Traction Control System (TCS) Operation

TRACTION CONTROL SYSTEM (TCS) DESCRIPTION
The main purpose of traction control is to reduce wheel slip and maintain traction at the driven wheels when road surfaces are slippery. The traction control system reduces wheel slip by braking the wheel that is losing traction. The system is designed to operate at speeds below 56 km/h (35 mph).

The Controller Antilock Brake (CAB) monitors wheel speeds. If, during acceleration, the module detects front (drive) wheel slip and the brakes are not applied, the CAB will enter traction control mode. Traction control works in the following order when drive wheel slip is detected.

1 Close the (normally open) isolation valves.
2 Start pump/motor and supply volume/pressure to front hydraulic circuits (pump runs continuously during traction control).
3 Open and close build and decay valves to maintain minimum wheel slip and maximum traction.

The cycling of the build and decay valves is similar to the ABS except that they work to control wheel spin by applying brakes. ABS, function is to control wheel skid by releasing brakes.

Two pressure relief valves allow excess fluid volume to return to the reservoir when not used by the build/decay cycles. These are required because the pump supplies more volume than the traction control system requires.

If at any time the brake pedal is applied during a traction control cycle, the brake lamp switch will trigger the CAB to switch off the traction control.

The traction control system will be enabled at each ignition cycle. It may be turned off by depressing the Traction Control Switch. The traction control system function lamp will illuminate "TRAC OFF" immediately upon depressing the traction control switch button. Only the "TRAC" portion of the "TRAC OFF" indicator will illuminate during a traction control event. If the CAB calculates that the brake temperatures are high, the traction control system will become inoperative until a time-out period has elapsed. When in this thermal protection mode, the traction control "TRAC OFF" lamp will illuminate; however, a fault will not be registered.

HYDRAULIC SHUTTLE VALVES

Two pressure relief hydraulic shuttle valves are included on vehicles with traction control. These valves are located inside the Hydraulic Control Unit (HCU) and cannot be serviced separately from the HCU.

TRACTION CONTROL LAMP

The traction control function lamp is located in the transmission range indicator display of the instrument cluster, displaying TRAC, TRAC OFF or neither depending on system mode.

The TRAC OFF lamp is controlled by a Traction Control Off switch that is a momentary contact type switch. The Traction Control Off switch is located on the steering column upper shroud.

OPERATION - TRACTION CONTROL SYSTEM

The traction control module monitors wheel speed. During acceleration, if the module detects front (drive) wheel slip and the brakes are not applied, the module enters traction control mode. Traction control operation proceeds in the following order:
1. Close the normally open isolation valves.
2. Start the pump/motor and supply volume and pressure to the front (drive) hydraulic circuit. (The pump/motor runs continuously during traction control operation.)
3. Open and close the build and decay valves to maintain minimum wheel slip and maximum traction.

The cycling of the build and decay valves during traction control is similar to that during antilock braking, except the valves work to control wheel spin by applying the brakes, whereas the Antilock Brake System (ABS) function is to control wheel skid by releasing the brakes.

If the brakes are applied at anytime during a traction control cycle, the brake lamp switch triggers the controller to switch off traction control.

HYDRAULIC SHUTTLE VALVES

Two pressure relief hydraulic shuttle valves allow pressure and volume to return to the master cylinder reservoir when not consumed by the build and decay valves. These valves are necessary because the pump/motor supplies more volume than the system requires.

TRACTION CONTROL LAMP

The traction control system is enabled at each ignition cycle. It may be turned off by depressing the Traction Control Off switch button when the ignition is in the ON position. The traction control function lamp (TRAC OFF) illuminates immediately upon depressing the button.

The traction control function lamp illuminates during a traction control cycle, displaying TRAC.

If the Controller Antilock Brake (CAB) calculates that the brake temperatures are high, the traction control system becomes inoperative until a time-out period has elapsed. During this "thermo-protection mode," the traction control function lamp illuminates TRAC OFF; note that no trouble code is registered.