When wheel slip is noted during a brake application, the ABS will enter the antilock mode. During antilock braking, the hydraulic pressure in the individual wheel circuits is controlled in order 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 the hydraulic pressure to each wheel brake. However, the ABS cannot increase the hydraulic pressure above the amount that the master cylinder transmits during braking.
During antilock braking, a series of rapid pulsations will be felt in the brake pedal. The rapid changes in the position of the individual solenoid valves as they respond to the desired wheel speed causes the pulsations. 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 nose 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 pulsations are normal during antilock operation. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal provides the shortest stopping distance while maintaining vehicle stability.
When the EBCM senses the wheel slip, the EBCM closes the inlet valve and keeps the outlet valve closed in the BPMV in order to isolate the system. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
If during the pressure hold mode the EBCM still senses wheel slip, the EBCM will decrease the pressure to the brake. The inlet valve is left closed and the outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cylinder.
If during the pressure hold or the pressure decrease mode the EBCM senses that the wheel speed is too fast, the EBCM will increase the pressure to the brake. The inlet is opened and the outlet valve is closed. The increased pressure comes from the master cylinder and is related to the pressure applied to the brake pedal.
The EBCM performs self-diagnostics during initialization.
The EBCM also verifies correct operation of the following components during initialization:
The EBCM inspects the wheel speed sensor circuitry for the following conditions during initialization:
If the EBCM detects a malfunction in itself or in other ABS components, the EBCM will perform the following actions:
The following indicators will remain on for approximately three seconds with the key in the ON position:
The initialization performs during engine cranking and engine starting. A slight mechanical noise may be heard during the system initialization. This noise is normal.
During this process, when the vehicle attains a speed of 5 km/h (3 mph). The EBCM will verify motor, hydraulic modulator and solenoids by using specific control schemes to verify all components are functioning properly.
If brake pedal pressure interrupts the system initialization, a slight movement in the brake pedal may exist.
The standard brake system uses a single red BRAKE warning indicator, which is located in the instrument panel cluster.
The antilock brake system uses the following indicators:
The following indicators will turn ON for approximately three seconds and then turn OFF when the ignition is turned to the ON position:
As the engine is cranked, the red BRAKE warning indicator and the amber ABS indicator should illuminate steadily.
Three seconds after the engine is started, the ABS indicator should be OFF. The red BRAKE warning indicator will go OFF immediately.
Using the compact spare supplied with the vehicle will not affect the operation of ABS. The EBCM software can compensate for this smaller tire. However, increased stopping distances may occur due to reduced tire tread depth on compact spare tires.
Tire size is important for proper performance of the ABS. Replacement tires should be the same size, load range, and construction as the original tires. Replace tires in axle sets and only with tires of the same Tire Performance Criteria (TPC) specification number. Use of any other tire size or type may seriously affect the ABS operation.
Refer to Tires and Wheels in Suspension for more information.
