Antilock Brakes / Traction Control Systems: Description and Operation
ABS Description and Operation
Fuse Block - Underhood, PCM, EBCM, and Aux. Battery
BPMV Hydraulic Flow
This vehicle is equipped with the Kelsey-Hayes EBC310 antilock braking system.
The following components are involved in the operation of the above system:
^ Electronic Brake Control Module (EBCM) - The EBCM controls the system functions and detects failures.
The EBCM contains the following components:
- System relay - The system relay is internal to the EBCM. The system relay is energized when the ignition is ON. The system relay supplies battery positive voltage to the solenoid valves and to the pump motor. This voltage is referred to as system voltage.
- Solenoids - The solenoids are commanded by the EBCM to operate the appropriate valves in the brake pressure modulator valve.
^ Brake pressure modulator valve (BPMV) - The BPMV uses a 3-circuit configuration to control the left front wheel, the right front wheel, and the combined rear wheels. The BPMV directs fluid to the left front and right front wheels independently. The BPMV directs fluid to both of the rear wheels on a single hydraulic circuit.
The BPMV contains the following components:
- Pump motor
- Three isolation valves (one for each hydraulic circuit)
- Three dump valves (one for each hydraulic circuit)
- A front low-pressure accumulator
- A rear low-pressure accumulator
^ Wheel Speed Sensors (WSS) - As the front wheels spin, a toothed ring located at each wheel hub interrupts a magnetic field in the wheel speed sensors. This causes the wheel speed sensors to generate an AC signal. The EBCM uses this AC signal to calculate the wheel speed. The wheel speed sensors are serviceable only as part of the wheel hub and bearing assemblies. Any imperfections in the toothed ring, such as a missing or damaged tooth, can cause an inaccurate WSS signal.
^ Vehicle Speed Sensor (VSS) - The input signal for rear wheel speed originates with the VSS. The Powertrain Control Module (PCM) or Vehicle Speed Sensor Buffer (VSSB-diesel only) receives rear wheel speed information from the VSS and supplies this information to the EBCM.
Initialization Sequence
The EBCM performs one initialization test each ignition cycle. The initialization of the EBCM occurs when the following conditions are met:
^ The ignition is ON.
^ The bulb check has been completed.
^ Vehicle speed is greater than 6 km/h (4 mph).
The initialization sequence briefly cycles each solenoid and the pump motor to verify proper operation of the components. The EBCM sets one or more DTCs in accordance with any malfunction that is detected.
The EBCM defines a drive cycle as the completion of the initialization sequence.
Antilock Brake System
When wheel slip is detected during a brake application, the ABS enters 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. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.
During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the EBCM responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and 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 cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during antilock operation.
Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.
Pressure Hold
The EBCM closes the isolation valve and keeps the dump valve closed in order to isolate the slipping wheel when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
Pressure Decrease
If a pressure hold does not correct the wheel slip condition, a pressure decrease occurs. The EBCM decreases the pressure to individual wheels during deceleration when wheel slip occurs. The isolation valve is closed and the dump valve is opened. The excess fluid is stored in the accumulator until the pump can return the fluid to the master cylinder or fluid reservoir.
Pressure Increase
After the wheel slip is corrected, a pressure increase occurs. The EBCM increases the pressure to individual wheels during deceleration in order to reduce the speed of the wheel. The isolation valve is opened and the dump valve is closed. The increased pressure is delivered from the master cylinder.
Brake Warning Indicator
The brake warning indicator illuminates when one or more of the the following occurs:
^ The park brake is set.
^ The pressure differential switch is off-center due to a base brake failure.
^ The EBCM performs the bulb check.
^ The EBCM detects an ABS disabling malfunction while DTC C0286 is set. This is because DTC C0286 disables the ABS indicator.
^ The ignition is turned to START (DRL equipped vehicles only).
ABS Indicator
The ABS indicator illuminates when one or more of the the following occurs:
^ The Electronic Brake Control Module (EBCM) detects an ABS-disabling malfunction.
^ The EBCM performs the bulb check.
^ The EBCM does not power up or loses power during the ignition cycle.