1962 Erratic Gas Gauge

[fasteddy]

Gas gauge is not operating.. I grounded out the sending unit and the gauge moved to FULL. This indicates a bad sending unit. I pulled the Sending Unit and the float was full of gas. I ran a resistance check on the sending unit and had a erratic reading while moving the float level up and down. I installed a NEW Sending Unit. No reading on the gauge. I grounded the sending unit, the gauge went to full, removed the ground wire and the gauge dropped to 1/4 tank and stayed there. Turned the key off and back on, NO MOVEMENT.
Any ideas would be appreciated.

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[biddle]

Since we are a Ford Thunderbird site, you can omit the make/model of your car from future subjects to shorten it. I updated it for you.

 

[ak7an]

Gas gauge is not operating.. I grounded out the sending unit and the gauge moved to FULL. This indicates a bad sending unit. I pulled the Sending Unit and the float was full of gas. I ran a resistance check on the sending unit and had a erratic reading while moving the float level up and down. I installed a NEW Sending Unit. No reading on the gauge. I grounded the sending unit, the gauge went to full, removed the ground wire and the gauge dropped to 1/4 tank and stayed there. Turned the key off and back on, NO MOVEMENT.
Any ideas would be appreciated.

On the old sending unit, when you move the float arm up and down you should see the resistance change, thats what it is designed to do, it is a variable resistor, and changes in fuel level changes the resistance and that results in a current change and that results in a change in deflection on the gauge. Sounds like the only issue with the old one was a compromised float bulb.
You say that when you ground the sending unit the gauge goes to full. Explain the process that you are using to ground the sending unit.
If my memory is correct (not usually) One wire on the sender goes to the gauge the other wire goes to ground. This completes the circuit thru the variable resistor in the sending unit. A shop manual would show the circuit in detail and is invaluable when working on these old cars, but it is a very simple circuit.

 

[fasteddy]

Thank you. I put a jumper wire from the top of the sending unit to ground on the frame. The gauge went to FULL. I put another jumper wire from the sending unit ring to ground to ensure a good ground , no change. I pulled the constant voltage regulator, which is new, and found a loose terminal. It is working but erratic. I am now checking for bad grounds in the instrument panel.

 

[ak7an]

Sounds like you are making progress. A note on the CVR, it cycles on and off rapidly and when hooked to meter it does look erratic. Should show 5 volts or a little more. Mine shows around 8. Explains why all my gauges read a little on the high side.