BRAKE SYSTEM ON HAULTRUCK 789D

789D Off-Highway Truck 

BRAKE SYSTEM.

Identify air system components

INTRODUCTION
The 789D truck is equipped with an air system similar to the 789C truck. The air system provides energy to perform the following functions:
* Engine start-up (if not equipped with optional electric starting system)
* Service and retarder brake control
* Secondary and parking brake control
* Automatic lubrication injection (grease)
* Horn operation and cab clean-out

The parking/secondary brakes are spring engaged and hydraulically released. The service/retarder brakes are engaged hydraulically and released with spring force.
The service/retarder brakes and the parking/secondary brakes are activated by the air system.

The 789D truck is equipped with a Hydraulic Automatic Retarder Control (HARC) and Traction Control System (TCS). The ARC and TCS are electronically controlled by the Brake ECM.

Changes to the air and brake system from the 789C truck include:
* Three air reservoirs if the truck is equipped with air starting (two air reservoirs on electric start            trucks)
* Component location changes
* New parking brake reset valve
* One air dryer only (the second air dryer has been removed)

This image shows the following main components in the air and brake systems: 
* Air start reservoir (air start trucks only) (1)
* Front slack adjuster (2)
* Brake retract pump (3)
* Parking/secondary brake release valve (4)
* TCS valve (5)
*  HARC valve (6)
* Brake makeup tank (7)
* Rear brake cylinders (8)
* Brake relay valve (9)
* Front brake cylinder (10)
* Main air reservoir (11)

Explain function of air system

AIRSYSTEM
This schematic shows the main components and air flow in the 789D air system. The components have been color coded to aid in explaining the air circuits.
The air supply circuit (yellow) supplies air to operate the air system components. The air compressor (1), which is driven by the engine, sends air to the air dryer (2) and main air reservoir (3). The main air reservoir supplies air to:
* Service brake relay valve (4) for operation of the service/retarder brakes  (blue)
* Starter relay valve (5) for operation of the air starter motor (6)
* Pressure protection valve (7) in the rear of the cab (17)
* Retarder valve (8) and service brake valve (9) in the front of the cab (18)

The service brake valve and retarder brake valve direct supply air to activate the service brake relay valve. The service brake relay valve directs air to the front brake hydraulic cylinder (27) and the rear brake hydraulic cylinders (28) to engage the service brakes. The double check valve (29) separates the service brake valve and the retarder valve so that air is provided to the service brake relay valve from only one source.

Supply air flows through the pressure protection valve to the secondary air reservoir (16), the air start solenoid valve (15), and the accessories. If the secondary air lines or an accessory circuit fails, the pressure protection valve maintains a minimum of 482 kPa (70 psi) in the service/retarder brake circuit.

The accessory components include:
* Green: autolube tank (10) and autolube solenoid (11)
* Light blue: Air horn (12) and air horn solenoid (13)
* Cab clean out gun (14)

If the truck is equipped with an air start system (orange), an optional air starter reservoir (19) supplements the main air reservoir to provide the air supply to the air start system.
The secondary air reservoir, in the rear of the cab, provides air to the following secondary/parking brake circuit components (red):
* Parking brake valve (20)
* Parking brake reset valve (21)
* Inverter valve (22)
* Secondary brake valve (23)

Secondary/parking brake release valve (24) .The secondary air reservoir also provides air to the optional brake cooling diverter solenoid (25) and diverter valve (26). The air pressure sensor (30) sends a signal to the Brake ECM indicating air system pressure. The retarder pressure (32) switch informs the Brake ECM when retarder air pressure is present and turns on the retarder lamp. The service/retarder pressure switch (31) informs the Transmission/Chassis ECM when retarder or service brake air pressure is present. The parking/secondary brake pressure switch (33) informs the Transmission/Chassis ECM when parking or secondary brake air pressure is present and turns on the parking brake lamp.

Identify air supply circuit components and explain air flow.

Air Supply Circuit 
The air supply circuit supplies air to operate the air system components. The air compressor (1), which is driven by the engine, sends air to the air dryer (2) and main air reservoir (3).
The governor (4) maintains the system pressure between approximately 660-830 kPa (95-120 psi).
The air dryer removes contaminants and moisture from the air system and includes a vent (5) and external air supply connection (6).
The main air reservoir includes a drain valve (7), a relief valve (8), and a check valve (9). The check valve prevents air loss if a leak upstream of the tank occurs.

The main air reservoir supplies air to: 
* Service brake relay valve (10) for operation of the service/retarder
* brakes Starter relay valve (11) for operation of the air starter motor (on trucks • with air starting)
* Pressure protection valve (12) in the rear of the cab (13)
* Manifold for the retarder/service brake valves (14)

Supply air flows through the pressure protection valve to the secondary air reservoir (15), the air start solenoid valve (16), and the accessory circuits (17). If the secondary air lines or an accessory circuit fails, the pressure protection valve maintains a minimum of 482 kPa (70 psi) in the service/retarder brake circuit. The air pressure sensor (18) sends a signal to the Brake ECM indicating air system pressure.

Identify air compressor and governor at left front of engine

The air system is charged by an air compressor (1) mounted on the left front of the engine.
System pressure is controlled by the governor (2). The governor maintains the system pressure between 660 and 830 kPa (95 and 120 psi).

The governor setting can be adjusted with a screw below the cover on top of the governor. Turn the adjustment screw OUT to increase the pressure, and IN to decrease the pressure.
To test the air compressor efficiency, lower the air system pressure to 480 kPa
(70 psi). Start the engine and raise the engine speed to HIGH IDLE. When the air system pressure reaches 585 kPa (85 psi), measure the time that it takes to build system pressure from 585 kPa (85 psi) to 690 kPa (100 psi).

The time to raise the pressure should be 50 seconds or less. If the time recorded is greater than 50 seconds, check for leaks or a restriction in the system. If no leaks or restrictions are found, the air compressor may have a problem.

Identify air dryer and ground level air connector behind left front tire.

Air flows from the air compressor to the air dryer (1) located behind the left front tire.
The air system can be charged from a remote air supply through a ground level air connector (2) inside the left frame.

The air dryer removes contaminants and moisture from the air system. The condition of the desiccant in the air dryers 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 approximately 830 kPa (120 psi), the governor sends an air pressure signal to the purge valve in the bottom of the dryers. The purge valve opens and air
pressure that is trapped in the air dryers is exhausted through the desiccant, an oil filter, and the purge valve. An air system relief valve is located on the air dryers to protect the system if the air compressor governor malfunctions. A heating element in the bottom of the dryers prevents moisture in the dryers from freezing in cold weather.

Identify main air reservoir and valves at left side of truck

Air flows through the air dryers and fills the main air reservoir (1), which is located on the left side of the truck.
Condensation should be drained from the tank daily through the drain valve (2).
A relief valve (3) is located in the top of the tank. This relief valve protects the air system when the air dryer has exhausted and the ball check valves in the air dryer
outlet ports close. The check valves separate the air system from the air dryer relief valves.

Identify pressure protection valve and air system pressure sensor behind operator’s station.

Located behind the operator’s station is a pressure protection valve (1). Supply air flows from the main air reservoir, through the pressure protection valve, to the secondary air system and accessories. The pressure protection valve opens at 550 kPa (80 psi) and closes at 482 kPa (70 psi). If the secondary air lines or an accessory circuit fails, the pressure protection valve maintains a minimum of 482 kPa (70 psi) in the service/retarder brake circuit.

To test the pressure protection valve, drain the air pressure to approximately 345 kPa (50 psi). Use the VIMS display to observe the brake air pressure. With the engine running at LOW IDLE, press the horn button. Record the air pressure when the horn  sounds. This pressure reading is the open setting of the pressure protection valve. Slowly drain the air pressure and record the air pressure when the horn turns off. This pressure reading is the setting of the pressure protection valve when it closes. The air system pressure sensor (2) provides an input signal to the Brake ECM. The Brake ECM sends a signal to the VIMS, which informs the operator if a problem exists in the air system.

Identify air starting system components.

Air Starting System 
The 789D truck can be equipped with an air starting system, as shown in this image, or an optional electric start system.
Air from the main air reservoir (1) supplies air to the air start reservoir (2). Air from both tanks supply air to the air start relay valve (3). Air from the secondary air tank in the rear of the cab supplies air to the air start solenoid valve (4).

When the ignition key is turned to START, the air start solenoid directs air from the secondary tank to the air start motor (5). The solenoid also activates the start relay valve, which directs air from the main air reservoir and air start reservoir to the start motor to crank the engine.
NOTE: If the truck is equipped with an electric starting system, the air start reservoir is not installed.

Identify accessory air circuit components and explain air flow

Accessory Air Circuit 
This schematic shows the components in the accessory air circuit. Air from the main air reservoir (1) flows to the pressure protection valve (2) in the rear of the cab (3). From the pressure protection valve air flows to the autolube solenoid (4) and the horn solenoid (5) in the rear of the cab, and to the clean-out gun (6) in the operator’s station. Air from the pressure protection valve also supplies air to the secondary air reservoir (7) and the air start solenoid (8).

The autolube solenoid air valve provides a controlled air supply for the autolube tank (9). The solenoid air valve is controlled by the Transmission/Chassis ECM. The ECM ENERGIZES the solenoid 10 minutes after the machine is started. The ECM keeps the solenoid ENERGIZED for 75 seconds and then DE-ENERGIZES it. Every 60 minutes thereafter, the ECM ENERGIZES the solenoid for 75 seconds until the machine is stopped (turned off). These settings are adjustable through the VIMS keypad in the cab. The horn solenoid controls the air supply to the air horn (10).

Explain operation of service brake/retarder air circuit with retarder lever released and service brakes engaged

Service Brake/Retarder Air Circuit 
This schematic shows the components and air flow through the service/retarder brake air circuit when the retarder lever is RELEASED, and the service brakes are ENGAGED.
Supply air flows from the main air reservoir (1) to the service brake relay valve (2), to the service brake valve (3), and to the retarder valve (4).

The retarder valve blocks air flow to the double check valve (5). The service brake valve directs air to the double check and moves the check ball. Air pressure from the service brake valve flows through the double check valve to the service brake relay valve. The service brake relay valve opens and metered air flows from the main air reservoir to the front brake hydraulic cylinder (6) and to the rear brake hydraulic cylinders (7). The relay valve reduces the time required to engage and release the
brakes. Air from the service brake valve also flows to the service brake/retarder pressure switch (8). Depressing the service brake pedal turns ON the brake lights and changes the transmission shift points and anti-hunt timer. When the manual retarder lever is moved, the retarder valve directs air to the double check and moves the check ball.

Air pressure from the retarder valve flows through the double check valve to the service brake relay valve. The service brake relay valve opens and metered air flows from the main air reservoir to the front brake hydraulic cylinder and the rear brake hydraulic cylinders.
Air from the retarder valve also flows to the retarder pressure switch (9) and the service brake/retarder pressure switch. Engaging the retarder turns ON the retarder dash lamp, the brake lights, and changes the transmission shift points and anti-hunt timer.

Identify retarder valve and retarder lever.

The retarder valve (1) is controlled by the retarder lever (2) in the cab. The retarder valve directs air to the service brake relay valve.
The retarder lever is used to modulate the service brake engagement by metering the amount of air flow to the service brake relay valve. The retarder engages the same brakes as the service brake pedal, but is easier to control for brake modulation.
The retarder system allows the machine to maintain a constant speed on long downgrades. The retarder will not apply all of the normal braking capacity.

Identify components in front of the cab.

The service brake valve (1) is controlled by the brake pedal in the cab. Supply air for the service brake valve and the retarder valve is supplied by the main air reservoir through a manifold (2).
The service brake valve engages the same brakes as the retarder, but does not control brake modulation as precisely as the retarder.

Air from either the service brake valve or the retarder valve flows through the double check valve (3) to the service brake relay valve. If the retarder and the service brakes are engaged at the same time, air from the system with the highest pressure will flow through the double check valve to the service brake relay valve.

Air from the retarder valve also flows through the double check valve to the retarder
pressure switch (4). The retarder switch turns on the amber retarder lamp on the dash in the operator’s station when the manual retarder is ENGAGED. Also visible in this image is the secondary brake valve (5).

Identify service brake relay valve and front brake cylinder

The service brake relay valve (1) receives metered air from the service brake valve
or the retarder valve. The relay valve is located on the crosstube inside the left frame rail. When the service brake valve or retarder valve is activated, the relay valve opens and metered air flows from the main air reservoir to the brake hydraulic cylinders. The front brake cylinder (2) is visible in this image. The brake relay valve reduces the time required to engage and release the brakes.

Identify parking/secondary brake air circuit components and explain air flow.

Parking/Secondary Brake Air Circuit 
This schematic shows the parking/secondary brake air circuit components and air flow.
Air flows from the main air reservoir (1) to the pressure protection valve (2). From the pressure protection valve, air flows to the secondary air reservoir (3). Supply air from the secondary air reservoir flows to the parking brake reset valve (4) and the secondary brake valve (5).

The parking brake reset valve prevents the truck from moving if the air pressure is too low. If the air pressure in the secondary air reservoir decreases to less than approximately 415 ± 35 kPa (60 ± 5 psi), the parking brake reset valve will pop out and block air flow to the parking brake release valve (6), which will engage the parking brake. The parking brake reset valve must be reset before the truck can be moved. To reset the parking brake reset valve, place the parking brake lever into the ON position and allow the system air pressure to increase to the maximum air pressure. When air pressure is above approximately 415 ± 35 kPa (60 ± 5 psi), air from the secondary air reservoir flows through the parking brake reset valve and inverter valve (7) to the parking brake valve (8).

When the parking brake valve is ON, air is blocked from flowing to the parking brake release valve, which will engage the parking brake. When the parking brake valve is OFF, air is directed from the secondary tank to the parking brake release valve, which will release the parking brake.
Normally, the secondary brake valve blocks air from the secondary air reservoir from flowing to the inverter valve. When the secondary brake pedal is depressed, the secondary brake valve directs air from the secondary tank to the inverter valve. The air pressure moves the inverter valve, and the inverter valve blocks the air from the secondary air reservoir from flowing to the parking brake release valve, which allows the parking brake to engage.

The secondary brake can be used to modulate parking brake engagement by metering the amount of air flow to the parking brake release valve. A parking/secondary brake pressure switch (9) is located in the air line between the parking brake valve and the parking brake release valve. The switch provides an input signal to the Transmission/Chassis ECM. When the parking or secondary brakes are ENGAGED, the switch signals the Transmission/Chassis ECM to allow rapid downshifts.

Identify components in rear cab compartment, left side

The secondary air reservoir (1) and the inverter valve (2) are located in the rear cab compartment on the left side.
A drain valve and the parking brake/secondary brake pressure switch are located on the right side of the rear cab compartment. Moisture should be drained from the air reservoir daily.
A check valve (3) prevents a loss of air if an air line breaks upstream of the air reservoir.

When the secondary brakes are engaged, air flows from the secondary brake valve
to the signal port (4) of the inverter valve. The inverter valve then blocks the air flow from the secondary brake tank to the parking brake release valve. Blocking the air from the parking brake release valve drains the oil from the parking brakes, which allows the springs in the parking brake to ENGAGE the brakes.

Identify brake hydraulic system components and explain oil flow

BRAKEHYDRAULIC SYSTEM
This schematic shows the main components and oil flow in the brake hydraulic system.
Oil from the parking brake release pump (1) flows through the parking brake release filter (2) to the parking brake release valve (3) and to the following components:
* Torque converter lockup clutch in the torque converter (19)
* HARC valve (5)
* TCS valve (6)
   Hoist valve (7)
The parking brake release valve controls the oil from the parking brake release pump to the TCS valve. When the parking brake is activated by the air system, the parking brake release valve directs the oil to the TCS valve. The TCS valve directs the oil to the parking brakes to release the brakes. The parking brake release valve also directs oil to the makeup tank. The parking brake release pump provides signal oil to the TCS valve, which opens a check valve in the TCS valve and allows oil from the parking brakes to the tank.

The HARC valve directs oil from the parking brake release pump to the rear slack adjuster (9) and rear brakes (14)-(15) when the HARC is activated. The brake hydraulic cylinders (10) are activated by the air system when the service brake pedal is depressed or the retarder lever is moved.
When activated, the brake cylinders direct oil from the makeup tank (8) to the front and rear brakes to stop the truck. The electrically driven brake retract pump (11) provides oil to release the parking brakes and lower the truck bed for towing.

Identify brake system components.

Service Brake/Retarder Hydraulic Circuit 
The brake cylinders (1) operate by air-over-oil. The rear brake cylinders are shown in this image. When the metered air enters the brake cylinders, a piston moves down and pressurizes the oil in the bottom of the cylinders. The front cylinder supplies oil to the front brakes through a slack adjuster. The rear brake cylinders supply oil to the rear brakes through a separate slack adjuster.
As the brake discs in the brake assemblies wear, more oil is needed from the brake cylinders to compensate for the wear. The brake makeup oil tank (2) supplies makeup oil for the brake cylinders.

Oil from the parking brake release valve flows through an orifice and a screen to provide a continuous supply of oil to the makeup tank. Low flow to the makeup tank can cause the makeup oil reserve to decrease and cause the brake cylinders to overstroke.
To check for makeup oil flow, remove the cover from the makeup oil tank. With the engine at HIGH IDLE, a stream of oil filling the tank should be visible. If a stream of oil is not visible, the filter or hose to the tank may be restricted, or pump flow may be low.

Keep the service brake ENGAGED for at least one minute. If air is in the system or a loss of oil downstream from the cylinders occurs, the piston in the cylinder will overstroke and cause an indicator rod to extend and open the brake overstroke switches (3). The overstroke switches provide an input signal to the Brake ECM. The Brake ECM sends the signal to the VIMS, which informs the operator of the condition of the service/retarder brake oil circuit. If an overstroke condition occurs, the problem must be repaired and the indicator rod pushed in to end the warning. The oil-to-air ratio of the brake cylinder is approximately 6.6 to 1.

To test the brake cylinder, install a gauge in the fitting on top of the brake cylinder and a gauge on the pressure tap on the slack adjuster. When the service brakes are ENGAGED, if the air pressure in the brake cylinder is approximately 690 kPa (100 psi), the oil pressure measured at the slack adjuster should be approximately 4560 kPa (660 psi).

When the brakes are RELEASED, both pressures should return to zero. Inspect the condition of the breathers (4) for the brake cylinders. Oil should not leak from the breathers. Oil leaking from the breathers is an indication that the oil piston seals in the brake cylinder need replacement. Air flow from the breathers during a brake application is an indication that the brake cylinder air piston seals need replacement.

Explain operation of brake cylinder with brakes engaged

This visual shows a sectional view of the brake cylinder when the brakes are ENGAGED.
Air pressure from the brake relay valve enters the air inlet (1). The air pressure moves the air piston (2) and the rod (3) closes the valve in the oil piston (4). When the valve in the oil piston is closed, the oil piston pressurizes the oil in the cylinder.

The pressure oil flows to the slack adjuster (5). If air is in the system or a loss of oil downstream from the cylinders occurs, the piston in the cylinder will overstroke, which causes the indicator rod (6) to extend and open the brake overstroke switch. If an overstroke condition occurs, the problem must be repaired and the indicator rod pushed in to end the warning.

When the air pressure is removed from behind the air piston, the spring (7) moves the air piston and the attached rod opens the valve (8) in the oil piston. Any makeup oil that is needed flows into the passage at the top of the oil chamber, through the valve, and into the oil chamber at the right of the oil piston.

Identify components below truck bed.

The truck is equipped with two slack adjusters; one for the front brakes and one for the rear brakes. The front slack adjuster (1) is located in front of the crosstube and the rear slack adjuster (2) is located above the rear axle housing. The slack adjusters compensate for brake disc wear by allowing a small volume of oil to flow through the slack adjuster and remain between the slack adjuster and the brake piston under low pressure. The slack adjusters maintain a slight pressure on the brake piston at all times.

Brake cooling oil pressure maintains a small clearance between the brake discs.
The service brake oil pressure can be measured at a pressure tap (3) located on each slack adjuster.
Air can be removed from the service brakes through two remote bleed valves (not shown) mounted on the rear axle housing.
Also visible in this image is the makeup tank (4), the rear brake cylinders (5), and the front brake cylinder (6).

NOTE: Air can be removed from the front service brakes through bleed valves located on each wheel.

Explain operation of brake slack adjuster.

This image shows sectional views of the slack adjuster when the brakes are RELEASED and ENGAGED.
When the brakes are ENGAGED, oil from the brake cylinders (1) enters the slack adjusters and the two large pistons (2) move outward. Each large piston supplies oil to one wheel brake (3). The large pistons pressurize the oil to the service brake pistons and ENGAGE the brakes.
Normally, the service brakes are FULLY ENGAGED before the large pistons in the slack adjusters reach the end of their stroke. As the brake discs wear, the service brake piston will travel farther to FULLY ENGAGE the brakes. When the service brake piston travels farther, the large piston in the slack adjuster moves farther out and contacts the end cover. The pressure in the slack adjuster increases until the small piston (4) moves and allows makeup oil from the brake cylinders to flow to the service brake piston.

When the brakes are RELEASED, the springs in the service brakes push the service brake pistons away from the brake discs. The oil from the service brake pistons pushes the large pistons in the slack adjuster to the center of the slack adjuster. Makeup oil that was used to ENGAGE the brakes is replenished at the brake cylinders from the makeup tank. The spring behind the large piston causes some oil pressure to be felt on the service brake piston when the brakes are RELEASED. Keeping some pressure on the brake piston provides rapid brake engagement with a minimum amount of brake cylinder piston travel.

The slack adjusters can be checked for correct operation by opening the service brake bleed screw with the brakes RELEASED. A small amount of oil should flow from the bleed screw when the screw is opened. The small flow of oil verifies that the spring behind the large piston in the slack adjuster is maintaining some pressure on the service brake piston.
Another check to verify correct slack adjuster operation is to connect a gauge to the pressure tap on top of the slack adjuster and another gauge at the service brake bleed screw location. With system air pressure at maximum and the service brake pedal depressed, the pressure reading on both gauges should be approximately the same.

When the brakes are RELEASED, the pressure at the slack adjuster should return to zero. The pressure at the service brake bleed screw location should return to the residual pressure held on the brakes by the slack adjuster piston. Low residual pressure may indicate a failed slack adjuster. High residual pressure may also indicate a failed slack adjuster or warped brake discs.

To check for warped brake discs, rotate the wheel to see if the pressure fluctuates. If the pressure fluctuates while rotating the wheel, the brake discs are probably warped and should be replaced. To check for brake cooling oil leakage, block the brake cooling ports and pressurize each brake assembly to a maximum of 138 kPa (20 psi). Close off the air supply source and observe the pressure trapped in the brake assembly for five minutes. The trapped pressure should not decrease.

Identify components inside right frame rail

Parking/Secondary Brake Hydraulic Circuit 
Oil from the parking brake release pump (1) flows through the parking brake release filter (2) to the brake release valve (3) located inside the right frame rail. Oil flows from the parking brake release valve to the TCS valve and to the parking brake piston in the brakes when the parking brakes are released.

Supply air from the parking brake air valve in the cab or the secondary brake valve flows to an air chamber in the brake release valve. The brake release valve contains an air piston that moves a spool. The spool either directs oil to RELEASE the parking brakes or drains oil to ENGAGE the parking brakes. A relief valve in the brake release valve limits the system pressure for releasing the brakes.
Supply oil also flows from the brake release valve through an orifice and a screen to the brake oil makeup tank (4).

To release the parking brakes for service work or towing, the electric motor (5) that turns the towing pump (6) can be energized by the brake release switch located in
the cab. The pump sends oil to the brake release valve to RELEASE the parking brakes. Towing pump pressure is controlled by a relief valve in the towing pump.

Explain oil flow through towing system

Normally, supply oil flows from the parking brake release pump (1), through the parking brake release oil filter (2), to the parking brake release valve (3). If air pressure (8) is present from the parking brake air valve or the secondary brake valve, supply oil flows past the relief valve (4), the check valve (5), and the spool (6) to RELEASE the parking brake (7). The relief valve limits the parking brake release oil pressure to approximately 4700 ± 200 kPa (680 ± 30 psi).

This schematic shows the oil flow through the parking brake release system when the towing motor and pump (11) are activated and oil from the parking brake release pump has stopped. When the towing motor is energized, and air pressure is present above the parking brake release valve piston, the air pressure moves the spool in the parking brake release valve down to block the drain port.
Oil flows from the towing pump to the parking brake release valve and the parking brakes. The check valve (9) blocks the oil from the towing pump from flowing to the parking brake release pump. During towing, the parking brake release pressure is limited by the towing pump relief valve (12) in the towing pump. When the relief valve opens, oil transfers from the pressure side to the suction side of the towing pump. The setting of the relief valve is approximately 4480 kPa (650 psi). A check valve (10) in the outlet port of the towing pump prevents oil from flowing to the towing pump during normal operation.

To check the brake release system used for towing, connect a gauge to the parking brake release pressure tap on the rear axle. Use a long gauge hose so the gauge can be held in the cab. With the parking brake air valve in the RELEASE position
and the key start switch in the ON position, energize the parking brake release switch in the cab. The parking brake release pressure should increase to approximately 4480 kPa (650 psi). Turn off the switch when the pressure stops increasing. The parking brake release pressure must increase to a minimum of 3790 kPa (550 psi). The parking brakes start to release between approximately 3100 and 3445 kPa (450 and 500 psi). During towing, the brake release switch on the dash must be energized whenever the parking brake release pressure decreases below this level or the brakes will drag. The parking brakes are fully released between approximately 3445 and 3860 kPa (500 and 560 psi).

NOTE: A minimum of 550 kPa (80 psi) air pressure must be available at the parking brake release valve to ensure full release of the brakes for towing.

Identify Brake electronic Control System input and output components

BRAKEELECTRONIC CONTROLSYSTEM
This image shows the input and output components of the brake electronic control system. The Brake ECM (26) controls the Automatic Retarder Control (ARC), the Traction Control System (TCS), and the Rear Axle Lubrication (RAXL) system on the 789D trucks.
* Other components controlled by the Brake ECM are:
* Optional Front Brake oil Cooler Diverter Solenoid (14)
* Optional Powered Stairway Lamp (15)
* Retarder Engaged Lamp (16)
* Stoplamp Relay (17)

NOTE: The RAXL system was covered in the “Power Train” module. The ARC and the TCS system will be covered later in this module.

Explain operation of hydraulic ARC system with ARC enabled

Hydraulic Automatic Retarder Control (HARC) 
This schematic shows the oil flow in the HARC system when enabled.
The parking brake release pump (1) provides oil flow for the HARC system. Oil
from the parking brake release pump flows through a check valve (2) to the HARC valve (3). The HARC valve modulates the amount of pressure to the service brakes to control the ground speed of the truck. Oil from the HARC valve flows to the shuttle valves (4), which separate the brake cylinders (5) and service/retarder brakes from the HARC system. The system (HARC or service/retarder brake system) with the highest pressure will control the service brakes. The accumulator (6) keeps a constant oil pressure in the HARC system.

Identify engine speed sensor

The engine speed sensor (arrow) provides the primary input signal used by the ARC.
The engine speed sensor is a passive (two wire) sensor and is located on top of the flywheel housing. The engine speed information is the main parameter that the Brake ECM uses to control retarding. The engine speed sensor is a frequency sensor that generates an AC signal from the passing flywheel gear teeth.

ARC also uses the camshaft speed/timing sensor for diagnostic purposes. If the Brake ECM receives an input signal from the camshaft speed/timing sensor, but not the engine speed sensor, the Brake ECM will log an engine speed fault. The ARC will not function without an engine speed signal from the engine speed sensor.

Identify HARC components inside right frame rail

The HARC valve (1) is located inside the right frame rail. The HARC valve contains a supply solenoid valve (2) and a control solenoid valve (3). A purge solenoid valve is located on the bottom of the HARC valve. The HARC accumulator (4) is located to the rear of the HARC valve.

Explain operation of hydraulic ARC valve with engine on and ARC off

This image shows the HARC valve condition when the engine is ON and the ARC is OFF. Supply oil from the parking brake release pump (1) flows through the check valve (2) and into the HARC valve. The oil flows to the spool (3), to the accumulator (4), to the supply solenoid (5), and to the purge solenoid (6).

With the ARC OFF, the supply solenoid is de-energized and oil flows through the supply solenoid valve to the left end of the spool. The oil pressure and spring force moves the ARC spool to the right, which blocks supply oil from flowing to the control solenoid (7) and the service brakes (8).

Explain operation of hydraulic ARC valve with engine on and ARC on

When the ARC is activated, the Brake ECM energizes the supply solenoid (5). The supply solenoid valve directs oil to the right end of the spool (3) and oil on the left end of the spool is directed to the tank (9). The spool shifts left against spring force and directs oil from the pump (1) to the control solenoid valve (7).

The Brake ECM will send varying levels of current to the control solenoid. This
variable current will modulate the spool within the proportional valve. The level of current is dependent on the brake requirements for the HARC valve to maintain a constant braking force. When the control solenoid is energized, the pin moves to the right and pushes against the ball.
The ball blocks the pump supply oil from flowing to the tank. Pressure increases in the chamber to the left of the spool and moves the spool to the right. When the spool moves to the right, pump supply oil flows to the service brakes (8). To maintain the correct brake pressure, the Brake ECM will vary the current to the control solenoid to open and close the oil drain port.

Explain operation of hydraulic ARC valve with engine off and ARC off

With the engine OFF, no current is supplied from the Brake ECM to the supply solenoid (5). The supply solenoid valve directs any pressurized oil acting on the spool (3) to the tank (9). Current is supplied from the steering bleed control to the purge solenoid valve (6) for approximately 70 seconds, which allows the pressure in the accumulator (4) to drain to the tank.

Identify steering bleed down control

The steering bleed down control (arrow) is located in the compartment behind the cab. The steering bleed down control is used to purge the ARC accumulator when the machine is shut down. When the control receives a signal from the key start switch, a timer built into the control will energize the purge solenoid for a period of approximately 70 seconds to purge the ARC accumulator.

Explain function of traction control system

Traction Control System (TCS) 
The Traction Control System (TCS) uses the rear parking/secondary brakes (spring engaged and hydraulically released) to decrease the revolutions of a spinning wheel. The TCS allows the tire with better underfoot conditions to receive an increased
amount of torque. The system is controlled by the Brake ECM (1) and operates the same as the 789C TCS.

The Brake ECM monitors the drive wheels through four input signals: the left rear wheel speed sensor (4), the right rear wheel speed sensor (3), and the transmission output speed sensors (6) (via the Transmission ECM). When a spinning drive wheel is detected, the Brake ECM sends a signal to the TCS proportional solenoid (7) and the TCS selector solenoids (8), which ENGAGE the brake of the affected wheel. When the condition has improved and the ratio between the right and left axles returns to 1:1, the Brake ECM sends a signal to RELEASE the brake.
The service brake/retarder pressure switch (5) provides an input signal to the Brake ECM and performs the following two functions:
* When the service brakes or retarder are ENGAGED, the TCS function  is stopped.
* The service brake pressure switch provides the indication to the Brake  ECM, which is needed to perform a diagnostic test. When the TCS test switch (2) and the retarder lever are ENGAGED simultaneously, the TCS will engage each rear brake independently. Install two pressure gauges on the TCS valve, and observe the pressure readings during the test cycle.

The left brake pressure will decrease and increase. After a short pause, the right brake pressure will decrease and increase. The test will repeat as long as the TCS test switch and the retarder lever are ENGAGED.
The TCS valve has left and right brake release pressure taps. When the proportional solenoid is ENERGIZED, Cat ET will show 44% when the brake is FULLY ENGAGED. NOTE: During the diagnostic test, the parking/secondary brakes must be released.

Identify right rear wheel speed sensor

Shown is the right rear wheel speed sensor (arrow). The TCS monitors the right and left rear wheel speed sensors to determine wheel speed. The transmission output speed sensors are located on the front of the output transfer gear. The TCS uses the transmission output speed sensors to disable the TCS when ground speed is above 19.3 km/h (12 mph).

Identify components inside rear of left frame rail

The Traction Control System (TCS) valve is mounted inside the rear of the right frame rail. Two solenoids are mounted on the valve.
Electrical signals from the Brake ECM cause the selector solenoid valve (1) to shift and select either the left or right parking brake. If the selector valve shifts to the left parking brake hydraulic circuit, the control oil is drained. The left reducing spool of the control valve can then shift and engage the parking brake.

The Brake ECM energizes the selector solenoid valve with + Battery voltage (24 volts). Normal resistance through the selector solenoid is between 18 and 45 ohms.
The proportional solenoid valve (2) controls the volume of oil being drained from the selected parking brake control circuit. The rate of flow is controlled by a signal from the Brake ECM.
The proportional solenoid receives a current between 100 and 680 mA (or 0 to 12 volts) from the Brake ECM. The more current that is sent, the more the proportional solenoid valve is open, and more oil pressure is drained from the brakes. Normal resistance through the solenoid is between 12 and 22 ohms.

The pressure taps (3) can be used to check the left and right brake release pressures when performing diagnostic tests on the TCS. The pressure at the taps in the TCS valve will be slightly less than the brake release pressure measured at the wheels.
NOTE: The brake release pressure sensors have been removed from the TCS valve and are no longer used on the 789D truck.

Explain operation of traction control system (TCS) with engine running and left brake released

This image shows the TCS with the engine running and the brakes RELEASED.
When the machine is started, oil flows from parking brake release pump (1) through the brake release oil filter (2) to the parking brake release valve (3), and to the signal port on the right end of the signal piston (4).

Oil flow to the TCS control valve signal port causes the ball check piston (5) to move to the left and unseat the drain ball check valve. Opening the drain ball check valve opens a drain passage to the hydraulic tank.

When the operator releases the parking brakes, air pressure is increased at the parking brake release valve, forcing the valve spool down, and parking brake release oil flows through the parking brake release valve to the TCS control valve. Oil closes the parking/secondary ball check valve (6) and flows through the screen (7) to the right and left brake control circuit orifices (8).
Oil flows to the ends of the left and right brake reducing valve spools (9). The reducing spools shift toward the center of the TCS control valve and parking brake release oil flows to release the parking brakes.

Explain operation of traction control system (TCS) with engine running and left brake engaged

This image shows the TCS with the engine running and the left brake ENGAGED.
When signals from the sensors indicate that the left wheel is spinning 60% faster than the right wheel, the Brake ECM sends a signal to the selector solenoid valve (10) and the proportional solenoid valve (11). The selector solenoid valve opens a passage between the outer end of the left brake pressure reducing valve (9) and the proportional solenoid valve.

The proportional solenoid valve opens a passage from the selector solenoid valve to drain. The proportional solenoid valve also controls the rate at which the oil is allowed to drain.
Control circuit oil drains through the selector valve and enters the proportional valve. The reducing valve spool for the left parking brake shifts and blocks the flow of oil to
the parking brake. Oil in the left parking brake control circuit begins to drain and the left parking brake begins to ENGAGE. The left brake orifice (8) restricts the flow of oil from the parking brake release valve (3).

When the signals from the sensors indicate that the left wheel is no longer spinning, the Brake ECM stops sending signals to the selector solenoid and the proportional solenoid.
The selector solenoid valve and proportional solenoid valve block the passage to drain, which increases the control circuit pressure. The left brake reducing valve spool shifts to the center position and blocks the passage to drain. Parking brake release oil is directed to the left parking brake and the brake is RELEASED.

Explain oil flow through brake cooling system

This schematic shows the flow of oil through the 789D brake cooling system. Three pump sections provide oil for rear brake cooling: the two-sections of the hoist pump (5) and the outboard section of the torque converter pump (1). The two center sections of the torque converter pump provide oil for front brake cooling. All the pumps pull oil from the hydraulic tank through suction screens.
Oil flows from the hoist pump sections through the hoist screens (6) to the hoist valve (7). In the HOLD and FLOAT positions, oil from the pump flows through the hoist valve to the rear brake oil coolers (8) and to the rear brakes (9).

Oil flows from the fourth section of the torque converter pump, joins with the oil from the hoist valve, and flows to the rear brake oil coolers. The rear brake oil coolers are cooled by the engine jacket water cooling system. The pressure in the rear brake cooling system is controlled by the oil cooler relief valve located in the hoist valve. Oil flows from the torque converter charging pump through the torque converter charging filter (10), the torque converter (11), and the torque converter outlet screen (12) to the optional front brake oil cooler diverter valve (13).

Oil flows from the brake release pump through the brake release filter (14) to the brake release valve (2). The brake release valve controls the oil pressure to release the parking brakes, lock up the torque converter and shift the directional spool in the hoist valve. These functions require minimal oil flow. Most of the oil from the brake release pump flows through the brake release valve and joins with the torque converter charging pump oil at the optional front brake oil cooler diverter valve.

When the service or retarder brakes are ENGAGED, the optional front brake oil
cooler diverter valve allows brake cooling oil to flow through the front brake oil cooler (3) to the front brakes (4). When the brakes are RELEASED, the oil bypasses the cooler and flows directly to the brakes. The front brake oil cooler is cooled by the engine aftercooler cooling system. The aftercooler cooling system does not have temperature regulators (thermostats) in the circuit.

NOTE: On trucks with the 3516C engine, the front brake oil cooler is cooled by jacket water provided by the auxiliary water pump. Normally, front brake cooling oil is diverted around the cooler and goes directly to the front brakes. Diverting oil around the cooler provides lower temperature aftercooler air, on trucks with the 3516B engine, during high power demands (when climbing a grade with the brakes RELEASED, for example).

Identify brake cooling oil pressure test port.

Shown is the left rear brake housing. Brake cooling oil pressure can be tested at the test port (arrow) located in each of the brake cooling oil tubes. One tap is located on the brake cooling inlet tube and another tap is located on the brake cooling outlet tube. The pressure measured at the brake inlet tube (from the oil coolers) will always be higher than the pressure measured at the brake outlet tube.
With the brake cooling oil temperature between 79°C to 93°C (175°F to 200°F), the pressure measured at the brake inlet tube should be above 14 kPa (2 psi) at LOW IDLE and below 172 kPa (25 psi) at HIGH IDLE.

Four brake oil temperature sensors, one for each brake, are located in the brake oil
cooling tubes. The brake oil temperature sensors provide input signals to the VIMS, which keeps the operator informed of the brake cooling oil temperature. The most common cause of high brake cooling oil temperature is operating a truck in a gear that is too high for the grade and not maintaining sufficient engine speed. Engine speed should be kept at approximately 1900 rpm during long downhill hauls. Also, make sure the pistons in the slack adjuster are not stuck and retaining too much pressure on the brakes.

LAB 1: Brake System Component Identification
Instructions: Identify the numbered components in the above illustration.

LAB 1: Brake System Component Identification (continued)
Instructions: Identify the numbered components in the above illustration.

LAB 1: Brake System Component Identification (continued)
Instructions: Identify the numbered components in the above illustration.

789D Off-Highway Trucks Brake System Post-Assessment
Instructions: Select the correct answer(s) for each question.
1. The tow pump ____________________________.
    a. is hydraulically driven.
    b. [provides oil to release the parking brakes].
    c. provides oil to the steering control valve if the steering pump fails.
    d. provides oil to the traction control valve
2. The Brake ECM controls the brake system and the ____________________. (select all that apply)
    a. hoist system.
    b. [TCS].
    c. [HARC ].
   d. compression brake.
3. Which of the following components compensates for brake disc wear?
    a. [Slack adjusters].
    b. Front brake manifold.
    c. Pressure reducing valve.
    d. Service brake piston.
4. What provides secondary brake control?
    a. Tow pump.
    b. [Air system].
    c. Secondary brake pump.
   d. Pilot pump.
5. Technician A says that high residual pressure at the slack adjusters could indicate a failed slack            adjuster. Technician B says that high residual pressure at the slack adjusters could indicate warped      brake discs. Who is right?
    a. Technician A.
    b. Technician B.
    c. [Both Technician Aand Technician B].
    d. Neither Technician A nor Technician B.
6. Which of the following are updates to the 789D trucks? (select all that apply)
     a. [Three air reservoirs if the truck is equipped with air starting (two air reservoirs on electric start           trucks)].
     b. Parking brake accumulator added.
     c. [New parking brake reset valve].
     d. [One air dryer]
7. In addition to rear wheel speed and transmission output speed, what parameter is monitored by the      Brake ECM for TCS operation?
    a. [Service Brake/Retarder Pressure Switch].
    b. Torque converter speed.
    c. Steering pressure.
    d. Brake pressure