SPRING APPLIED HYDRAULIC RELEASE (SAHR) SERVICE BRAKES AND PARKING BRAKE. The SAHR
(spring applied hydraulic release) brakes are located in the pilot drive motors and are engaged by spring tension
when the machine is stationary. The brakes are supplied hydraulic fluid from a multivalve which releases the
brakes when the parking brake button is released. When the machine stops, the hydraulic fluid to the brakes is cut
off. The brakes are automatically engaged by spring tension.
HEATING, VENTILATION, AND AIR CONDITIONING (HVAC) SYSTEM
1. HVAC HEATING OPERATION. The heating system consists of the engine, coolant pump, thermostat,
radiator, heater core, temperature control valve, and blower motor. When the engine reaches operating
temperature the thermostat opens, allowing coolant to be circulated through the heating system by the coolant
pump. Hot coolant then moves from the engine through hoses to the temperature control valve where the
amount of coolant flowing through the heater core is controlled. Then coolant returns through hoses back to
the engine. The blower motor then circulates air over the heater core, extracting heat from the coolant and
warming the cab.
2. HVAC COOLING OPERATION. The cab cooling system consists of an engine-driven refrigerant compressor,
a condenser mounted in front of the engine cooling pack, a refrigerant dryer unit, an expansion valve, and an
evaporator coil. The compressor pumps refrigerant and oil around the system. It raises the temperature and
pressure of the refrigerant gas and forces it to the condenser where it changes state and becomes a liquid. The
compressor also sucks the vaporized refrigerant out of the evaporator and back inside itself in the form of gas.
The refrigerant leaves the compressor and moves through a high-pressure hose to the condenser. Inside the
condenser the gas changes state and becomes a liquid. It is still hot and under pressure. The liquid refrigerant
continues to move inside the system, out of the condenser to the receiver/dryer. The receiver/dryer serves as a
storage tank and filter for the refrigerant. When the refrigerant moves from the receiver/dryer, it travels to an
expansion valve at the inlet to the evaporator coil. The expansion valve bleeds high-pressure refrigerant into
the evaporator coil, where the pressure is low. The refrigerant expands rapidly in this low-pressure environ-
ment. When it expands it changes state to a gas and rapidly cools and absorbs heat from the air as the blower
forces air through the fins.
Any moisture in the air (humidity) condenses on the fins of the evaporator as water droplets, which drain
through a drain tube from the A/C unit. This action dehumidifies the air in the cab, contributing to driver com-
fort. Cab air forced across the evaporator coil gives up its heat energy to the cold refrigerant inside the coil.
The cooled air circulates through the cab. The refrigerant continues to expand and absorb heat energy in the
evaporator coil. The refrigerant changes from a liquid to a gas before it leaves the evaporator coil on its way
back to the compressor to start the cycle again.
3. CONDENSER. Allows the refrigerant gas contained in the machine A/C system to be cooled and condensed.
As the refrigerant gas flows through the system, air is drawn over the condenser by machine movement and
the action of the engine cooling fan removes excess heat.