797B Off-Highway Truck Braking System Caterpillar

Illustration 1	g00988097

Illustration 2	g00789417
(1) Electronic Control Module (ECM)

The Electronic Control Module (ECM) (1) for the brakes and for cooling is located in the compartment at the rear of the cab. The Brake/Cooling ECM has several input signals and output signals. Refer to the Illustration 1 for the input signals and for the output signals.

The Brake/Cooling ECM receives current through a circuit breaker. This current is used to activate the solenoids.

The Cat Data Link is used to share information with the other Electronic Control Modules on the machine.

Cab Controls for the Brakes

Illustration 3	g00987962

The retarder control is hand operated by the lever (2) near the steering wheel. When the operator needs to retard the machine, the retarder control causes signal oil to flow to the service brake/retarder relay valve. This causes the service brakes to be applied.

The retarder control is a position switch. The retarder lever (2) controls the flow of signal oil with the supply solenoid and with the control solenoid. The supply solenoid and the control solenoid are located in the cab brake valve. The same solenoids are also used for the Automatic Retarder Control (ARC).

If the retarder lever (2) is used, the supply solenoid will be energized. This allows the oil from the brake actuation pump section to flow to the control solenoid. The control solenoid is a proportional solenoid. The position of the retarder lever (2) will determine the amount that the proportional solenoid will open. This sends the needed amount of oil to the port for the signal oil.

When the retarder lever (2) is in the OFF position, neither solenoid will be energized.

The manual retarder lever (2) is located on the right side of the steering column. The manual retarder lever (2) is used in order to modulate engagement of the service brakes on all four wheels. The retarder lever (2) can control the modulation more precisely than the service brake pedal that is located on the cab floor.

When manual retarder lever (2) is engaged, the sensor for the manual retarder lever (2) sends a PWM signal to the Brake/Cooling ECM. The PWM signal will be higher when the manual retarder lever (2) is moved further. signal. The ECM for the Brake Cooling then turns On a supply solenoid.

Illustration 4	g00987964

Right of the Speedometer/Tachometer module are three rocker switches that are used to control the brake functions. Switch (3) is for the Automatic Retarder Control (ARC) and switch (4) is for the Brake Release and the Hoist Pilot. Switch (5) is for the Traction Control System (TCS).

Switch (4) is used to provide pilot oil for the hoist in order to lower the body with a dead engine. Also, switch (4) is used in order to release the parking brakes for towing.

Brake Actuation

Illustration 5	g00987987

Right of the operator's seat is the shift console. Parking brake switch (6) is located on the shift console.

The Brake/Cooling ECM uses the input signal from the parking brake switch (6) to determine the amount of current for the parking brake solenoid valve. If the parking brake switch (6) is in the ON position, the parking brake solenoid valve will be deactivated. Signal oil will be blocked and the springs will engage the parking brakes. If the parking brake switch (6) is in the OFF position, the parking brake solenoid valve will be activated. Oil will be permitted to flow through the parking brake solenoid valve.

When it is necessary to retard the machine, the Brake/Cooling ECM will energize the supply solenoid and the control solenoid. If the ARC is not needed and if the retarder lever is in the OFF position, the supply solenoid valve will not be energized. The control solenoid will not have any oil supply. The supply solenoid valve will be energized if the ARC is turned on and the engine speed is within 100 rpm of the programmed setting or if the retarder lever is being used. The control solenoid is a proportional solenoid. Either the required level of braking or the position of the retarder lever will determine the amount of current that is supplied in order to open the control solenoid. When more current is supplied to the control solenoid, the control solenoid will open further.

Three pressure switches monitor the signal oil for the service brakes. The pressure switch for the retarder lever and for the ARC can be used for diagnostic purposes. If the supply solenoid and the control solenoid are deactivated and pressure is present at the pressure switch, then one of the solenoids is leaking.

A pressure switch monitors the oil pressure of the service brake accumulator. The operator will be notified if the oil pressure in the service brake accumulator drops below the deactuation pressure of the pressure switch. If the ground speed is higher than 8 km/h (5 mph) and this switch is active, a Level III warning will occur.

Automatic Retarder Control

The Automatic Retarder Control (ARC) switch (3) activates the ARC system. If the ARC On/Off switch (3) is moved to the On position and the throttle pedal is not depressed the ARC will be activated and the parking and secondary brakes are released. The ARC system is disabled when the throttle is depressed.

The Automatic Retarder Control (ARC) is designed to modulate the service brakes of the machine during the descent of a long grade. The ARC modulates the service brakes in order to maintain a constant engine speed. A constant engine speed improves production and brake cooling. The ARC will be activated when the following conditions exist:

• The ON/OFF switch for the ARC is in the ON position.

• The throttle pedal is NOT depressed.

• The parking brakes are in the RELEASED position.

The ARC uses the supply solenoid and the control solenoid in the cab brake valve. The ARC is set at the factory in order to maintain a constant engine speed of 1900 rpm. The tolerance for the engine speed is ± 50 rpm. The engine speed setting is programmable. The engine speed may oscillate beyond ± 50 rpm of the target when the ARC turns on. The engine speed should stabilize within a few seconds. The Engine ECM provides the engine speed information over the Cat Data Link.

For proper operation of the ARC, the operator needs only to turn on the ON/OFF switch for the ARC. The operator should then select the correct gear for the grade, the load, and the ground conditions. The ARC is designed to permit the transmission to upshift into the selected gear of the shift lever. The ARC will apply the service brakes after the transmission shifts into the selected gear and the engine speed exceeds 1900 rpm ± 50 rpm.

The ARC also provides Engine Overspeed protection. The ARC will engage the service brakes if an unsafe engine speed is attained. The service brakes will be applied regardless of the position of the ON/OFF switch for the ARC. The service brakes will be applied regardless of the position of the throttle.

Machines that approach an overspeed condition will activate a horn and the retarding indicator lamp at 2100 rpm. The ARC will activate the retarder at 2180 rpm if the operator ignores the retarding indicator lamp and the horn. The Transmission ECM will upshift one gear if the engine speed reaches 2300 rpm. The Transmission ECM will only upshift one gear above the position of the shift lever. If the shift lever is in the top gear and the engine speed reaches 2300 rpm, the Transmission ECM will unlock the torque converter.

The ARC also provides service personnel with enhanced diagnostic capabilities through the use of onboard memory. The onboard memory stores information that includes possible faults and cycle counts for the solenoids.

Service personnel can access stored diagnostic information with a laptop computer with software for Electronic Technician (ET).

Traction Control System (TCS)

The TCS switch (5) is used when you turn in a tight circle with the engine at a low idle and the transmission in First Gear.

Two dash indicator lights are located above the key switch and to the right of the Speedometer/Tachometer module. The indicator lights are used to indicate when a brake function is active.

The TCS uses the rear parking brakes in order to decrease the revolutions of a spinning wheel. The rear parking brakes are activated by springs and released by oil pressure. The TCS allows the tire with better underfoot conditions to receive an increased amount of torque.

The TCS will be activated when the following conditions exist:

• The throttle pedal is depressed.

• The retarder is not being used.

The Brake/Cooling ECM monitors the drive wheels through four input signals. The Brake/Cooling ECM monitors one signal from each drive axle and two signals from the transmission output shaft. The Brake/Cooling ECM sends a signal to the appropriate selector solenoid valve and to the proportional solenoid valve when the spinning of a drive wheel is detected. The parking brake release oil will be modulated in the affected wheel. When the ratio between the right axle and the left axle returns to 1:1, the proportional solenoid valve will be deactivated. The full amount of parking brake release oil will flow to the parking brakes.

The Traction Control System (TCS) was formerly referred to as the Automatic Electronic Traction Aid (AETA). The operation of the system has not changed. The main difference is the appearance of the electronic control and the TCS is now on the CAT? Data Link. The ET can communicate with the TCS.

The traction control valve has pressure sensors for the oil pressure of the left rear parking brake and for the oil pressure of the right rear parking brake.

Diagnostic Test

The input signal from the service brake switch can be used to perform a diagnostic test. When the TCS test switch and the retarder lever are ENGAGED simultaneously and speed is zero, the TCS will engage each rear brake independently.

Install two pressure gauges on the traction control valve. Observe the pressure readings during the test cycle. Watch for a decrease in left brake pressure and an increase in left brake pressure. After a short pause, watch for a decrease in the right brake pressure and an increase in the right brake pressure. The test will repeat while the TCS test switch and the retarder lever are in the ENGAGED position. Use a laptop computer with the ET in order to view the left parking brake pressure and the right parking brake pressure.

Note: The parking brakes must be released during the diagnostic test.

Brake Cooling

The speed of the piston motor for the brake cooling pump sections is determined by three factors.

• Temperature status of the brake oil

• High speed braking status

• Temperature status of the engine coolant and of the power train oil

The Brake/Cooling ECM analyzes the data. The Brake/Cooling ECM sends current to the displacement solenoid in order to control the output of the piston pump that drives the piston motor for the brake cooling pump sections.

The same data is used to control the solenoid valve for the brake diverter valve. The solenoid valve for the brake diverter valve is located in the chassis/brake valve. When the Brake/Cooling ECM energizes the solenoid valve, oil from the gear pump for the hoist and for brake cooling flows through the rear brake oil cooler.

Temperature Status

Illustration 6	g00988057

Illustration 7	g00988058

The Brake/Cooling ECM analyzes the temperatures from the four brake oil temperature sensors that are shown in Illustration 6 and Illustration 7. The maximum brake temperature is converted into a temperature status. The temperature status can be COOL, HOT or VERY HOT.

• If the brake temperature is unknown, the temperature status will default to the COOL status.

• If the temperature rises above 102 °C (216 °F), the temperature status will be HOT.

• If the temperature rises above 107 °C (225 °F), the temperature status will be VERY HOT.

• If the temperature falls below 105 °C (221 °F), the temperature status will become HOT.

• If the temperature falls below 100 °C (212 °F), the temperature status will be COOL.

High Speed Braking Status

The Brake/Cooling ECM will look at the high speed braking status if any of the brake actuation inputs are present. This prevents the piston motor from driving the brake cooling pump sections during low speed braking applications.

Any of the following brake actuation inputs must be present.

• The switch for the retarder lever or any of the solenoids for the retarder and for the Automatic Retarder Control (ARC) are energized.

• The pressure switch for the signal oil to the service brake/retarder relay valve is activated.

• Parking brake oil pressure indicates that the parking brake is engaged and the Traction Control System (TCS) is not active.

• The Brake/Cooling ECM is energizing the parking brake solenoid.

If no brake actuation inputs are present, the high speed braking status will be inactive.

The Brake/Cooling ECM also analyzes the ground speed of the machine. The ground speed is converted into a LOW status or a HIGH status. If the ground speed is unknown, the ground speed defaults to the HIGH status.

• If the ground speed is above 6.4 km/h (4 mph), the ground speed will have a HIGH status.

• If the ground speed falls below 4.8 km/h (3 mph), the ground speed will have a LOW status.

If the ground speed has a low status, the high speed braking status will be inactive. If the ground speed has a high status and brake actuation inputs are present, the high speed braking status will be active.

Temperature Status of Other Systems

The Brake/Cooling ECM also checks the temperature status in the following other machine systems.

• Engine jacket water

• Engine aftercooler

• Transmission lubrication

• Torque converter output

If the temperature status of any of the above systems is VERY HOT, the Brake/Cooling ECM will not energize the displacement solenoid on the piston pump. This gives cooling priority to the engine and the power train. In this situation, brake temperature will not have any effect on the speed of the brake cooling pump sections.

Factors For Brake Cooling

Engine Cooling Fan

If the temperature status for all of the temperature sensors is COOL, the engine cooling fan will be turned off. If the temperature status for any of the temperature sensors is HOT, the speed of the engine cooling fan will depend on the system. A preset speed has been determined for each temperature sensor. If the temperature status for any of the temperature sensors is VERY HOT, the speed of the engine cooling fan will be fully on. When the speed of the engine cooling fan is fully on and the retarder is not being used, the speed of the engine cooling fan will depend on engine speed. When the speed of the engine cooling fan is fully on and the retarder is being used, the speed of the engine cooling fan will be a maximum speed of 575 rpm.

The speed of the engine cooling fan will be fully on when the high speed braking status is active.

Lubrication Oil for the Differential and for the Final Drives

The amount of oil that lubricates the differential and the final drives is controlled by the Brake/Cooling ECM. Two solenoids direct oil flow away from the final drive and the differential until the oil temperature reaches a certain status.

At oil temperatures below -4 °C (25 °F), the rear axle oil pump will not be driven. At oil temperatures below -4 °C (25 °F), the Transmission ECM will limit the top gear of the machine until the oil temperature is high enough to ensure proper lubrication.

When the oil temperature is below 60 °C (140 °F), lubrication oil will spray on the differential. This will warm the oil. When the oil temperature is above 60 °C (140 °F), lubrication oil will flow to the final drives and to the wheel bearings in addition to the differential.