AUTOMATIC ELECTRONIC TRACTION AID 775 OFF-HIGHWAY TRUCK CATERPILLAR

AUTOMATIC ELECTRONIC TRACTION AID
775D OFF-HIGHWAY TRUCK

Traction Control System (TCS)

The Traction Control System (TCS) uses the rear parking and 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 (see Slides No. 162 and 163).



The TCS electronic control 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 electronic control 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 electronic control sends a signal to release the brake.
The 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.  Also, the ECAP and ET Service Tools can communicate with the TCS.

A service and retarder brake switch (see Slide No. 142) provides an input signal to the TCS 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 and retarder brake 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 a left and right brake release pressure sensor.  A laptop computer with the ET software installed 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, ET will show 35% when the brake is FULLY ENGAGED.

NOTE:  During the diagnostic test, the parking and secondary brakes must be released.

Wheel speed sensor (arrow)

Shown is the left wheel speed sensor (arrow) on the 773D/775D update trucks.  The TCS monitors the drive wheels through three input speed signals:  one at each drive axle, and one at the transmission output shaft.

The Transmission Output Speed (TOS) sensor (see Slide No. 104) monitors the ground speed of the machine and provides input signals to the TCS through the CAT Data Link.  The TCS uses the TOS sensor to disable the TCS when ground speed is above 19.3 km/h (12 mph).

TCS valve

The Traction Control System (TCS) valve is mounted inside of the rear 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 Pulse Width Modulated (PWM) signal from the brake ECM.  The longer the duty cycle, the more time the proportional solenoid valve is open, and more oil pressure is drained from the brakes.  Voltage to the proportional solenoid increases proportionally from zero to approximately 10 Volts with the demand for more braking. Normal resistance through the proportional solenoid is between
12 and 22 Ohms.

The pressure taps (3) or pressure sensors (4) can be used to test 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 tested at the wheels.

TCS operation with brakes RELEASED

Shown is the TCS with the engine running and the brakes RELEASED. When the machine is started:
* Oil from parking brake release pump flows through the brake release oil filters where the flow is divided.  One line from the filters directs oil to the parking brake release valve.  The other line sends oil to the pump signal port (right end of signal piston) in the TCS control valve.

* Oil flow to the TCS control valve causes the ball check piston 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.

* Parking brake release oil can then flow through the parking brake release valve to the TCS control     valve.

* In the control valve, oil closes the parking and secondary ball check valve and flows through the         screen.

* Oil flows through the right and left brake control circuit orifices.


* Oil flows to the ends of the left and right brake reducing valve spools.

* When the control circuit pressure is high enough, the reducing spools shift toward the center of the     TCS control valve and parking brake release oil flows to release the brakes.

TCS operation with left brake ENGAGED

Shown is 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 following sequence of events occurs:

* The brake ECM sends a signal to the selector solenoid valve and the proportional solenoid valve.

* The selector solenoid valve opens a passage between the outer end of the left brake pressure               reducing valve 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 flow 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.

* The left parking brake begins to ENGAGE.

* The left brake orifice restricts the flow of oil from the parking brake release valve.

When the signals from the sensors indicate that the left wheel is no longer spinning, the following occurs:

* 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 center position and blocks the passage to drain.

* Parking brake release oil is directed to the left parking brake and the brake is RELEASED.

CONCLUSION

This presentation has provided an introduction to the Caterpillar 769D/771D/773D/775D Update Off-highway Trucks.  All the major component locations were identified and the major systems were discussed.  When used in conjunction with the service manual, the information in this package should permit the serviceman to analyze problems in any of the major systems on these trucks.

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