ABS System Description

Hydraulic Flow:

PURPOSE
The purpose of the Delco Bosch 5 (TM) Antilock Brake System (ABS) is to minimize wheel slip during heavy braking. The Delco Bosch 5 performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during a braking event. This allows the driver to retain directional stability and better steering capability.

ANTI-LOCK BRAKING MODE
When wheel slip is noted during a brake application, the ABS will enter Antilock mode. During Antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping. A separate hydraulic line and specific solenoid valves are provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressure to each wheel brake. However, it cannot increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking. During Antilock braking, a series of rapid pulsations will be felt in the brake pedal.

Normal Signs Of ABS Operation
Pulsations are caused by the rapid changes in position of the individual solenoid valves as they respond to their desired wheel speed. This pedal pulsation is present during antilock braking and will stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves rapidly cycle. During antilock braking on dry pavement, the tires may make intermittent "chirping" noises as they approach slipping. These noises and pedal pulsation's should be considered normal during Antilock operation.

Obtaining Shortest Stopping Distance
Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal will provide the shortest stopping distance while maintaining vehicle stability.

PRESSURE HOLD MODE
When the Electronic Brake/Traction Control Module (EBCM/EBTCM) senses a wheel slip the EBCM/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 MODE
If during the pressure hold mode the EBCM/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 MODE
If during the pressure hold or decrease mode the EBCM/EBTCM senses that the wheel speed is too fast it will increase pressure to the brake. It does this by opening the hold valve and closing the release valve in the BPMV. The increased pressure comes from the master cylinder and is related to the pressure applied to the brake pedal.