Front tank guage not working after sending unit change.

I realize the obvious answer would be that the sending unit is bad out of the box, but before dropping the tank I figure I would check with you guys and see if there is anything external, (besides the connection to the tank), which I should check.  Looks like the wires go from front tank to the rear tank before going up front. The guage did work before the change, but was trashed. Rear tank guage works.  Front tank shows past full.
Thanks.

Showing past full probably means there is a short to ground in the dark blue/yellow wire.  I say that because even if the sender is stuck in the fully "up" position the gauge should only read "Full".  So check the wiring and see if it might be caught between the tank and the frame, or something like that.

Pull the connector off the tank and see if the reading changes.  If it does then it isn't in the wiring but is in the sender.  Maybe it is shorted.  If it does change then the problem is in the wiring.

What are we working on again as different year / motors have different fuel systems for dual tanks.
In your case if the new sender is showing full there is a short to ground.
Now could it be the new sender or the wiring for that tank you will have to check to find out. If you um-plug the sender and the gauge does not move to full it is the sender.

If it still goes to full it is the wiring any where from the tank to gauge but depending on where the gauge switches for the tank (some systems do this at the frame tank switching valve and why I asked on the system you have) then you only need to check to that point.


IIRC on my 81 with a 300 six both tank wiring go up to the dash switch as dose a 3rd wire for the tank switching valve.

The dash switch sends power to the tank switching valve or not, spring loaded valve so 1 tank is online with no power.

The dash switch for the gauge then picks what tank it is on, front or rear, to send that signal to the gauge.
Good luck and hope its an easy fix
Dave ----

Thanks guys, you have definitely pointed me in the right direction. I appreciate it.
It is a 1984 setup, all stock, and I have only the front tank connected, I bypassed the valve since i need to replace fuel line on the rear tank.
I have the simpler valve, (no return lines). Worth mentioning although not related to the issue at hand.
Thanks very much guys.