Lesson 4/Learning Event 3
Electronic speedometers and tachometers are self-contained units that use an electric signal from
the engine or transmission as an indicator to formulate a reading. They differ from electric units
that use the generated signal as the driving force. The gage unit usually is transistorized and will
supply information through either a magnetic analog (dial) or a light emitting diode (LED) digital
gage display. The gage unit derives its input signal in the following ways:
An electronic tachometer can obtain a pulse signal from the ignition distributor that switches the
coil on and off. This is the most popular signal source for a tachometer that is used on a gasoline
engine. The pulse speed at this point will change proportionally with engine speed.
A tachometer that is used with a diesel engine can use the alternating current generated at the
stator terminal of the AC generator as a signal. The frequency of the alternating current will
change proportionally with engine speed.
An electronic speedometer derives its signal from a magnetic pickup coil that has its field
interrupted by a rotating pole piece. The signal unit's operation is the same as the operation of the
reluctor and pickup coil. The pickup coil is located strategically in the transmission case to interact
with the reluctor teeth located on the input shaft.
Principles of Operation
The most common type of horn is the vibrator type, in which the general principle of operation is
the same as that of a vibrating coil. A vibrating diaphragm is operated by the coil that also
operates the contacts that break the circuit.
Magnetism from the coil pulls the diaphragm toward it when the contacts are closed. The contacts
then are pulled open by the coil, reducing the magnetism and allowing the diaphragm to return to
its normal position. When the contacts are closed again, a new surge of current induces magnetism
in the coil and starts a second movement of the diaphragm. This cycle is repeated rapidly. The
vibrations of the diaphragm within an air column produce the note of the horn. Tone and volume
adjustments are made by loosening the adjusting locknut and turning the adjusting nut. This very
sensitive adjustment controls the current consumed by the horn. Increasing the current increases
the volume. However, too much current will make the horn sputter and may lock the diaphragm.