System Description: Notes

This vehicle is equipped with the Lucas/Sumitomo hybrid non-integral Anti-lock Brake System (ABS). The system employs a single valve per channel, 4-channel system to reduce the tendency of any wheel to lock up. This improves driver control of the vehicle during braking. The purpose of the ABS is to minimize wheel lockup during heavy braking on most road surfaces. The ABS performs this function by monitoring the speed of each wheel and then controlling the brake fluid pressure to each wheel independently during a braking maneuver. This allows the driver to retain directional stability and allows the driver better steering capability.

The purpose of the ABS is to minimize wheel slipping during braking, thus improving vehicle stability and steerability. The ABS accomplishes this objective by controlling the hydraulic pressure applied at each front wheel brake and both rear wheel brakes as a pair.

The ABS operation is available from approximately 6.3 km/h (4 mph) to the maximum vehicle speed. During braking the ABS is available from the maximum vehicle speed to 5 km/h (3 mph).

Fig 1: ABS Components

Courtesy of GENERAL MOTORS CORP.

The BPMV assembly is a compact integrated unit that contains the following components:

• An EBCM
• Four solenoid valves, one for each wheel brake hydraulic channel
• Four flow control valves in-series with each wheel brake hydraulic channel, one for each solenoid valve
• Buffer chambers
• A pump motor
• Check valves

The BPMV assembly is located on the left rear corner of the engine compartment just below the master cylinder. The BPMV assembly is mounted on a bracket.

The controlling element of the ABS is a microprocessor based EBCM. The inputs to the system include the following components:

• The four wheel speed sensors
• The stoplamp switch
• The ignition switch
• The battery voltage
• The OBD2 Data Link Connector (DLC)
• The deceleration sensor on vehicles equipped with all-wheel drive.

The outputs include the following components:

• The four solenoid valves
• The pump motor
• The ABS indicator
• The DLC

The EBCM monitors the speed of each wheel. If any wheel begins to slip during braking, the EBCM actuates the solenoid valves and the pump motor to modulate hydraulic pressure to the wheel brake(s) approaching slip. Once the wheel regains traction the hydraulic pressure is increased until the wheel again begins to slip. This cycle repeats until any of the following happens:

• The vehicle comes to a stop.
• The brake pedal is released.
• No wheels approach slippage.

Additionally the EBCM monitors each input and each output for proper operation. The EBCM sets a Diagnostic Trouble Code (DTC) when it detects a system malfunction. The DTC will not clear when the battery is disconnected because the EBCM stores DTCs in nonvolatile memory. The EBCM is an integral part of the BPMV assembly and is not serviceable separately.

Fig 2: Front Wheel Speed Sensors and Rings

Courtesy of GENERAL MOTORS CORP.

The front wheel speed sensors are of a variable reluctance type. Each sensor is attached to the knuckle assembly in close proximity to a toothed ring. The toothed ring is pressed onto the outer Constant Velocity (CV) joint. As the toothed ring rotates, the wheel speed sensor generates an AC voltage. The magnitude and the frequency are proportional to the speed of the wheel and both will increase with increased speed. The sensor is not repairable nor is the air gap adjustable.

Fig 3: Rear Wheel Speed Sensors and Rings

Courtesy of GENERAL MOTORS CORP.

The rear wheel speed sensors operate in the same manner as the front wheel speed sensors. The rear wheel speed sensors are mounted in the rear suspension knuckles. The rear wheel speed sensor reluctor ring is part of the rear hub and bearing assembly. Replace the rear hub and bearing assembly when the rear wheel speed sensor ring is damaged.