Two separate brake systems are used on the 793C Off-highway Truck.
The two brake systems are: the parking/secondary brake system and the service/retarder brake system.
The parking/secondary brakes are spring engaged and hydraulically released. The service/retarder brakes are engaged hydraulically by an air-over-oil brake system.
The 793C Truck is also equipped with an air system. An engine driven air compressor supplies the air and fills two reservoirs. Air from the
reservoirs provides energy to perform several functions:
1. Engine start-up
2. Service and retarder brake control 3. Secondary and parking brake control 4. Windshield washer and wiper
5. Automatic lubrication injection 6. Horn
7. Exhaust bypass (wastegate) control
793C
AIR SYSTEM AND BRAKES
• Two brake systems:
- Parking/secondary brake system - Service/retarder
brake system
• Air system functions
• Oil cooled brake assembly
• Duo-Cone seals prevent oil from leaking or transferring
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Shown is a cutaway illustration of an oil cooled brake assembly. The brakes are environmentally sealed and adjustment free. Oil continually flows through the brake discs for cooling. Duo-Cone seals prevent the cooling oil from leaking to the ground or transferring into the axle housing. The wheel bearing adjustment must be maintained to keep the Duo-Cone seals from leaking.
The smaller piston (yellow) is used to ENGAGE the secondary and parking brakes. The parking brakes are spring ENGAGED and hydraulically RELEASED.
The larger piston (purple) is used to ENGAGE the retarder/service brakes.
The retarder/service brakes are engaged hydraulically by an air-over-oil brake system.
• Small piston
ENGAGES secondary and parking brakes
• Large piston ENGAGES retarder/service brakes
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Operator Controls
Several brake system control components are located on the center console at the right of the operator's seat.
The parking brake air switch (1) controls the flow of air to the parking brake release valve.
The windshield wiper/washer switch (2) controls the flow of air to the pump in the windshield washer reservoir and to the wiper motor in front of the cab.
The brake retraction switch (3) is an electrical switch used to activate the electric pump that supplies oil for towing.
The Traction Control System (TCS) switch (4) is used to test the TCS (formerly referred to as the "Automatic Electronic Traction Aid").
1 2 3
4
3. Brake retraction switch
4. TCS switch 2. Windshield
wiper/washer switch 1. Parking brake air
switch
• Secondary brake lever (red)
• Retarder lever (black)
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Operator controls on the steering column are the secondary brake lever (red) and the retarder lever (black).
The secondary brake lever allows the operator to modulate the
engagement of the parking brakes. The secondary brake lever engages the same brake system as the parking brake air switch.
The retarder brake lever allows the operator to modulate the engagement of the service brakes. The retarder brake lever engages the same brake system as the service brake pedal (shown in the next slide).
• Service brake pedal (arrow)
• Horn control button (not shown)
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On the floor is the service brake pedal (arrow).
Located on the floor to the left of the steering column is the horn control button (not shown).
• Air compressor
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Air Charging System
The air system is charged by an air compressor mounted on the left front of the engine.
System pressure is controlled by the governor (arrow). The governor maintains the system pressure between 660 and 830 kPa (95 and 120 psi).
• Air compressor governor (arrow)
1. Air dryer
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Air flows from the air compressor to the air dryer (1) located in front of the left front tire. The air dryer removes contaminants and moisture from the air system. The condition of the desiccant in the air dryer should be checked every 250 hours and changed periodically (determined by the humidity of the local climate).
When the air compressor governor senses that system air pressure is at the cut-out pressure of 830 kPa (120 psi), the governor sends an air pressure signal to the purge valve through the hose (2). The purge valve opens and air pressure that is trapped in the air dryer is exhausted through the
desiccant, an oil filter and the purge valve.
An air system relief valve (3) is located on the air dryer. A heating element (4) prevents moisture in the dryer from freezing in cold weather.
2 4 1
3
3. Air system relief valve
4. Heating element 2. Purge valve signal
hose
1. Service/retarder brake reservoir
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Air flows through the air dryer and fills two reservoirs. The
service/retarder brake reservoir (1) is located on the right platform. This reservoir also supplies air for the air start system.
The second reservoir is located behind the cab and supplies air for the parking/secondary brake system.
A relief valve (2) is installed in the service/retarder brake reservoir. This relief valve serves as a back-up for the relief valve on the air dryer.
Condensation should be drained from the tank daily through the drain valve (3).
2 1
3
3. Condensation drain valve
2. Relief valve
• Parking/secondary brake reservoir
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Located behind the operator’s station is the parking/secondary brake air reservoir. A drain valve is located on the right side of the cab. Moisture should be drained from the reservoir daily through the drain valve (see Slide No. 28).
A check valve (arrow) prevents a loss of air if an air line breaks upstream of the air reservoir.
• Check valve (arrow)
1. Pressure protection valve
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Located behind the operator’s station is a pressure protection valve (1). If one of the accessory circuits fails, the pressure protection valve maintains a minimum of 482 kPa (70 psi) in the service brake circuit.
Also located behind the operator’s station is the air system pressure sensor (2). The air system pressure sensor provides an input signal to the VIMS which informs the operator if a problem exists in the air system.
The solenoid air valve (3) provides controlled supply air for the automatic lubrication system. The solenoid air valve is controlled by the VIMS.
The VIMS ENERGIZES the solenoid ten minutes after the machine is started. The VIMS keeps the solenoid ENERGIZED for 75 seconds and then DE-ENERGIZES it. Every 60 minutes thereafter, the VIMS ENERGIZES the solenoid for 75 seconds until the machine is stopped (turned off). These settings are adjustable through the VIMS keypad in the cab.
2 1
3
2. Air system pressure sensor
3. Automatic
lubrication solenoid air valve
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