ABS Operation

NORMAL BRAKING
Under normal braking conditions, the ABS functions virtually like a conventional vacuum boost brake system. If a wheel slip tendency is noted during a brake application, the ABS will enter antilock mode. During antilock braking, hydraulic pressure in that wheel circuit is modulated to prevent the wheel from slipping. Pedal operation during normal braking should be no different than that of a conventional vacuum boost brake system. During antilock operation, the brake pedal may rise and pulsate, even as the brakes are being applied. This pedal feedback is normal. Ticking and/or popping noises may also be heard as the ABS valves cycle to modulate the hydraulic pressure. On dry pavement the tires may make intermittent chirping noises as the wheel(s) approach slip.

PRESSURE HOLD
When the Electronic Brake/Traction Control Module (EBTCM) senses a wheel slip the EBTCM isolates the system by closing the Hold valve and keeps the Release valve closed in the Brake Pressure Modulator Valve (BPMV). This holds the pressure steady on the brake so hydraulic pressure does not increase or decrease.

PRESSURE DECREASE
If during the pressure hold mode the EBTCM still senses wheel speed slip it will decrease pressure to the brake. It does this by leaving the Hold valve closed and opening the Release valve in the BPMV. The excess fluid is stored in the accumulator until it can be returned to the reservoir by the return pump.

PRESSURE INCREASE
During the pressure hold or decrease mode, if the EBTCM senses excessive wheel speed, the brake pressure is increased. This is done by opening the Hold valve and closing the Release valve in the BPMV. The increased pressure is in addition to the manually applied brake pressure.