This schematic shows the major components of the brake system with the standard oil cooled front brakes. The front slack adjuster is not included on the optional caliper type front brake system.
Oil is drawn from the hydraulic tank by the brake charging pump. Oil flows through the brake filter to the accumulator charging valve. The accumulator charging valve directs supply oil to the brake accumulators. The accumulator charging valve also controls the cut-in and cut-out pressure. When the accumulators are charged, the charging valve will direct excess pump flow to the brake cooling system.
The service brake accumulators provide oil flow through the cab manifold to the service brake control valve. Oil flowing into the cab manifold also flows to the ARC control solenoid and front brake solenoid. When the operator depresses the service brake pedal, the service brake control valve directs pump flow to the rear service brakes to stop the truck.
The front brakes are only engaged when the Brake ECM energizes the front brake solenoid.
With the standard oil cooled front brakes, the Brake ECM determines when to energize the front brake solenoid when the service brake pedal is depressed. With the optional caliper type front brakes, the Brake ECM will energize the front brake solenoid when the front brake lockout switch in the cab is activated.
The Brake ECM controls the modulation of the ARC solenoid and front brake solenoid, which controls truck braking when the ARC system is ON.
Oil from the parking brake accumulator flows to the parking brake valve and the towing diverter valve. When the parking brake is activated, the supply oil for releasing the parking brakes is directed to the tank and the parking brakes are engaged by spring force. When the parking brake solenoid is energized (parking brake de-activated), the parking brake valve directs oil to the TCS valve. The pressure reducing valves in the TCS valve direct oil to release the parking brakes.
The diverter valve, under normal operation, is closed and blocks the oil flow from the electric brake retract pump. If the truck is to be towed with a dead engine, the diverter valve must be shifted manually. When manually shifted, the diverter valve directs oil flow from the electric brake retract pump to the parking brake valve to release the rear brakes.
Brake Electronic Control System
The 777F Trucks are equipped with a Brake ECM for controlling the parking brake and front service brake applications, the ARC system, and the TCS. Two possible arrangements can be installed on a truck:
- ARC only
- ARC and TCS
The Brake ECM receives information from various input components such as the engine speed sensor, the service brake pedal position sensor, the ARC switch, and the wheel speed sensors.
Based on the input information, the Brake ECM controls the front service brake application when the service brake pedal is depressed or the front and rear service brake application when the ARC system is activated. The Brake ECM also controls when the parking brakes should engage for the TCS and parking brake application when the parking brake is manually activated.
Output components include the ARC solenoid, the front service brake solenoid, the TCS selector and proportional solenoids, and the parking brake solenoid.
The compression brake switch is also an input to the Brake ECM. When the compression brake switch is activated, the Brake ECM sends a signal over the Cat Data Link to the Engine ECM. The Engine ECM controls the engine compression brake, which was covered earlier in the presentation.
The Brake ECM (arrow) is located in the compartment at the rear of the cab. The Brake ECM performs the brake control functions, and controls the ARC system and TCS. The Brake ECM is an A4M1 module with two 70-pin connectors. The Brake ECM communicates with the Engine ECM, Transmission/Chassis ECM, and monitoring system over the CAT Data Link and can communicate with some attachments over the CAN Datalink.
When the service brake pedal is depressed, the service brake valve directs oil from the service brake accumulators to the rear brakes and sends a PWM signal to the Brake ECM via the service brake pedal position sensor. The Brake ECM then determines what signal to send to the front service brake solenoid based on the following conditions:
1. If the truck is equipped with the standard oil cooled front brakes, the Brake ECM signals the front service brake solenoid to direct oil from the service brake accumulators to the front and rear brakes.
2. If the truck is equipped with the optional caliper type front brakes, the Brake ECM receives a signal from the front brake lockout switch in the cab. If the lockout switch is OFF, the Brake ECM signals the front service brake solenoid to direct oil from the service brake accumulators to the front and rear brakes the same as the oil cooled front brakes.
NOTE: Oil flow to the front and rear brakes may not be proportional. When the pedal is initially depressed, more oil is directed to the rear brakes. As the pedal is depressed farther more oil is sent to the front brakes in proportion to the rear until full front brake pressure is present at full pedal travel.
3. If the truck is equipped with the optional caliper type front brakes, and the lockout switch is ON, the Brake ECM de-energizes the front service brake solenoid. Oil flow to the front brakes is blocked and only the rear brakes are used to stop the truck.
The Brake ECM also de-energizes the ARC solenoid when the ARC switch in the cab is OFF and the manual retarder lever is in the NEUTRAL position. The manual retarder lever also controls the service brake application using the front brake solenoid and the ARC solenoid.
When the retarder lever is moved, a PWM signal is sent to the Brake ECM. The Brake ECM then determines what signal to send to the ARC solenoid and front service brake solenoid based on the following conditions:
1. If the truck is equipped with the standard oil cooled front brakes, the Brake ECM signals the ARC solenoid and the front service brake solenoid to divide the oil flow from the service brake accumulators between the front and rear brakes.
2. If the truck is equipped with the optional caliper type front brakes, the Brake ECM de-energizes the front service brake solenoid. Oil flow to the front brakes is blocked and only the rear brakes are used to stop the truck with the retarder lever.
Automatic Retarder Control (ARC)
The ARC system receives signals from several switches and sensors. The main inputs to the Brake ECM for the ARC system are the ARC switch and engine speed sensor. The Brake ECM analyzes the various input signals and sends output signals to the ARC solenoid and front service brake solenoid.
NOTE: If the truck is equipped with the optional front caliper type brakes, the Brake ECM will de-energize the front service brake solenoid when the ARC system is activated.
The ARC system function is to modulate truck braking (retarding) when descending a long grade to maintain a constant engine speed. The ARC system engages the rear service brakes and the front oil cooled service brakes. If the ARC switch is moved to the ON position, the ARC system will be activated if the throttle pedal is not depressed and the parking/secondary brakes are RELEASED. The ARC system is disabled when the throttle is depressed or when the parking/secondary brakes are ENGAGED.
The ARC is set at the factory to maintain a constant engine speed of 1900 ± 50 rpm (engine speed setting is programmable). When the ARC initially takes control of retarding, the engine speed may oscillate out of the ± 50 rpm target, but the engine speed should stabilize within a few seconds.
For proper operation of the ARC system, the operator needs only to activate the control with the ARC switch and select the correct gear for the grade, load, and ground conditions. The ARC system is designed to allow the transmission to upshift to the gear selected by the shift lever. After the transmission shifts to the gear selected by the operator and the engine speed exceeds 1900 rpm, the ARC system will apply the retarder as needed to maintain a constant engine speed.
The ARC system also provides engine overspeed protection. If an unsafe engine speed is reached, the ARC will engage the brakes, even if the ARC switch is in the OFF position and the throttle is depressed.
Trucks approaching an overspeed condition will sound a horn and activate a light at 2100 rpm.
If the operator ignores the light and horn, the ARC will engage the retarder at 2180 rpm. If the engine speed continues to increase, the Transmission/Chassis ECM will either upshift (one gear only above shift lever position) or unlock the torque converter (if the shift lever is in the top gear position) at 2300 rpm.
The ARC also provides service personnel with enhanced diagnostic capabilities through the use of onboard memory, which stores possible faults, solenoid cycle counts and other service information for retrieval at the time of service.
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 and operates the same as the 777D TCS.
The Brake ECM monitors the drive wheels through three input signals: one at each drive axle, and one at the transmission output shaft. When a spinning drive wheel is detected, the Brake ECM sends a signal to the selector and proportional valves 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 pressure switch provides an input signal to the Brake ECM from the Transmission/Chassis ECM through the CAT Data Link and performs two functions:
1. When the service brakes or retarder are ENGAGED, the TCS function is stopped.
2. The service brake pressure switch provides the input signal needed to perform a diagnostic test. When the TCS test switch 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. Cat ET can also be used to view the left and right parking brake pressures during the test discussed above in function No. 2. 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.
Shown is the right rear wheel speed sensor (arrow) looking toward the rear of the truck. The TCS monitors the drive wheels through four input speed signals: one at each drive axle, and two at the transmission output shaft.
The transmission output speed sensors monitor the ground speed of the machine and provide input signals to the Brake ECM through the CAT Data Link. The TCS uses the transmission output speed sensors to disable the TCS when ground speed is above 19.3 km/h (12 mph).
The TCS valve is mounted inside the left frame rail toward the rear of the machine. 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 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 pressure taps (3) can be used to test the left and right brake release pressures when performing diagnostic tests on the TCS. At HIGH IDLE, the pressure at the taps in the TCS valve will be approximately 138 kPa (20 psi) less than the brake release pressure tested at the wheels.
The pressure taps are also used to provide parking brake dragging information to the service technician. If the parking brakes are released, as sensed by the secondary brake pressure switch on the parking brake control valve, and parking brake pressure is below 3445 kPa (500 psi), a parking brake dragging event will be logged in the Brake ECM. The event can be viewed with Cat ET.
This illustration shows the TCS valve with the engine running and the brakes RELEASED.
With the engine running, oil flows from the brake charging pump to the parking brake valve.
When the operator moves the transmission lever out of the PARK position, the Brake ECM energizes the parking brake solenoid which directs oil flow to the TCS valve.
In the TCS valve, oil flows through a screen and orifices to the selector solenoid and the brake reducing valves. When the TCS is not activated, the oil is blocked at the selector solenoid. Oil pressure moves the brake reducing solenoids to the left and oil from the brake charging pump is directed to the parking brakes. The parking brakes are RELEASED.
This illustration shows the TCS valve 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 and the proportional solenoid valve. The selector solenoid valve shifts up to open a passage between the right end of the left brake pressure reducing valve and the proportional solenoid valve.
The torque converter lockup pump oil provides signal oil to the drain ball check which allows oil from the TCS valve to return to the tank.
The proportional solenoid valve opens a passage from the selector solenoid valve to drain through the drain ball check. 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 oil flow 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 restricts the flow of oil from the parking brake valve.
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 and allow the control circuit pressure to increase.
The left brake reducing valve spool shifts to the left and blocks the passage to drain. Parking brake oil is directed to the left parking brake and the brake is RELEASED.
See You Soon!!!
MARYGAR
0 comments:
Post a Comment