I have a 1986 F150 with a front and rear tank. The rear tank has been leaking for quite some time so I have been using only the front tank. I took both tanks off and decided to replace only the front tank. The sending unit on the back tank was badly rusted, so I just took all the connections to the back tank, tied them off, wrapped them in plastic and duct taped it to keep them clean in case I want to put a rear tank on the truck in the future. After replacing the front tank, the gas gauge quit working. Do they both need to be hooked up, or is there a problem with the connection on the front tank?

 

If you still have the old one, hook it up and see. Something is telling me that it may need something in it's place. When you flip the switch from front to back, what does it do when you look at the back tank? does the guage show empty or full? I'm not even sure where I'm going with this...but in my mind, I'm trying to visualize if it needs it or not. Simple enough, hook up the old one....

 

Thanks Tex! Seemed to have it figured out. Looks like a bad connection at the plug. The plug is in pretty bad shape. Cleaned it up some and it works. Not sure how long it will hold.