Traction Control System (TCS)
DESCRIPTION - TRACTION CONTROL SYSTEMTraction control reduces wheel slip and maintains traction at the driving wheels at speeds below 64 km/h (40 mph) when road surfaces are slippery. The traction control system reduces wheel slip by braking the wheel that is losing traction.
The traction control system may be turned off or on by depressing the traction control switch button located on the instrument panel (Fig. 1).
OPERATION - TRACTION CONTROL SYSTEM -MK20E
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.
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.
The traction control function indicator 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 indicator (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 indicator illuminates TRAC OFF; note that no trouble code is registered.
OPERATION - TRACTION CONTROL SYSTEM -MK25E
The ABM monitors wheel speed. During acceleration, if the ABM detects front (drive) wheel slip and the brakes are not applied, the ABM 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.
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.
The traction control function indicator 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 indicator (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 ABM 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 indicator illuminates TRAC OFF; note that no trouble code is registered.
CAUTION
The ABS uses an electronic control module. This module is designed to withstand normal current draws associated with vehicle operation. Care must be taken to avoid overloading the circuits.
CAUTION: In testing for open or short circuits, do not ground or apply voltage to any of the circuits unless instructed to do so for a diagnostic procedure.
CAUTION: These circuits should only be tested using a high impedance multi-meter or the designated scan tool as described in this section. Power should never be removed or applied to any control module with the ignition in the ON position. Before removing or connecting battery cables, fuses, or connectors, always turn the ignition to the OFF position.
CAUTION: The CAB (MK20e) 25-way connector should never be connected or disconnected with the ignition switch in the ON position.
CAUTION: The ABM (MK25e) 47-way connector should never be connected or disconnected with the ignition switch in the ON position.
CAUTION: This vehicle utilizes active wheel speed sensors. Do not apply voltage to wheel speed sensors at any time.
CAUTION: Use only factory wiring harnesses. Do not cut or splice wiring to the brake circuits. The addition of aftermarket electrical equipment (car phone, radar detector, citizen band radio, trailer lighting, trailer brakes, etc.) on a vehicle equipped with antilock brakes may affect the function of the antilock brake system.
CAUTION: When performing any service procedure on a vehicle equipped with ABS, do not apply a 12-volt power source to the ground circuit of the pump motor in the HCU. Doing this will damage the pump motor and will require replacement of the entire HCU.
CAUTION: An attempt to remove or disconnect certain system components may result in improper system operation. Only those components with approved removal and installation procedures in this manual should be serviced.
CAUTION: If welding work is to be performed on the vehicle using an electric arc welder, the CAB (MK20e) or ABM (MK25e) connector should be disconnected during the welding operation.
CAUTION: Many components of the ABS System are not serviceable and must be replaced as an assembly. Do not disassemble any component which is not designed to be serviced.
CAUTION: Only the recommended jacking or hoisting positions for this vehicle are to be used whenever it is necessary to lift a vehicle. Failure to raise a vehicle from the recommended locations could result in lifting a vehicle by the hydraulic control unit mounting bracket. Lifting a vehicle by the hydraulic control unit mounting bracket will result in damage to the mounting bracket and the hydraulic control unit.
CAUTION: Brake fluid will damage painted surfaces. If brake fluid is spilled on any painted surface, wash off with water immediately.