Ok. I have a 1979 f-250 XLT 2wd. New front tank and send unit. Original rear tank and new send unit.
New selector valve and switch in dash works. Truck runs great on either tank.
Fuel gauge does not register for either tank.
A test light glows dim and flashes at the plugin to sending unit on both tanks. We have cleaned, repaired or replaced every connection that was questionable. Before front tank showed up, we did have the rear tank reading the fuel level and at the sending unit the test light was bright and solid.
I'm horrible at auto electric, so I run out of ideas pretty quick.
Also if anyone knows where I can print off a wiring diagram that will be readable and show dual tank wiring that would be great.

Thank You!

 

http://www.fordification.net/tech/wiring.htm

Plus ?, does both sending units have 2 wires each, one to send to the gauge switch and the other a ground wire that attaches to one of the mounting screws.

 

Yes Both do. Both wires are in good condition, and we did take grounds loose and wire brush so there was no chance of a bad ground.

 

Do have a DVOM? check continuity between the 2 ground wires. Then if you can locate where the 2 ground wires join and are mounted/bolted to the frame or sheet metal, check continuity of ground wire at tank to the bolt securing the wires and the metal to which the bolt is attached. Just some possibilities.

 

I do have one and will give that a try. Interesting idea. I will let you know though it will be a few days due to work.. Thank you!