Temperature Gauge and Warning Light

The operator should be warned if the temperature of the coolant in the cooling system goes too high. For this reason, a temperature gauge or warning light is installed in the instrument panel of the vehicle. An abnormal heat rise is a warning of abnormal conditions in the engine. The warning lights alert the operator to stop the vehicle before serious engine damage can occur.

Temperature gauges are of two general types—the balancing-coil (magnetic) type and the bimetal-thermostat (thermal) type.

1.      The balancing-coil consists of two coils and an armature to which a pointer is attached. An engine-sending unit, that changes resistance with temperature, is placed in the engine so that the end of the unit is in the coolant. When the engine is cold, only a small amount of current is allowed to flow through the right coil; the left coil has more magnetism than the right coil. The pointer, attached to the armature, moves left indicating that the engine is cold. As the engine warms up, the sending unit passes more current. More current flows through the right coil, creating a stronger magnetic field. Therefore, the pointer moves ‘to the right to indicate a higher coolant temperature.

2.      The bimetal-thermostat is similar to the balancing-coil type except for the use of a bimetal thermostat in the gauge. This thermostat is linked to the pointer. As the sending unit warms up and passes more current, the thermostat heats up and bends. This causes the pointer to swing to the right to indicate that the engine coolant temperature is rising.

A temperature warning light informs the operator when the vehicle is overheating. When the engine coolant becomes too hot, a sending unit in the engine block closes, completing the circuit and the dash indicating light comes ON. The indicating light warns of an overheating condition about 5°F to 10°F below coolant boiling point.

In some construction equipment a "prove-out" circuit is incorporated in the system. When the ignition switch is turned from OFF to RUN, the light comes on, proving that the system is operating. If the light does not come on, either the bulb is burned out or the sending unit or connecting wire is defective. The light will go out normally after the engine starts.