THEORY OF OPERATION - CONTINUED
AIR SYSTEM - CONTINUED
Air Compressor. The engine runs
a two-cylinder air compressor directly off the timing gears. The engine water pump
moves coolant through the air compressor when the engine is running. Lubrication for the air compressor bearings is
provided by the engine oil pump.
Governor. The governor keeps the air pressure in the air reservoirs constant. When the engine is started, the compressor
runs until the reservoir pressure is at the cutout setting of approximately 122 psi (841 kPa). At the cutout setting, air
pressure moves the governor piston, letting air go through the governor to the unloading valves in the compressor. Air
pressure holds the unloading valves open to stop the delivery of air from the compressor. When the air pressure in the
system is at the cut-in setting of approximately 101 psi (696 kPa), the force of the governor spring moves the governor
piston, stopping the air flow to the unloading valves. The unloading valves close and the compressor again sends air to
Relief Valve. The relief valve, mounted on one of the tractor reservoirs, protects the entire system in the event of gover-
nor failure. This valve will open at approximately 150 psi (1,034 kPa).
Relay Valve. The relay valve for the
brakes controls the flow
of air from the reservoir
Quick Release/Holding Valves. The quick release/holding valves release the air from the tractor brake actuators. Each
valve has three positions. In the OPERATE position, pressurized air goes through the valve into the brake actuators. In
the HOLD position, pressurized air is held in the brake actuators. In the RELEASE position, air in the brake actuators is
released through an exhaust passage. This quick release/holding valve ensures that the parking/emergency brake does
not partially engage during service brake operation.
Double Check Valves. Each double check valve lets air from only one of the two inlet passages go through the outlet
passage to the brake actuators. When air goes through one inlet passage, a shuttle cock moves against the other inlet pas-
sage to stop the flow of air. This double check valve is used as an anti-compounding valve to ensure that the service
brake and the parking/ emergency brake cannot be operated at the same time.
Retarder Hand Control Valve. The retarder hand control valve controls the braking capacity of the retarder by releas-
ing pressurized air to the hydraulic control valve at a specific rate. In the released position, no pressurized air is sent to
the hydraulic control valve. As the handle for the control valve is moved toward the operator, the braking capacity of the
retarder increases. The control valve controls the pressure of the air between approximately 27 and 80 psi (186 and 552
Regulator for Retarder Control Valve. The regulator for the retarder control valve is in the air line to the hydraulic
control valve for the retarder. It controls the air pressure from the retarder control valve to the maximum of 65 psi (448
Pressure Protection Valve. All air flow from the service brake circuit to the accessory air circuit must go through the
pressure protection valve. Air will not go through the valve until the pressure at the supply passage is 75 psi (517 kPa).
This ensures pressure for braking.
Differential Control Lock Valve. The control valve for the differential lock controls the supply of air to the piston in
the differential lock and to the control passage of the pilot valve.
Transmission Control Valve. The hold control valve controls the supply of air to the hydraulic control valve in the
transmission, preventing automatic shifting.
Brake Actuator. The brake actuators receive pressurized air from this brake control valve. These actuators turn air pres-
sure into mechanical force. They apply braking action to the wheels in direct proportion to the air pressure.
Brake Control Valve. The brake control valve located in the operator compartment directly controls brake actuation.
When the brake control valve is opened, air is directed through a metered orifice to the double check valves, then to the
quick release valves, and finally to the tractor-portion brake actuators.
The air is also sent to valves on the scraper-portion. Upon actuation, the valves open to supply pressurized air to the
scraper-portion brake actuators. The scraper brakes operate before the tractor brakes, due to the metered orifice in the air