gauge unit derives its input signal in the following
An electronic tachometer obtains a pulse signal
from the ignition distributor, as it switches the coil on
and off. The pulse speed at this point will change
proportionally with engine speed. This is the most
popular signal source for a tachometer that is used on a
A tachometer that is used with a diesel engine
uses the alternating current generated by the stator
terminal of the alternator as a signal. The frequency of
the ac current will change proportionally with engine
An electronic speedometer derives its signal
from a magnetic pickup coil that has its field interrupted
by a rotating pole piece. The signal units operation is the
same as the operation of the reluctor and pickup coil
described earlier in this TRAMAN. The pickup coil is
located strategically in the transmission case to interact
with the reluctor teeth on the input shaft.
The horn currently used on automotive vehicles is
the electric vibrating type. The electric vibrating horn
system typically consists of a fuse, horn button switch,
relay, horn assembly, and related wiring. When the
operator presses the horn button, it closes the horn
switch and activates the horn relay. This completes the
circuit, and current is allowed through the relay circuit
and to the horn.
Most horns have a diaphragm that vibrates by
means of an electromagnetic. When the horn is
energized, the electromagnet pulls on the horn
diaphragm. This movement opens a set ofcontact points
inside the horn. This action allows the diaphragm to flex
back towards its normal position. This cycle is repeated
rapidly. The vibrations of the diaphragm within the 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
When a electric horn will not produce sound, check
the fuse, the connections, and test for voltage at the horn
terminal. If the horn sounds continuously, a faulty horn
switch is the most probable cause. A faulty horn relay is
another cause of horn problems. The contacts inside
the relay may be burned or stuck together.
The windshield wiper system is one of the most
important safety factors on any piece of equipment. A
typical electric windshield wiper system consists of a
switch, motor assembly, wiper linkage and arms, and wiper
blades. The description of the components is as follows:
The WINDSHIELD WIPER SWITCH is a
multiposition switch, which may contain a rheostat.
Each switch position provides for different wiping
speeds. The rheostat, if provided, operates the delay
mode for a slow wiping action. This permits the
operator to select a delayed wipe from every 3 to 20
seconds. A relay is frequently used to complete the
circuit between the battery voltage and the wiper motor.
The WIPER MOTOR ASSEMBLY operates on
one, two, or three speeds. The motor (fig. 2-85) has a
worm gear on the armature shaft that drives one or two
gears, and, in turn, operates the linkage to the wiper
arms. The motor is a small, shunt wound dc motor.
Resistors are placed in the control circuit from the
switch to reduce the current and provide different
The WIPER LINKAGE and ARMS transfers
motion from the wiper motor transmission to the wiper
blades. The rubber wiper blades fit on the wiper arms.
The WIPER BLADE is a flexible rubber
squeegee-type device. It may be steel or plastic backed
and is designed to maintain total contact with the
windshield throughout the stroke. Wiper blades should
be inspected periodically. If they are hardened, cut, or
split, they are to be replaced.
When electrical problems occur in the windshield
wiper system, use the service manual and its wiring
diagram of the circuit. First check the fuses, electrical
connections, and all grounds. Then proceed with
checking the components.
REVIEW 5 QUESTIONS
Which type of battery condition gauge provides
the most accurate indication of the condition of
the electrical system?
How ispointerposition obtained in a differential
type thermostatic fuel gauge?
The signal generator for an electric tachometer
used on a gasoline engine is driven by what