I have 1965 mustang and the gas gauge does not work. The tank was replaced before i bought the car. What could be the problem? most tell me it is a ground wire, but where could it be?

 

"Most" is wrong, there is no ground wire. Power flows from the battery to the CVR (stepping down to about 7V), then to the gauge, then to the sender, which, through the rheostat, grounds the circuit. Now you have to find which of these is at fault. Use a test probe and work your way along the path I described. When you get somewhere that has no power, you've found the problem.

 

 

It doesn't ground the CVR, it uses the CVR screw to attach the instrument cluster ground wire. Read the schematic. You're wrong. Sometimes I'm wrong, but I have tested and repaired way, way too many of these systems, so this is not one of those times.

 

under that screw is a black wire. Im just saying...It says " Constant Voltage Unit ground to dash wire" and its black. Looks like the one in my car. And I wasnt smooshing it in your face, I was just pointing it out. But I am wrong because I have a 66 not a 65. So, your right and I am wrong.

 

The instrument cluster is always grounded to the chassis, really odd things can happen to the cluster lights when it is not connected. Since the CVR was screwed to the cluster, Ford saved themselves .04¢ by using the same screw to attach the wire. You should see the same thing on your 66.

 

I have the black wire.

 

You seem to have two wires, the factory ground and a doubled hand-crimped add-on. What's that one for?

 

I have a different voltage regulator. I dont have the can type.

 

If the oil pressure and water temp are working, it's not the IVR or the ground of the cluster. 2+2 has it right, the ground for the fuel gauge is the sending unit, just like the ground for the oil pressure and water temp gauges are the sending units.

Could be a bad sending unit or bad gauge.