Anti-Lock Brake System: Operation
The EBCM is located in left rear storage compartment, behind driver's seat. Wheel speed sensors supply information about wheel acceleration, deceleration and slip value. The EBCM controls braking by activating and deactivating modulator valve. The EBCM is protected from voltage spikes by electronic brake control relay. Electronic brake control relay is located in rear storage compartment, between battery fuses and EBCM.
Modulator valve consists of 3 switching solenoid valves, 2 accumulator chambers and a return pump. Valves increase and decrease brake fluid pressure to each wheel (rear wheels are on one circuit) to prevent wheel lockup.
Modulator valve regulates brake pressure based on road conditions as determined by EBCM, regardless of master cylinder output pressure. Two relays are mounted on top of modulator valve. The pump motor relay controls pump motor, and solenoid relay controls power supply to solenoid valves. See Fig 1. Modulator valve is located in left rear storage compartment, behind driver's seat.
The lateral accelerometer is used when vehicle is cornering faster than a given curve speed. Information from lateral accelerometer is used by EBCM to control braking action to prevent vehicle spin. The lateral accelerometer is located on instrument panel carrier, beneath radio control. See Figure.
Wheel sensors send a signal to EBCM to indicate speed at each wheel. The EBCM monitors this signal to determine anti-lock brake system operation. When ignition is turned on, Amber SERVICE ABS light on instrument panel will come on. After engine is started, light should go out. If light fails to go out or comes on during vehicle operation, a system malfunction exists and system must be checked.
If SERVICE ABS light is turned on by EBCM, ABS system will generally be turned off. System will remain off until vehicle is restarted. The SERVICE ABS light will also come on if supply voltage to EBCM drops below a specified value. The light will go off once voltage reaches specified value.
The Amber ABS ACTIVE light on instrument panel will activate when pump motor relay applies voltage to pump motor, and ABS ACTIVE indicator relay energizes to indicate ABS operation. When vehicle is first started and vehicle speed reaches approximately 4 MPH, modulator valve pump motor operation can be heard, and ABS ACTIVE light will come on while a system operating test is being performed. This is done to ensure all components are functioning properly.
If a system failure exists, a trouble code will be stored in EBCM memory and EBCM will turn on SERVICE ABS light. The ABS system will be turned off.
The EBCM contains a self-diagnostic capability to detect and isolate system failures. Trouble codes can be retrieved by the FLASH CODE METHOD or TECH I METHOD. See appropriate RETRIEVING CODES method under DIAGNOSIS & TESTING. Trouble code is referred to as current code if it is currently present. If trouble code existed previously, but is not present at time of testing, it will be referred to as a history code.