Antilock Brakes / Traction Control Systems: Description and Operation

The Antilock Brake System maintains vehicle stability and shortens stopping distance during braking by preventing the vehicle's wheels from locking up. The Antilock Brake System primary component operations are outlined below.

Wheel Speed Sensors

The Wheel Speed Sensors send wheel speed information to the Electronic Brake Control Module (EBCM) in the form of AC voltage. The induced AC voltage signal has a frequency proportional to wheel speed.

TCC/Antilock Brake Switch

The TCC/Antilock Brake Switch is a pedal travel input to the EBCM. The normally closed switch opens when the brake pedal is depressed approximately 40% down. During ABS braking, the EBCM turns on the Pump Motor. The EBCM turns off the Pump Motor when brake pedal height sufficiently rises to close the TCC/Antilock Brake Switch.

Pressure Modulator Valve (PMV) Fluid Level Switch

The normally closed switch opens when the PMV reservoir fluid level is low. This signals the EBCM to illuminate the "ANTILOCK" indicator alerting the driver of the low fluid condition.

Pump Motor Run Sensor

The EBCM checks Pump Motor operation using the Pump Motor Run Sensor input. When the pump is operating, voltage is induced into the sensor's coil, and the EBCM monitors this voltage to ensure the Pump Motor is working properly.

Electronic Brake Control Module (EBCM)

The Electronic Brake Control Module (EBCM) controls hydraulic pressure to wheel cylinders by controlling inlet and outlet valve solenoids. Hydraulic pressure is controlled by pulsing the inlet and outlet valves on and off. The EBCM monitors each wheel speed sensor signal and the Cruise/Shift Interlock/Brake Switch to determine wheel lockup. The EBCM also controls the Pump Motor during an ABS stop based on input from the TCC/Antilock Brake Switch and Wheel Speed Sensor Signals. The EBCM also monitors all antilock brake components and circuitry faults, and controls the "ANTILOCK" Indicator.

Pressure Modulator Valve (PMV) Assembly

The Pressure Modulator Valve (PMV) Assembly contains inlet and outlet valves and the Pump Motor, The individual components are not serviceable and must be replaced as an assembly. The inlet valves, outlet valves and Pump Motor operation are controlled by the EBCM. The Pump Motor replenishes the PMV Assembly's fluid reservoir and applies fluid pressure to the Master Cylinder. This prevents excessive pedal travel during an ABS stop. This low pressure system does not need an accumulator to boost pressure.

TCS CIRCUIT OPERATION

In addition to antilock brakes, some vehicles are equipped with a Traction Control System (TCS). Traction control prevents the drive wheels from spinning excessively during acceleration. The system, designed to operate at speeds under 40 km/h (25 mph), reduces wheel slip by braking the wheel that is losing traction. The Traction Control System's primary component operations are outlined below.

Electronic Brake Control Module (EBCM)

The EBCM controls the isolation valve solenoids. These valves isolate the front wheel brake hydraulic circuit from the Master Cylinder and the rear brake hydraulic circuits during a traction control situation. The EBCM then turns on the Pump Motor and operates the proper front inlet and outlet valve to control hydraulic pressure. The EBCM monitors both front wheel speed signals to determine wheel slip. The EBCM also monitors the Transaxle Temperature Switch, PMV Pressure Switch and isolation valves, and all related circuitry for system faults. The EBCM controls the "TRACTION OFF" indicator.

Transaxle Temperature Switch

The EBCM monitors transaxle temperature through the Transaxle Temperature Switch. When the transaxle becomes hot, about 1600°C (3200°F), the normally closed switch opens signaling the EBCM to disable the Traction Control System and turns on the "TRACTION OFF" indicator. The system remains disabled for approximately 3 to 5 minutes after the switch closes. This allows the brakes and transaxle to cool down. NOTE: When performing tests on the system, if the Transaxle Temperature Switch is disconnected, the Traction Control System becomes disabled for approximately 3 to 5 minutes.

Pressure Modulator Valve (PMV) Assembly

The PMV Assembly contains the Pump Motor, isolation valves and the Pressure Switch. The above components are not serviced separately and must be replaced as an assembly.
During traction control mode, the EBCM energizes the isolation valves. The valves isolate the front wheel brake hydraulic circuit from the Master Cylinder and the rear wheel brake hydraulic circuits in order to apply pressure to only the drive wheels. Once the front wheel brake circuits are isolated, the EBCM, applies pressure by turning on the PMV Pump Motor and cycling the proper inlet and outlet valves.

Pressure Modulator Valve (PMV) Pressure Switch

The normally closed switch opens whenever the brakes are applied. If the brakes are manually applied during traction control mode, the PMV Pressure Switch input along with the brake switch input, signal the EBCM to disable traction control and allow manual braking.

Traction Control Switch

Located in the console center trim plate, the switch allows the driver to select the traction control mode (on/off). When the Traction Control System is turned off, the "TRACTION OFF" indicator remains illuminated.