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| Illustration 1 | g02316913 |
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Schematic of the Brake Hydraulic System with the retarder enabled and the service brakes engaged (1) Brake ECM (2) Automatic Retarding Control (ARC) switch (3) Manual retarder lever (4) Engine speed sensor (5) Service Brake Valve (Foot Control) (6) Rear Service Brakes (7) Slack Adjusters (Rear Brakes) (8) Oil from the Control Manifold (Brake Accumulator Charge) (9) Directional spool for the Brake Valve (Front Service Brake) (10) Front brake solenoid for the Brake Valve (Front Service Brake) (11) Front Service Brakes (12) Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (13) Rear brake supply solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (14) Brake reducing valves for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (15) Resolver valves for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (16) Pressure control solenoids for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (17) Brake Valve (Front Service Brake) (18) Slack Adjuster (Front Brakes) (19) Hydraulic Tank (Brake, Brake Cooling, Hoist) | |
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| Illustration 2 | g02316555 |
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Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (7) Slack Adjusters (Rear Brakes) (12) Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (13) Rear brake supply solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (16) Pressure control solenoids for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (20) Return hose to tank (21) Supply hose from Service Brake Valve (Foot Control) (22) Supply hose from the service brake accumulator | |
Brake ECM (1) controls truck retarding and the TCS (Traction Control System). Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) is a new design in the 777G Off-Highway Truck.
The main components in the retarding system are Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) and Brake Valve (Front Service Brake) (17). Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) is mounted to the inside of the rear frame rail. Brake Valve (Front Service Brake) (17) is mounted to the left upper frame rail below the cab. Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) contains three solenoids which control oil flow from the service brake accumulator to Slack Adjusters (Rear Brakes) (7) and rear service brakes (6). Brake Valve (Front Service Brake) (17) controls oil flow to front service brakes (11) for the retarding system. Front service brakes (11) are not used in the TCS.
Rear brake supply solenoid (13) directs oil from the service brake accumulator to pressure control solenoids (16) when the retarding system or TCS is active. Pressure control solenoids (16) control oil flow to brake reducing valves (14) inside Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12), which control the oil flow to Slack Adjusters (Rear Brakes) (7) and rear service brakes (6). Oil from the service brake accumulator enters Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) through supply hose (22) and returns to hydraulic tank (19) through return hose (20). Oil from Service Brake Valve (Foot Control) (5) enters Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) through hose (21) at the top of Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12).
When the service brake pedal is depressed, Service Brake Valve (Foot Control) (5) directs oil to brake reducing valves (14) inside Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12), which control the oil flow to Slack Adjusters (Rear Brakes) (7) and rear service brakes (6) as previously described.
Front brake solenoid (10) controls the oil to front service brakes (11) when the service brake pedal is depressed or the retarding system is activated.
Illustration 1 shows the retarding system components in the brake hydraulic system with the service brakes engaged by the retarding function. Retarding control is a feature of the 777G Off-Highway Trucks. Retarding control is used to control the truck's downhill speed. The feature consists of manual retarding and Automatic Retarding Control (ARC). Manual retarder lever (3) is used for retarding when ARC is not desired by the operator, such as a speed slower than the speed that initiates the ARC.
When manual retarder lever (3) is activated, the lever sends a proportional signal to Brake ECM (1). Brake ECM (1) sends a current to front brake solenoid (10), rear brake supply solenoid (13), and pressure control solenoids (16). When the ARC is activated by pressing ARC switch (2) in the cab, and engine speed sensor (4) senses an increase above the desired engine speed, Brake ECM (1) sends a current to front brake solenoid (10), rear brake supply solenoid (13), and pressure control solenoids ().
Oil from Control Manifold (Brake Accumulator Charge) (8) flows into Brake Valve (Front Service Brake) (17). With front brake solenoid (10) energized, the solenoid shifts to the right which directs oil from Control Manifold (Brake Accumulator Charge) (8) to the left side of directional spool (9). Directional spool (9) shifts to the right and brake pump oil is directed through Slack Adjuster (Front Brakes) (18). Front service brakes (11) are engaged.
Oil from Control Manifold (Brake Accumulator Charge) (8) also flows into Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12). With rear brake supply solenoid (13) and pressure control solenoids (16) energized, oil from Control Manifold (Brake Accumulator Charge) (8) is directed to resolver valves (15). Resolver valves (15) shift and oil flows to the right end of brake reducing valves (14). Brake reducing valves (14) shift to the left and oil from Control Manifold (Brake Accumulator Charge) (8) flows through Slack Adjusters (Rear Brakes) (7) and rear service brakes (6) are engaged.
Note: If manual retarder lever (3) and ARC switch (2) are ENGAGED at the same time, front service brakes (11) and pressure control solenoids (16) will receive a current that corresponds to the system that is requiring the most braking.
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| Illustration 3 | g02318521 |
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TCS input and output components (1) Brake ECM (23) TCS test switch (24) Left rear wheel speed sensor (25) Right rear wheel speed sensor (26) Position Sensor (Foot Brake Control) (27) Transmission output speed sensors (28) Steering cylinder position sensor (29) Rear brake supply solenoid (30) Pressure control solenoids | |
The Traction Control System (TCS) uses rear service brakes (6) 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 Brake ECM (1) and operates similar to the 777F Off-Highway Truck TCS. However, the 777G Off-Highway Truck uses rear service brakes (6) rather than the rear parking brakes to slow the wheels.
Brake ECM (1) monitors the drive wheels and the steering angle. The drive wheels are monitored through four input signals:left rear wheel speed sensor (24), right rear wheel speed sensor (25) and transmission output speed sensors (27) (via the Transmission ECM). The steering angle is monitored through steering cylinder position sensor (28).
When a spinning drive wheel is detected, Brake ECM (1) sends a signal to rear brake supply solenoid (29) and pressure control solenoids (30) which ENGAGE rear service brake (6) of the affected wheel. When the condition has improved and the ratio between the right and left axles returns to 1:1, Brake ECM (1) sends a signal to RELEASE the brake.
Position Sensor (Foot Brake Control) (26) provides an input signal to Brake ECM (1) and performs the following two functions:
(1) When rear service brakes (6) or manual retarder lever (3) are ENGAGED, the TCS function is stopped.
(2) Position Sensor (Foot Brake Control) (26) provides the input signal needed to perform a diagnostic test. When TCS test switch (23) and manual retarder lever (3) are ENGAGED simultaneously, the TCS will engage each rear service brake (6) independently. Install two pressure gauges on Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12), and observe the pressure readings during the test cycle. The left rear service brake pressure will decrease and increase. After a short pause, the right rear service brake pressure will decrease and increase. The test will repeat as long as TCS test switch (23) and manual retarder lever (3) are ENGAGED.
Pressure ports can be installed on Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) to check left and right rear service brake release pressure. When the proportional solenoid is ENERGIZED, Cat ET will show 68% when the brake is FULLY ENGAGED.
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| Illustration 4 | g02319176 |
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TCS wheel spin ratio vs. steering angle | |
Illustration 4 is an example of the steering angle versus the spin ratio.
Note: Illustration 4 is only for explanation purposes and does not depict the actual TCS software map in the Brake ECM.
When the front wheels are straight, the TCS is active when one rear wheel is spinning approximately 1.2 times as fast as the other (spin ratio). The TCS provides more traction when the truck is traveling straight.
The TCS becomes active at a higher spin ratio as the steering angle increases during a turn. When the TCS is active, the spin ratio is locked in until the TCS is de-activated. For example, if a wheel starts slipping and the operator turns the wheel slightly, the spin ratio will hold at the lower value regardless of the steering wheel angle.
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| Illustration 5 | g02319360 |
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View from front of the truck facing the rear of the truck (24) Left rear wheel speed sensor (25) Right rear wheel speed sensor | |
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| Illustration 6 | g02320120 |
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View of the head end of the left side steering cylinder Cover for steering cylinder position sensor (28) | |
Illustration 5 shows left rear wheel speed sensor (24) and right rear wheel speed sensor (25). The TCS monitors the drive wheels through four input speed signals: one at each drive axle, and two at the transmission output shaft.
Transmission output speed sensors (27) monitor the ground speed of the machine and provide input signals to Brake ECM (1) through the CAT Data Link. The TCS uses transmission output speed sensors (27) in order to disable the TCS when ground speed is above
Steering cylinder position sensor (28) is located inside the left side steering cylinder. The sensor connector for steering cylinder position sensor (28) can be accessed by removing the cover shown in Illustration 6. The cover is located on the head end of the left side steering cylinder.
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| Illustration 7 | g02316413 |
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Schematic of Brake Valve (Rear Service Brake, Retarder Control, Traction Control) with engine running and service brakes released (1) Brake ECM (5) Service Brake Valve (Foot Control) (6A) Left Rear Service Brake (6B) Right Rear Service Brake (8) Oil from the Control Manifold (Brake Accumulator Charge) (12) Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (13) Rear brake supply solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (14) Brake reducing valves for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (15) Resolver valves for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (16) Pressure control solenoids for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) | |
Illustration 7 shows Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) with the engine running and rear service brakes (6) RELEASED.
With the engine running, oil flows from Control Manifold (Brake Accumulator Charge) (8) into Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12).
In Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12), oil flows through a screen to rear brake supply solenoid (13) and to brake reducing valves (14). When the TCS is not activated, rear brake supply solenoid (13) and pressure control solenoids (16) are deenergized by Brake ECM (1). Oil from Control Manifold (Brake Accumulator Charge) (8) is blocked at rear brake supply solenoid (13). Spring force moves brake reducing valves (14) to the right and oil from left rear service brake (6A) and right rear service brake (6B) is directed to hydraulic tank (19) through brake reducing valves (14). Rear service brakes (6) are RELEASED.
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| Illustration 8 | g02320735 |
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Schematic of Brake Valve (Rear Service Brake, Retarder Control, Traction Control) with engine running and left rear service brake engaged (1) Brake ECM (5) Service Brake Valve (Foot Control) (6A) Left Rear Service Brake (6B) Right Rear Service Brake (8) Oil from the Control Manifold (Brake Accumulator Charge) (12) Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (13) Rear brake supply solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (14A) Left brake reducing valve for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (14B) Right brake reducing valve for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (15A) Left resolver valve for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (15B) Right resolver valve for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (16A) Left pressure control solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (16B) Right pressure control solenoid for the Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (24) Left rear wheel speed sensor (25) Right rear wheel speed sensor | |
Illustration 8 shows Brake Valve (Rear Service Brake, Retarder Control, Traction Control) (12) with the engine running and left rear service brake (6A) ENGAGED.
When signals from left wheel speed sensor (24) and right wheel speed sensor (25) indicate that the left wheel is spinning 60% faster than the right wheel, Brake ECM (1) sends a signal to rear brake supply solenoid (13) and left pressure control solenoid (16A). Rear brake supply solenoid (13) shifts down and directs oil to pressure control solenoids (16). Left pressure control solenoid (16A) shifts to the left and oil is directed to left resolver valve (15A). The resolver shifts and oil flows to the right end of left brake reducing valve (14A). Left brake reducing valve (14A) shifts to the left and oil is directed to left rear service brake (6A). Pressure control solenoids (16) control the rate at which the oil is allowed to flow to rear service brakes (6).
Left rear service brake (6A) begins to ENGAGE. The left rear service brake orifice restricts the flow of oil to left rear service brake (6A).
When signals from left wheel speed sensor (24) and right wheel speed sensor (25) indicate that the left wheel is no longer spinning, Brake ECM (1) stops sending signals to rear brake supply solenoid (13) and left pressure control solenoid (16A). Oil from Control Manifold (Brake Accumulator Charge) (8) is blocked at rear brake supply solenoid (13). Spring force moves left brake reducing valve (14A) to the right and oil from left rear service brake (6A) is directed to hydraulic tank (19) through left brake reducing valve (14A). Left rear service brake (6A) is RELEASED.