Anti-Lock Brake System & Traction Control: Operation
When pressure is applied to brake pedal, ECU monitors input signals from each wheel speed sensor. If ECU measures a rate of reduction greater than what is programmed in ECU, the ECU will output a signal to appropriate solenoid valve.
On Corrado SLC and Passat with 4-cylinder engines, there is one pair of solenoids for each front disc brake. A third pair of solenoids control hydraulic pressure to both rear brakes. Each solenoid valve allows hydraulic pressure to increase or decrease to the appropriate wheel cylinder.
On all other models, hydraulic line pressure is controlled by one pair of solenoids for each disc brake. Each solenoid valve allows hydraulic pressure to increase or decrease to the appropriate wheel cylinder. If wheel lock-up is detected by ECU, inlet valve closes. This prevents further pressure increase. If wheel lock-up continues, ECU opens outlet valve and fluid pressure returns to brake fluid reservoir. See Figure . Each front brake is controlled separately by ECU. Rear brakes are controlled together based on first wheel which starts to lock.
On Corrado SLC and Passat with 4-cylinder engines, a hydraulic modulator/pump is used. This system uses a pressure sensor which maintains hydraulic pressure in an accumulator. This provides boost pressure for normal braking and ABS operation.
On all other models, a vacuum booster provides pressure assist for normal braking. During ABS operation, some hydraulic fluid is returned to the reservoir. A hydraulic pump runs whenever the brakes are being applied and ABS is in operation. This maintains pressure to the control solenoids and prevents internal loss of brake fluid (brake pedal goes to floor).
On models with traction control, 2 differential locking solenoids are included. If ECU detects wheel spin while vehicle is accelerating less than about 25 MPH, traction control prevents excess wheel spin. Under these conditions, ECU applies brake (on faster spinning wheel) to slow wheel down to the speed of slower spinning wheel.
If a system failure occurs, ANTILOCK warning light, located on instrument panel, will come on. System will be deactivated, but conventional brake system will still operate. If brake fluid level drops too low, BRAKE warning light, located on instrument panel, will come on.