Whenever I have the switch on the rear tank it shows me how much fuel I have but when I switch to the front tank it always stays empty even when the tank is full. What could be causing that? Any help would be greatly appreciated

 

You can try smacking the bottom of the tank in case your float assy. is stuck.The float could be bad or fell off the arm assy of the sender... Usually if the sender or wiring is open, the gauge will swing all the way past full.

 

I would suggest double checking this .
With older vehicles the gauge will usually go to full if the info to the gauge is shorted to ground.
If the circuit is open it usually doesn't register anything and stays at empty..

 

I can say with experience on my F150 that an open circuit will deflect the gauge all the way past full.

 

Hi raski,
Can I ask how you checked the gauge in your vehicle?
What did you disconnect and where?
Did you ground the signal terminal at the gauge ? Or at the tank?
What year is your F150?

 

I did my tests at the tank when the bed was off. I had a sending unit with a "bad spot" where the sender would go open when gas level was between 3/4-7/8 full as a bad potentiometer would. I verified it with my meter when I had the sender out of the tank, working it in my hands.The gauge would swing past full when there was 3/4-7/8 gas in the tank. The gauge then worked normally below 3/4 and from 7/8 to full. With the connector off, gauge swung full. My truck is an 87 but I doubt there are any major differences between mine and the OP's.

Anyway, the OP's gauge works normally with the rear tank but not properly with the front tank.So I would rule out an actual gauge issue and look at wiring from the front tank or its sending unit. I would also be suspicious of the gauge if he only had a single tank but he stated it works fine with the other tank.So I see no reason to waste time with the gauge itself.His symptoms are way different than mine.

 

It's been a while since I worked on a Ford those years.
Would you maybe remember what the ohm readings did when you raised and lowered the float arm on your test unit?
Some went up when the arm was raised and some went down.
It appears the newer sending units don't work the same as the older ones..

 

Sorry, I don't remember how the actual resistance readings were while working the sender. I wouldn't think there would be much difference between an 87 or a 90 but it can be confirmed if they share the same part numbers.

 

The ohm readings are not that important, what is important is which way the readings go when the arm is moved. What we are trying to confirm is: when the float arm is down and you get a reading, now when you raise the arm does the resistance go up or down?