are designed to maintain a fairly constant
working temperature, the actual
temperature can vary for a number of
reasons. It may even reach such a high level that engine damage becomes a
provides an early warning of overheating, allowing you
to stop the car before any damage occurs. In very cold weather, the gauge can
also tell you if the engine is being overcooled (which will increase
consumption and engine wear). You can then take preventative means, such as
blocking off part of the
or changing the
The temperature measuring system normally consists of two
gauge itself and the sensor unit that controls it, the two being connected by a
Types of gauge
There are two common types of gauge mechanism - magnetic gauges and
bimetallic gauges. You can tell which type your car has by the way it reacts
. With magnetic instruments the needle
immediately jumps to give a reading; bi-metallic gauges move slowly to the
reading after being switched on.
Temperature gauges are built into the car's instrument housing on the
. The sensor unit, however, may be in one of several locations - the
thermostat housing, the
or the upper radiator
. In all cases
the sensor is arranged so the coolant flows over it on the way out of the
Magnetic gauges, also called moving iron gauges, have a pair of
on each side of a pivoted iron
that carries the needle. Sometimes the
iron armature is weighted to hold the needle in the rest position; in other
cases a light hairspring does this.
The coils are connected directly to the car's electrical supply - one is
earthed direct and the other earths through the sensor, whose
with engine temperature. The
passing through the coils produces a
which moves the armature against the weight or spring. The
amount of movement depends on the difference in the
produced by the two
coils. This difference varies with the size of the current let through by the
With bi-metallic strip gauges, the current allowed through by the sensor is
fed to a coil of resistance wire wound around a bi-metallic strip that's linked
to the needle.
Current flowing through the bimetallic strip causes the strip to heat up. As
it does so it bends because the two metals in the strip expand with the heat by
different amounts. The bending strip deflects the needle across the
amount the strip bends depends on the amount of current arriving at the gauge,
which in turn depends on the heat of the engine.
To avoid errors caused by variations in the car's supply voltage due to
electrical load and
speed, a voltage stabilizer is included in the
. The voltage stabilizer also works on the bi-metallic strip
principle and keeps the instruments supplied at a steady 8 or 10
There are two types of sensor units: the
type and the
bimetallic strip type.
Semiconductor sensors are by far the most common type and consist of a
element in a metal capsule. The resistance of the
semiconductor decreases as the temperature increases. As the engine heats up,
the sensor resistance reduces, increasing the current flow to the gauge and
giving a higher reading.
The bi-metallic principle is used in a rarer type of sensor. Movement of a
bi-metallic strip inside a heating coil in the sensor opens a pair of contacts,
cutting off the current to the heater and the gauge. With the current off the
strip cools and straightens out, remaking the contact so that current flows
again. This sequence is repeated rapidly, with the amount of time the contacts
are closed (and the amount of time that currents flows to the gauge) depending
on the overall temperature of the sensor unit.
When the engine and sensor are hot, less electrical heating is needed to
bend the strip and open the contacts and the cooling process takes longer. This
means that the contacts remain open for longer so less overall current flows in
the circuit. The needle is linked in such a way that a low current equals a
high gauge reading.
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Almost all car engines work on the four-stroke cycle, so called because it
takes four strokes of the piston induction, compression, ignition and exhaust -
to produce one firing of the fuel/air mixture. This means that the crankshaft
rotates twice to complete each cycle.