I have a 1979 F-150 with rear and side tanks. The gauge only works for the rear tank, but not the side. I put a new sending unit in the side tank and put a ground from it to the frame and it pegs the gauge out at full. I am at a loss for what to do. Someone said that I maybe needed to clean the ground, but there is only one wire in the pigtail that connects to that sending unit, so how on earth would it be grounded. Does anyone have a wiring diagram of this to help me out? I am at a complete loss! Thanks in advance!

 

did you buy the sending unit brand new, used, or reman?

 

Brand new and I have tested that its good

 

Find the ground wire that is in the wiring harness to the sending unit.

 

That's what I don't get, there is only one wire that goes to the sending unit. It is wrapped in rubber and nylon from the factory, so I know its correct.

 

Yes...there is only one sending wire for each tank. I believe one is blue and the other is green/red stripe. The sending units ground to the tank....just like the windshield wiper motors if I'm not mistaken. I know they don't tie into any grounds anywhere. Check to make sure you sending unit has the correct ohms.

 

Haha turns out there IS a ground wire in that pigtail, it had just pulled completely out. I had to cut the entire rubber connector open to reattach a ground wire. Good news is that my gauge works! Bad news is I bought a new sending unit for nothing, and I have to buy a new connector. Oh well, thanks for the help!

 

Congratulations on sticking with it till you found it.

 

Hmm...my schematic shows it grounding to the chassis...not in the harness, but I'm guessing it comes out of the harness and grounds to the chassis somewhere close by....yep...they have a pic in the Haynes manual it goes in the harness and then comes out and grounds...it looks like to the bed in the pics? Glad you got it corrected.

 

The ground wires comes out of the plug and grounds to the frame right beside the tank. It is actually separate from the wire that goes into the harness and up to the gauge

 

Cool...that makes more sense. I knew it shouldn't go back in the harness and travel far as my schematic shows it grounding right at the tank, they just don't tell you exactly where, but that it is a chassis or body ground.

 

The wire that comes from the guage is a ground. However, it goes thru the sending unit and back out to a chassis ground. The sending unit varies the "amount of ground" which allows the guage to read accordingly....................Hope that makes sense..........

 

Perfect sense.