AUXILIARY POWER RECEPTACLES
The control switch has four positions--OFF, LO,
MED, and HI. Wiper speed is controlled by increasing
Auxiliary power receptacles vary from vehicle to
or decreasing the amount of current in the shunt field.
vehicle. In some applications, the auxiliary power
At low speed, the motor has no external shunt field
receptacle is mounted near the equipment battery. The
resistance, and maximum shunt field current flows. For
receptacle has two large pins; one pin is connected to
medium speed, resistors R1 and R2 are connected in
the positive terminal of the battery, and the other pin is
parallel with each other and in series with the shunt
connected to the negative terminal.
winding, thereby reducing the shunt field current. In
high speed, only R1 is in series with the shunt field,
The auxiliary power receptacle is used to obtain
resulting in maximum shunt field resistance, minimum
power from an external source for charging batteries,
shunt field current, and the least opposition to the series
engine starting, and for operating other components on
field. When the switch is placed in the "OFF" position,
the vehicle.
the motor continues to operate until a cam on the crank
The NC-8 and a few other power units provide
arm strikes a switch button (not shown), opening the
convenience outlets (power receptacles) of ac and dc
circuit and placing the wiper in the park position.
for use by the maintenance crews in performing their
Some windshield wiper systems have a windshield
duties. In this application, the power generating system
washing unit. The washers are energized by depressing
of the vehicle provides power to the outlets. Through
a button, which is part of the control switch. When the
proper placement of switches, power can be obtained
button is depressed, the washer relay coil is energized,
for test equipment, soldering irons, etc., so tests and
placing the pump in operation.
repairs can be made without having to move to shop or
hangar spaces.
NOTE: Electrical windshield wipers, heater fans,
defroster fans, and other accessories are driven by small
LIGHTING SYSTEM
electrical motors. The amount of current drawn will
depend on the particular motor and the load placed on
The lighting system consists of all the lights, control
it. The motors, with the exception of the starting motor,
devices, protection devices, and wiring harnesses neces-
should be connected to the ammeter so that any battery
sary for a piece of equipment to do its assigned job. The
current used to run them is indicated by the ammeter.
following paragraphs describe the components of the
The construction and theory of operation of ac and dc
lighting system and suggested maintenance procedures.
motors are covered in detail in NEETS, Module 5.
Headlights
HEATERS AND DEFROSTERS
A cutaway view of a sealed beam headlight is
Hot water heaters are part of the engine cooling
shown in figure 6-93. In this type of light, the filament,
system, since they circulate the cooling liquid from the
reflector, and lens form a single unit. The unit is
engine through the heater radiator. The heater has an
completely sealed so the reflector does not become
electrically driven fan, which circulates air through the
tarnished and the light output is not seriously affected
heater radiator so that the output air is warmed.
by age. The filament is correctly focused in relation to
The defroster operates in a manner similar to the
the lens and reflector by the manufacturer, and the
heater, usually deriving heat from the same heater
alignment (aiming) is the only adjustment that can be
radiator. The defroster, however, directs the flow of
made. Connection into the circuit is made by blade-type
warm air against the windshield to prevent condensa-
connectors and a bayonet plug.
tion or freezing of moisture.
Two filaments are provided in the sealed beam unit.
The only service the heater will normally require is
One provides an upper or high beam; the other provides
a periodic flushing out when the engine radiator is
a depressed or low beam. When either filament burns
flushed. A defective heater fan motor is usually
out, corrective action requires that the whole unit be
replaced with a new one. However, it is possible to
replaced. However, the sealed beam unit has a greater
replace bearings, armature, brushes, and certain other
filament life expectancy than other types of bulbs and
small parts under emergency conditions.
requires no maintenance during its lifetime.
6-74
