TM 9-2350-247-20-1
THEORY OF OPERATION — Continued
0003 00
ENGINE COOLANT HEATER KIT
Engine coolant heater kit provides heat for starting the carrier during extreme cold weather operation between –25
and –65
F
(–31
and –54
C). Heater warms and circulates the engine coolant through the engine and a battery box heat exchanger which
warms the engine block, lubricating oil, and battery electrolyte when the engine is not in operation.
Engine coolant heater produces heat by burning a mixture of fuel and air in a heat exchanger. Air is supplied for combustion
by a blower through a connecting tube to the burner air inlet in the heat exchanger. Electrical components are connected to the
wiring harness through a bracket mounted terminal strip located on the side of the heater.
Heater has fuel, exhaust, and electrical systems. Fuel system consists of a fuel pump, fuel manifold, fuel control valve, and
related fuel lines and fittings. Exhaust system consists of an exhaust pipe with a removal moisture trap, which carries exhaust
gases out of the heater. Electrical system consists of a coolant heater control box and a wiring harness which supplies power
to heater control box, coolant heater fuel pump, and coolant pump.
Engine coolant heater kit has a coolant circulating system, which consists of an electrically operated coolant pump. The pump
circulates coolant through the heater and engine battery box heat exchanger plate and back through the coolant heater.
AIR BRAKE KIT (M548A1)
The air brake kit provides regulated, pressurized air to operate the trailer equipment air brakes. The kit’s main components
are a compressor, reservoir, governor, safety valve, and stop light and air low pressure switches. Other components are a
treadle valve pedal, dial-type air pressure indicator and warning light, disconnect and air couplings, and dummy fittings.
Compressor is engine driven and air cooled. It charges a 1,100 cubic inch (0.016 cu m) storage reservoir at the rate of 7 1/4
cfm (0.2 cu M) at an engine speed of 1250 rpm. Air is drawn through a strainer into compressor cylinders, compressed by
pistons, and then forced through discharge valves and a hose and a tube into a reservoir.
Reservoir stores pressurized air for brake operation and is a place for air, heated during compression, to cool. Cooling causes
oil and water to condense to form an oil-water emulsion.
The governor receives air from the reservoir at one of it reservoir ports. Air acts on the piston and the inlet and discharge
valve. When air pressure reaches a cutout setting of the governor, piston and inlet and discharge valve move up. The
discharge stem of the inlet passage opens and allows reservoir air to flow by the open inlet valve through a drilled passage in
the piston and out to the unloading mechanism in the compressor. Air flows around the piston and acts on an additional area
of the piston, assuring full opening of the inlet passage. As air pressure drops to governor cut-in setting, force exerted by air
setting spring moves the piston down, which causes the inlet stem of the inlet and discharge valve to set. The discharge
passage opens and allows air at the compressor unloader pistons to escape back through the piston and discharge stem and out
the discharge port.
A spring-loaded ball check safety valve protects the air brake system against air pressure above 150 psi (1034 kN/sq cm). The
valve lifts and lets air discharge if pressure in reservoir rises above 150 psi (1034 kN/sq cm).
The air brake kit has a stop light and air low pressure switches. The stop light switch is an electro-pneumatic switch, which
operates in conjunction with the brake valve and stop light and closes the stop light electrical circuit when the brakes are
applied. The air low pressure switch is a safety device, which lights a warning light on the air brake instrument panel when
reservoir air pressure falls below 60 psi (418 kN/sq cm) and closes electrical contacts on the rubber diaphragm with spring
pressure.
Other components are a treadle valve pedal, dial-type air pressure indicator and warning light, disconnect and air couplings,
and dummy fittings. Compressed air flow to the towed load is controlled by foot pressure on the treadle valve pedal, which
bears down on a plunger in the treadle valve. Treadle valve pressure also actuates the stop light switch. A dial-type air
pressure indicator and warning light, which show condition of the air brake system, are on the air brake instrument panel.
Disconnect couplings are located on the rear cab bulkhead above the left power plant compartment grille and connect to
service and emergency air brake hoses that run along left side of hull to rear of carrier. Two air couplings on the hose ends
permit connections to towed equipment brake couplings, are closed by dummy fittings when not in use, and are stowed on the
cargo door.
0003 00-16
