Fuel Gauge reads full when truck is off! Help

[Cesar Diaz]

1972 C10 Cheyenne: Original gauge cluster

I replaced the gas tank and the fuel sending unit in this C10 we just got. When the truck is turned off, the tank reads full, When I turn the truck on, the Gas Gauge goes down to empty, not sure whats causing this. Before I installed the sending unit I tested it and everything was fine, down it would read empty and if i raised it, the needle would move up.

Any help would be appreciated.

I also have a second tank who's sending unit also works and when I switch to that tank it does the same, I am using an off-on-off switch. When I bypass the switch the same is happening.