SAFARI BSVI - SERVICE MANUAL

BRAKES (Non EPB) 13 7. WORKING OF ANTI-LOCK BRAKING SYSTEM (ABS): 1. ABS is an aid to retain steering control and stability while braking. 2. ABS will not prevent accidents resulting from excessive cornering speeds, following another vehicle too closely or aquaplaning, i.e. where a layer of water prevents adequate contact between tyre and road surface. 3. ABS will not improve braking performance or stopping distance beyond that which is physically possible in relation to road conditions and the vehicles tyres or brakes. 4. The additional control provided by ABS must never be exploited in a dangerous or reckless manner which could jeopardize the safety of driver or other road users. The ABS ECU takes wheel speed information from sensors, located within the hub carrier, and monitors the relative deceleration/acceleration of each wheel at all times. These signals are used to calculate the rotational deceleration (negative acceleration) rates for each wheel during braking operation. In the event of a wheel slip condition (i.e. rotational deceleration being outside of allowable limits) the hydraulic system will control the brake line pressure by operating the appropriate solenoid valves within the hydraulic modulator and thus releasing the brake pressure from that particular wheel. Once the wheel deceleration has recovered to within allowable limits the modulator then allows pressure to be re-applied to the particular brake caliper or wheel cylinder from which the brake pressure was released. When ABS intervention is necessary the warning lamp will not illuminate but the driver will feel the brake pedal pulsating and may experience audible feedback from the modulator indicating that ABS control is taking place. The ABS and EBD warning lamps will illuminate for a system check when the ignition is switched on. Both lamps will then extinguish to indicate the system is healthy. (As the EBD lamp is shared with the Parking brake lamp this will not extinguish until the parking brake is released.) If a fault occurs with the system or any of its constituent components the relevant warning lamp in the instrument cluster will illuminate to inform the driver for the respective fault. A. BRAKING WITHOUT ABS IN OPERATION: In case of normal braking operation when no wheel speed differences are detected and neither ABS nor EBD are active, when the driver presses the brake pedal, brake pressure developed in tandem master cylinder (TMC) reaches to each wheel through hydraulic control unit (HCU). During this phase, both inlet & outlet solenoid valves are switched OFF (or not excited) by the ECU. Hence the inlet valve is open and outlet valve remains closed allowing the brake fluid to flow freely through the inlet valve and to the relevant wheel. When the driver releases brake pedal, the brake fluid flows back from the wheel, through the respective one-way valve towards TMC. Solenoid Valve Electricity Status Valve Open Open- Close channel close INLET OFF OPEN Master cylinder Wheel cylinder OUTLET OFF CLOSE Wheel cylinder Reservoir B. BRAKING WITH EBD INTO OPERATION: In case of a braking where the ABS ECU detects a slip difference between the front and rear wheels, EBD comes into operation. If the difference in wheel slip between the front and rear wheels exceeds a predetermined threshold the ABS activates the pressure inlet valves for the rear wheels, closing the valves and preventing further pressure build up at that rear axle. If during the braking event, this slip ratio between the front and rear axle reduces below the threshold level the inlet valves will be de-activated and switch back to their normally open state allowing further pressure increase at the rear axle. In this way the rear axle can follow more closely to the Ideal braking force distribution curve for the rear axle. If the driver releases the brake, the inlet valves will be deactivated and fluid can return to the TMC via the inlet valve and the one-way valve. EBD control only acts upon the rear axle and can only limit pressure being applied to a rear wheel; it cannot perform pressure reductions.