Troubleshooting Fuel and Temperature Gauges

Proper Troubleshooting Techniques

1.  Remove the screws that retain the instrument cluster to the dash. Disconnect the speedometer cable from the rear of the instrument cluster and pull the cluster forward. You should be able to access the rear of the cluster and lie it on the steering column. A towel would help prevent the column paint from scratches.
2. Turn the Ignition Key to the "ON" position. Check the voltage on the "Green-Black" wire at one of the gauge connections. The voltage should fluctuate between Zero and Ten Volts. This is the same Voltage that will be seen at the connection of both sending units.
3. If the voltage is incorrect the instrument panel voltage regulator may be defective.
4. Connect 2 "D" cell batteries in series so you will have a 3 volt output. Connect the battery output to the terminals of the Fuel or Temperature gauge. Connect the Positive Battery output to the "Black-Green" terminal and the Negative Battery output to the "Yellow-White" terminal. The gauges should read "Full" or "Hot" respectively. If not, replace the gauge.
5. One item that is often overlooked is the Grounding System. The Engine and Chassis must both be properly grounded for the gauges to work properly. Ensure the Battery Ground Wire is properly connected to the Engine and that the Engine Ground Strap is properly connected to the Firewall. Just because the Engine Temperature Gauge is functioning correctly, you can't assume the Fuel Gauge Circuit Ground is O.K., because the Fuel Level Circuit gets it's ground from the Engine - Firewall strap and the Engine Temperature circuit gets it's ground from the Battery Ground Cable.
6. Now that you have checked and made certain the Gauges and Ground Circuits are correct we can move on to the Sending Units. The Sending Units are nothing more than Variable Resistors. The Temperature Resistance is changed with Engine Heat and the Fuel Resistance is changed with the the level of Fuel in the Tank. If you were to disconnect either wire going to the respective sending units and momentarily touch that wire to the electrical ground with the Ignition Key in the "on" position, the gauge should go to full. If not, there is an open circuit between that sending unit and it's respective gauge.
7. Most folks want to immediately challenge the accuracy of the sending units when the gauges don't read where they think they should. Yes, both sending units are immersed in some type of moisture of varying temperatures and conditions and can be problematic. However, the Gauges and the Constant Voltage Regulator are subjected to constantly changing voltages and currents and rely on the difference in the electrical properties of different metals for proper operation. These metals will also deteriorate over time and affect the accuracy of the circuits. So, after you have done all of these checks and the gauge still doesn't read correctly, the sending unit is probably defective and should be replaced.
8. DO NOT connect any 1960 - 1965 Ford Falcon sending units to a Positive 12 Volts at any time. Your unit will not be repairable as it not designed to operate at that voltage for a sustained period of time.
9. The Fuel Sending unit is manufactured and your Ford Falcon Fuel circuit is designed to operate at the following Ohmic values. Empty; ~75 Ohms, Full; ~8-12 Ohms. If your sender has these values for full and empty scenarios, it may be required you bend the float rod for accuracy of the gauge.