where do I start troubleshooting? Thanks!

 

May want to see if the switch even works in the dash.

 

How many miles did you put on the rear tank? Mine show full at about 12 gallons added from empty, and sometimes can be filled beyond that. I think most are listed as 19 gallons capacity, and a lot of the "extra" capacity seems to be on the full end.

If yours was over-full, it may take a while to use it down to "full", where the gage starts dropping. Depending on how the filling station nozzle and your truck's fill tube and overflow vent get along, you may be able to add a fair amount even with the tank showing full. So if your rear tank is 5 gallons over "full", you may need to do like 80 miles before the gage starts dropping. This is probably extreme over-fill, but the point is that you may need to do more than a few miles before seeing the gage drop. 12 gallons of usable capacity from a "normal" fill should be about 180 miles of range assuming 15 mpg, so even from a grossly over-full tank you should see the gage start dropping long before running out of fuel.

A possible failure mode is that the tank gages switch, but not the fuel valve. If you start with both tanks normally full, you should be able to get drive something like 120 miles with the rear tank selected. Even if the rear was grossly over-full, if the dash selector switch and frame valve work you should see the rear tank start to drop somewhere in that drive. If rear tank gage doesn't drop, switch to front tank, and see if that has dropped from full. Even if the front tank was grossly over-full, it should have dropped by 120 miles, and you'll know you have an issue with the selector switch on the dash or valve on the frame rail. If the front tank was just normally full, you should still have enough range to make it to a fuel stop.

 

Check the fuse first, it is a 15amp fuse in your dash panel fuse box. I believe it is number 6 and will also be the fuse to the ac clutch as well.