AIR OVER HYDRAULIC BRAKE SYSTEM - CONTINUED
EMERGENCY RELAY VALVE controls the flow of air from the towing vehicle to the different parts of the
semitrailer braking system. It speeds brake application by releasing air from the air reservoir on the
semitrailer directly to the brake air chamber. This valve controls the flow of air to and from the
semitrailer air reservoir and automatically applies the brakes if the semitrailer breaks away from the
truck tractor or if there is a serious leak in the emergency air line. The emergency relay valve allows
applied air to vent through its exhaust port.
WHEEL BRAKE MECHANISM involves an air chamber attached to a hydraulic master cylinder which
provides the means of converting the energy of compressed air into the hydraulic pressure necessary
to operate the semitrailer wheel brakes. The wheel brake mechanism is located within the
brakedrum. When the brakes are applied, the wheel cylinder pistons apply equal pressure against
each brakeshoe. As the shoe linings come into contact with the drum, friction develops. The
rotation of the drum pulls the shoes against the drum surface to add to the hydraulic force acting on the
shoes and produces additional braking action. Springs pull shoes away from the drum when
pressure is released.
ELECTRICAL SYSTEM
This shop van semitrailer is equipped with two separate electrical systems. One is a 24-volt system,
the other is a 110-volt system. The 24-volt system receives its power from the towing vehicle through
an intervehicular connector which attaches to a receptacle at the front of the semitrailer. The 110-
volt system receives its power from an outside source, and its receptacle is located at the right front of
the semitrailer.
24-VOLT SYSTEM - EARLY AND LATE M146
The 24-volt system has a single wire harness which leads from the receptacle to the lights.
SERVICE CLEARANCE LIGHTS are located on the upper corners of the right and left sides of the van
body for the early M146. The late M146 has additional clearance lights located on the upper front
and rear corners, with three mounted on the top middle of the rear of the van body. They go on when
either the towing vehicle clearance lights or service lights are turned on. They go off automatically
when the blackout lights are turned on.
CLEARANCE BLACKOUT LIGHTS are directly below the service clearance lights. They goon only
when the blackout light switch in towing vehicle is turned on.
TAIL, TURN, STOP, AND BLACKOUT LIGHTS appear at the rear of the semitrailer. There are two
service taillights at each rear corner of the semitrailer containing three lamps. One lamp functions
as a taillight when the service lights are turned on, and one lamp functions as both turn and stoplight.
One lamp functions as a blackout light when the blackout light switch is turned on. The blackout stop
light is located on the right rear corner of the semitrailer. The blackout lights automatically turn off
the tail, stop, turn, and clearance lights if both switches are on at the same time.
Some semitrailers may have composite light assemblies which control stop, tail, turn, blackout tail,
and blackout stop with four lamps in one housing.
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