Where does the gas tank sending unit connector get its power from for a 78 f250? I just check the hot wire at the sending unit with a test light and i have no power. I unplugged the bullet connector on the frame and checked again and still no power.

 

The gauge gets +5V from the instrument panel voltage regulator on the back of the cluster. The sending unit is on the ground side of the circuit. If you unplug the sending unit and ground the wire that goes from it towards the cab, it should push the gauge all the way to full - if it does, your gauge and wiring are fine. If not, work back in that direction.

 

I want to know where the hot wire from the sending unit goes. Does it go into the cab and into the dash cluster connector directly or does it go onto something else before the connector?

 

If you've got multiple tanks, the signal wire will go through the fuel gauge selector switch; if not, it'll go straight to the cluster. I don't know what I'd expect to see from a test light, as it's only +5V, with the gauge sitting in between the power source and the sending unit ... suggest using a digital multimeter for testing. Again, if you ground the wire that feeds the gauge, the gauge should peg to full - if it doesn't, the problem is up that way; if it does, everything is good as far as the sender.