ANTILOCKING BRAKE SYSTEM (ABS)  To prevent individual wheel locking when braking, the pedal should take the form of a series of impulses caused by rapidly depressing and releasing the pedal.

 An antilock brake pedal application does not rely on the skill of the driver to control wheel lock, instead it senses individual wheel slippage

SPEED SENSOR AND EXCITOR

 The speed sensor uses the variable reluctance magnetic sensing principle.

PRESSURE HOLD POSITION

PRESSURE REDUCING POSITION

ABS FOR PNEUMATIC BRAKES

Pressure increasing

PRESSURE REDUCING

Pressure Reducing  As soon as wheel deceleration or wheel threshold values are exceeded, the sensor transmits this information to the ECM which signals to the solenoid valve unit to reduce the wheel actuator pipe line pressure.  Both solenoids are energized. This opens inlet valve (I) whilst inlet valve (ii) is closed and exhaust (II) is opened. The open inlet valve (I) allows air to enter and pressurize pilot chamber (I) so that diaphragm (I) closes the outlet passage, thus preventing anymore air from the foot valve passing through to the outlet port.

Pressure reducing continues  At the same time, solenoid (II) closes inlet valve (II) and opens exhaust valve (II). This exhausts air from pilot chamber (II) permitting compressed air from the wheel actuator to push open diaphragm (II) outlet exhaust passage, causing the air pressure on the actuator pipe line to reduce quickly

PRESSURE HOLD

Pressure Hold  When the road wheel acceleration reaches a predetermined value, the sensor relays this information to the ECM, which in turn signals the solenoid control valve to hold the remaining pipe line actuator pressure  Solenoid (I) remains energized but solenoid (II) is de- energized. Therefore solenoid (I) inlet valve (I) and exhaust valve (I) remain open and closed respectively. Inlet valve (II) allows compressed air into pilot chamber (I) so that diaphragm (I) closed the outlet passage leading to the wheel actuator pipe line.

Pressure Hold continue  Conversely, solenoid (II) is now de-energized causing its return spring to move the armature so that the inlet valve (II) opens and exhaust valve (II) closes.  Compressed air from the foot valve now flows through the open inlet valve (II) along the passage leading to the underside of diaphragm (II) thus keeping the outlet exhaust passage closed.  Compressed air at constant pressure is now trapped between both closed diaphragm outlet passages and the wheel actuator pipe line. This pipe line pressure is maintained until the sensor signals that the wheel is accelerating above the threshold, at which point the ECM signals the solenoid control valve to its rising pressure mode