To reduce voltage loss in the wiring, the relays are
and keeps all other lights off. The plunger knob cannot
placed in such a position that only short lengths of
be pulled out to its third position until the safety lock
heavy gauge wire is necessary to connect them. Since
button is pushed in. This safety feature prevents any
the main light switch and dimmer switch carry only low
lights visible from above being accidentally turned on
control current, voltage loss due to burned switch
during a blackout. In the third position, with the
contacts is eliminated. Figure 6-99 shows a wiring
plunger knob pulled all the way out, the service lights
diagram for a system using two relays--one for high
are on and operate normally. A trailer connection is
beam and one for low beam. In some systems, these two
provided to operate taillights on the rear of the trailer.
relays are incorporated into one unit.
These are on when the switch is in either the second or
third position.
Another type of relay used in some automotive
lighting systems is the current limiting relay. These are
Vehicles that are equipped with dual filament
sometimes referred to as overload relays or circuit
headlights have a switch that allows the operator to
breakers. They can be found in two types. The thermal
select high or low beams (dimmer switch). It is a
overload relay is incorporated as part of the main light
foot-operated push button located on the floorboard. In
switch; the solenoid relay is mounted separately on the
newer vehicles, the dimmer switch may be combined
vehicle fire wall.
with the directional switch on the steering column.
The stoplights are controlled by a switch that is
FUSES AND CIRCUIT BREAKERS
actuated whenever the vehicle's service brakes are
applied. It may be located on the master cylinder
Fuse boxes contain fuses and/or circuit breakers
mount brackets. It is actuated by mechanical linkage; in
that protect the electrical circuits. When an overload
the hydraulic system, it is actuated by hydraulic
condition exists, the fuse element melts or the circuit
pressure.
breaker opens to relieve the overload condition. When
this happens, do not automatically replace the fuse or
RELAYS
reset the circuit breaker. It is important to first correct
the condition that caused the overload. Otherwise, the
Because of the limited voltage available to
situation will repeat itself; or worse, the circuit will be
automotive lighting circuits, resistance of the wiring
damaged. When replacing a fuse or circuit breaker, be
must be kept to a minimum to provide lighting of
sure that the rating of the replacement component is no
sufficient intensity. Some systems do this by using
larger than that specified in the technical manual for
heavy gauge wire; others use relays.
that circuit.
RIGHT
HEADLIGHT
BAYONET CONNECTOR
HIGH
DIMMER
BEAM
SWITCH
RELAY
BAYONET
CONNECTOR
TO MAIN LIGHT
SWITCH
TO BATTERY TERMINAL
ON STARTER RELAY
LOW
BEAM RELAY
LEFT
HEADLIGHT
WIRE SIZES
RELAY-TO-HEADLIGHTS: 12 GAUGE
RELAY-TO-DIMMER SWITCH: 16 GAUGE
RELAY-TO-STARTER RELAY: 10 GAUGE
DIMMER SWITCH-TO-MAIN SWITCH: 16 GAUGE
ASf06099
Figure 6-99.--Light relay wiring diagram.
6-78
