Temperature Sending Units

Since it is critical for you to keep an eye on the temperature of the coolant in your cooling system at all times, your car will have either a gauge or a warning light located on the instrument panel or dashboard (see temperature gauge). The question is, how does the information about your coolant get to the gauge? It gets there, or is sent by the temperature sending unit. The temperature sending unit is a device that is placed so that it can determine the temperature of the engine coolant. Simply put, its resistance to electricity changes with increases and decreases in the temperature of the coolant. The electric resistance changes control the movement of the indicator needle on the temperature gauge. If you have an indicator light, or lights, these changes will cause the bulb to be connected to the battery if the temperature of your coolant gets too high. If this happens, the light goes on. There are two types of sending units. One type uses a Bourdon tube instrument, a capillary tube filled with a special gas, and a capsule, or bulb. The other type uses an electric sender receiver. The Bourdon tube type works by having one end of the tube attached to the gauge fitting, and the free end fastened to the needle indicator. A Bourdon tube is a round, hollow metal tube. Putting pressure on the hollow end causes it to try to straighten, so that the other end moves the needle on the gauge. Because it is placed in an engine water jacket, the pressure from the coolant temperature causes it to move, which, in turn forces the other end to move the gauge needle. When the coolant cools, the lack of pressure allows the needle to swing back to cold on the gauge. The electric sender receiver type has a bimetal thermostat in the dashboard. This thermostat is linked to the gauge needle, so that when the engine gets warmer and passes more current, the thermostat, getting hotter itself, bends. When the thermostat bends, it moves the gauge needle, which indicates that the coolant temperature is rising. As it cools off, the thermostat "unbends" again, and the needle drops back to the cold indicator.