Brakes (Operation): Description and operation

Operation

Refer to illustration. BRAKES, DESCRIPTION AND OPERATION, Brake system control diagram.

When the ignition is switched on, the SLABS ECU performs a check of the brake related warning lamps as part of the power up procedure. The warning lamps are illuminated for approximately 3 seconds and then extinguished. If a fault warning lamp remains illuminated after the lamp check, a fault has been detected and repair action is required.

ABS

The ABS function prevents the road wheels locking during brake application, thus maintaining vehicle stability even under emergency conditions.

WARNING: ABS is an aid to retaining steering control and stability while braking: l ABS cannot defy the natural laws of physics acting on the vehicle. l ABS will not prevent accidents resulting from excessive cornering speeds, following another vehicle too closely, aquaplaning, etc. l The additional control provided by ABS must never be exploited in a dangerous or reckless manner which could jeopardise the safety of driver or other road users. l The fitting of ABS does not imply that the vehicle will always stop in a shorter distance.

NOTE: During normal braking the feel of the brake pedal on vehicles equipped with ABS will be the same as that on non ABS vehicles. During anti-lock braking operation the driver will experience feedback in the form of a pulsating brake pedal and solenoid/pump motor noise from the ABS modulator.

The anti-lock braking function is automatically enabled whenever the ABS modulator is in the normal braking mode. While the anti-lock braking function is enabled, if the SLABS ECU detects a wheel decelerating faster than the average and at the calibrated wheel slip limit for ABS operation, it operates the ABS modulator in the ABS braking mode for the affected wheel.

EBD

The EBD function optimises the distribution of hydraulic pressure between the front and rear axles, under all vehicle load configurations and road conditions, to maintain vehicle stability during braking. EBD operates in forward and reverse and is automatically enabled whenever the ABS modulator is in the normal braking mode at vehicle deceleration rates of 0.3 g and above (i.e. medium to high brake pedal loads). EBD operation is similar to that of ABS, but is calibrated to intervene at lower wheel slip limits and operates the brakes in axle pairs instead of individually.

During braking, if the SLABS ECU detects the wheels of one axle going slower than those of the other axle, i.e. a potential wheel slip situation, it signals the ABS modulator to close the inlet solenoid valve for the brakes of the slower wheels. This prevents any further increase in hydraulic pressure to those brakes, while allowing the hydraulic pressure to the brakes on the other axle to increase and so maximise the overall braking effort. If the wheel speeds of the axle being subjected to EBD control return within the calibrated wheel slip limits, the SLABS ECU signals a stepped opening of the inlet solenoid valves, which allows a progressive increase of hydraulic pressure to the related brakes.

Operation of EBD is detectable from a stiffening of brake pedal movement as the inlet solenoid valves close and a slight pulsing of the brake pedal as the inlet solenoid valves open. EBD operation ceases immediately the brake pedal is released.

The wheel slip limit for EBD operation varies with vehicle speed. During normal operation, the inlet solenoid valves always operate in axle pairs, with only one axle pair closed at any one time. Since the most lightly loaded wheel during a braking manoeuvre will usually be the first to reach the slip limit, under most vehicle load configurations and road conditions EBD control occurs on the trailing axle. However, EBD control can occur on the leading axle or switch between axles during the braking manoeuvre.