Lesson 4/Learning Event 3
The warning light receives battery power through the ignition switch. The circuit to ground is
completed through the engine sender switch. The sender switch contains a set of contact points
that are operated by the bending of a bimetallic strip that is calibrated to turn on the warning light
at approximately 230F (110C). Some models also use a cold indication light that indicates the
engine has not yet reached operating temperature. The light usually is green in color, whereas the
hot indication is red. The sending unit for this application has an extra terminal. Internally, the
sender has an extra contact. The bimetallic strip simply completes the circuit to the cold light until
engine temperature reaches approximately 150F (65.6C). The strip then will open the cold light
circuit. As long as the temperature of the engine is normal, the bimetallic strip will remain between
the contacts for the cold and hot indicator lights.
SPEEDOMETERS AND TACHOMETERS
Speedometers and tachometers are used in virtually all types of automotive equipment.
Speedometers are used to indicate vehicle speed in miles per hour or kilometers per hour. In most
cases, speedometers contain an odometer, a device that keeps a permanent record of the amount of
mileage that a vehicle has been used. Some speedometers also contain a trip odometer that can be
reset to zero at anytime so that individual trips can be measured. The odometer is calibrated to
measure distance in miles or kilometers, depending on the application. A tachometer is a device
used to measure engine speed in RPM. The tachometer may also contain a device known as an
engine-hours gage, which is usually installed on equipment that uses no odometer to keep a record
of engine use.
Electric
The electric speedometer or tachometer uses a mechanically driven permanent magnet generator to
supply power to a small electric motor. The electric motor then is used to rotate the input shaft of
a mechanical speedometer or tachometer. The voltage from the generator will increase
proportionally with speed and the motor speed will likewise increase proportionally with voltage,
enabling the gages to indicate speed. The signal generator for the speedometer usually is driven by
the transmission output shaft through gears. The signal generator for the tachometer usually is
driven by the distributor through a power take-off on gasoline engines. When the tachometer is
used with a diesel engine, a special power take-off provision is made, usually on the camshaft drive.
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