I just got a 65 Elky, non stock 350, 3 wire Alt, new gas tank. Fuel guage reads full when ignition is off, and goes off the scale on full side when you start it. Guy I got it from knows nothing about the vehicle. Any help would be appreciated.
Probably, the ground wire rusted off the sending unit on top of the gastank. It has to be grounded to work properly. There are 2 wires going from the sending unit to the wires near the back bumper. The orange one goes to the sending unit connection and the black one grounds onto the car frame.
Remove the sender wire from the sender, with the key on, note the direction that it pegs..(usually full)
Then Ground the sender wire , It should drive the gauge in the opposite direction..
If you have that , the gauge and circuit are whole..
Next, hook up the sender wire and run a jumper to the tank/sender egress panel to the frame..note the gauge indication..If it indicates correctly, install a new ground wire..If not..
Next, using your DVOM, set to OHMS, R x 1 Calibrated to 000, Pull the sender and measure it from the sender terminal to the sender ground and slowly move the float up and down..watching the meter as you do..It should give a range of OHMS.. like 0 to 90 or 90 to 0..should be smooth and have no drop outs..the float should be dry and not slosh also..
If that checks, the new sender may be out of range, check what sender resistance is required for your gauge and install a new proper sender..