Classic/Antique Car Repair: 1967 GTX GAS GUAGE NOT WORKING, digital voltmeter, jumper wire

Question
I have checked and confirmed that the sending unit in the tank is good. I do not know if the problem is the guage or the circuit board.  How can I test them?  Can the board be repaired or does it have to be replaced?  Thanks in advance for your advice.

Answer
First, if you are handy with a soldering iron, most circuit board problems can be repaired. Now, a digital voltmeter can be used to check the under dash circuits. There should be voltage at both sides of the fuel gauge behind the dash. Ford uses a constant voltage control behind the dash between the ignition switch feed and the gauge unit. This stops voltage fluctuations from causing false readings. The voltage into this unit is battery voltage and somewhere around 6 or 7 volts out. That is the voltage that you should see at the input side of the fuel gauge. To test the gauge operation I use a small jumper wire across the out terminal of the fuel gauge once the tank wire is disconnected. With the jumper connected to ground the gauge should go to full and to empty when disconnected.