How Ford Gauge Unit Work

[JSeery]

Based on several resent post there seems to be a lot of confusion about how early Ford gauges work. They are NOT resistant units and are unique to Ford as far a I know. A lot of the following is directly lifted from a Ford Shop Manuel. There are also a lot of older post on the Barn on this same issue.

The 6v Ford gauges (Fuel Level, Oil Pressure & Temperature) all work on similar principles. They have two major components, a Sender Unit and a Gauge Unit. The senders use a bimetal element and a heating coil to control the average current flow through both units. The Gauge Unit pointer is controlled by another bimetal and heating coil unit.
When the ignition switch is on, current flows through the circuit and warms the Sending Unit bimetal by means of a heating coil, causing the bimetal strip to bend and open a set of contact points. When the points open the current is interrupted allowing the bimetal to cool and close the contact points again. This cycle then repeats and the points vibrate open and closed pulsing the current in the circuit.
Because the current through the heating coil in the Sending Unit also flows through the heating coil in the Gauge Unit, the amount of heat supplied to the gauge unit is about the same as the heat in the Sending Unit. The amount of heat in both units is controlled by the average current flowing through the circuit due to the repeated opening and closing of the contact points.
The Sending Unit varies the current required to open and close the contact points by varying the pressure on the bimetal strip making it harder or easier for the points to open and close. The more pressure on the points the more heat (current) that is required to open the points. Likewise, the less the pressure on the points the less heat (current) that is required. The different gauges use slightly different mechanical systems to accomplish this.
Fuel Level Sender Unit:When the tank is filled, the float rises with the fuel level in the tank and a cam moves the ground contact toward the bimetal arm, increasing the tension holding the contacts closed. A greater amount of current is required to heat the Sending Unit bimetal arm enough to cause it to open the contacts. A similar greater bending of the bimetal arm occurs in the Gauge Unit and results in a movement of the needle toward the full position on the scale.
Oil Pressure Sender Unit: When there is no oil pressure, the contact points are just touching and the gauge pointer register at the “0” position. Any increase in oil pressure bends a diaphragm, which in turn increases the tension on the bimetal arm. More heat must be supplied to cause the contacts to open and a resulting increase in the average current flow to supply this heat. This increase in average current flow in the circuit heats the coil in the Gauge Unit which bends the bimetal strip and moves the pointer.
Temperature Sender Unit(s): The Temperature Sender is coupled with a Thermal switch. When the engine is cool the bimetal arm in the Sending Unit has maximum tension holding the contacts closed. The Maximum average current is necessary to open the contacts. The heating effect of the current causes the Gauge Unit bimetal arm and pointer to defect toward the “C” position of the scale. As the engine temperature increases, less current is required to keep the contacts at the break point since the increase in engine temperature causes the Sending Unit bimetal to bend away from the grounded contact. The Gauge Unit pointer then registers toward the “H” position of the scale.
The Sending Unit has one electric terminal. The Thermal Switch can be identified by the two terminal connectors on it. The switch is set to open at 200-212° F. With a sending unit in one cylinder bank and the switch in the other cylinder bank, the Gauge Unit will indicate a boiling condition in either bank.
Testing a Gauge System
Fuel Level & Oil Pressure System Tests: Gauge Unit can be tested by disconnecting the wiring to the Gauge Unit and connecting it to a 1 ½ volt source (a “D” cell battery works well). The gauge should display mid-scale with a 1 ½ volt input and full-scale with 3 volt input (two “D” cell batteries). The suggested test of a Sending Unit is to test it by substituting a know good gauge to see if it will read correctly with the suspect Sending Unit. If it does not work correctly with a know good gauge the problem could be a bad Sender Unit or a bad connection between the Sender Unit and the Gauge Unit.
Oil Pressure System Test: The gauge test is the same as for the Fuel Level and Oil Pressure Gauge Unit test. The Sending Unit is checked the same as the Fuel Level and Oil Pressure Sending Unit. The Thermal Switch can be tested to see if it is closed at normal temperatures. It can be tested with a test light or an ohm meter. It should show continuity. The switch can then be tested to determine if it opens at boiling temperature by placing the bulb in boiling water and testing it. At temperature it show test as an open.

Edit: on another post it was pointed out that by shorting the electrical terminal on the sending unit you are causing full current to flow through the Gauge Unit which would move the pointer on the gauge to full scale. I would be very carful doing this test because it would be easy to damage the heater coil in the Gauge Unit. It should be a momentary test. If the Gauge Unit responds then the Sending Unit may be the problem. If not it could be the wire between the two or the Gauge Unit itself. I would lean toward the "D" cell battery test, much easier on the Gauge Unit.