Traction Control System (TCS) Operation

OVERVIEW
Traction control reduces wheel slip and maintains traction at the driving wheels at speeds below 56 kph (35 mph) when road surfaces are slippery. The traction control system reduces wheel slip by braking the wheel that is losing traction.
The CAB monitors wheel speed. During acceleration, if the CAB detects front (drive) wheel slip and the brakes are not applied' the CAB enters traction control mode. Traction control operation proceeds in the following order:
1. Close the normally open traction control (TC) 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 solenoid 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 ABS function is to control wheel skid by releasing the brakes.

HYDRAULIC SHUTTLE VALVES
Two pressure relief 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.
If the brakes are applied at anytime during a traction control cycle, the brake lamp switch triggers the control module to switch off traction control.

TRACTION CONTROL FUNCTION LAMP
The traction control function lamp illuminates during a traction control cycle, displaying TRAC on the instrument panel.
The traction control system is enabled at each ignition cycle. It may be turned off by depressing the traction control switch button. The traction control function lamp (TRAC OFF) illuminates immediately upon depressing the button. Pressing this button again, or turning off and restarting the vehicle will enable the traction control system.
If the 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.