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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.
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