Anti-Lock Brake System: Operation

After initial engine start-up, when vehicle speed reaches 4 MPH in forward or reverse, the Electronic Brake Control Module (EBCM) performs an automatic test. The automatic test cycles each valve solenoid, pump motor and relay to check component operation. It is normal to both feel and hear the automatic test procedure.

The ABS/ASR system functions differently to control braking or acceleration slip. The ABS function uses a 4-way hydraulic circuit; one circuit for each wheel. However, when ABS is activated, modulator valve assembly activates both rear wheels simultaneously, regulating them as one circuit. 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. Four wheel speed sensors provide input to EBCM for ABS/ASR operation. The wheel speed sensors produce an AC voltage signal proportional to wheel speed.

To prevent wheel slip during acceleration, the ASR function uses (in order) spark timing retard, throttle close-down and rear brake application. The Electronic Control Module (ECM) is part of ASR operation. The ECM is located in left rear of engine compartment above battery. EBCM monitors difference between front wheel speed and rear wheel speed to detect rear wheel slip during acceleration. During ASR operation, EBCM requests ECM to retard spark timing as necessary to prevent wheel slip. The ECM also monitors Throttle Position Sensor (TPS) and disables Torque Converter Clutch (TCC), if equipped. After EBCM retards spark timing, if rear wheels still slip, EBCM will use throttle close-down to limit slip. The EBCM uses TPS module to monitor TPS signal. The TPS sensor provides necessary input to TPS module for EBCM input. The EBCM will use adjuster assembly to close-down throttle, lowering engine torque. After both spark timing retard and throttle close-down are applied, if rear wheels still slip, EBCM will apply rear brakes as necessary. During ASR operation, rear brake circuits are controlled individually to prevent rear wheel slip. When ASR is activated with cruise control on, the cruise control cut-off relay (located behind instrument panel, left of steering column) disables cruise control and TCC (if equipped).

The lateral accelerometer (located in instrument panel carrier below radio) is used when vehicle is performing a severe turning or cornering maneuver at 30 MPH or more. When vehicle experiences severe rear-end yaw, EBCM utilizes ASR to prevent further rear-end yaw.

ASR can be switched off by activating ASR switch located above headlight switch. After vehicle is started, ASR switch can be activated to manually override ASR control. ASR will remain off during entire ignition on cycle. When ignition is turned off, then on again, ASR will resume control until turned off by driver again. If ASR switch is activated during an ASR operation, ASR will not deactivate until ASR operation is complete. If a problem is detected by EBCM with ASR switch, EBCM will default to leaving ASR on. Driver will be unable to deactivate ASR. EBCM will set a code for ASR switch malfunction. If ASR switch is depressed for more than 15 seconds, EBCM will default to leaving ASR on and set a code.

The Electronic Brake Control Module (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 assembly.

The EBCM contains a self-diagnostic capability to detect system failures. Trouble codes can be retrieved by using Central Control Module (CCM) or Tech 1 "Scan" tester (94-00101-A).

Modulator valve assembly consists of 5 electronically controlled solenoid valves and 2 hydraulically controlled valves. The 5 electronically controlled solenoid valves are 4 wheel solenoid valves and a pilot valve. The 4 wheel solenoid valves have 3 positions; pressure hold, pressure decrease and pressure increase. Wheel solenoid valves are spring loaded to pressure increase (neutral position).

The pilot valve isolates master cylinder from pump during ASR function. The pilot valve closes when pump directs brake fluid to rear brake circuit for ASR operation. Excess fluid passes through the pressure limiting valve.

The 2 hydraulically controlled valves are a load valve and pressure limiting valve. The load valve isolates brake fluid prime pipe from modulator valve assembly to master cylinder reservoir during brake application. Load valve is spring loaded to open position. The pressure limiting valve regulates pump pressure by allowing excess fluid to return to master cylinder through brake fluid prime pipe. The pressure limiting valve only operates during ASR operation.

The modulator valve assembly transfers brake fluid from calipers to master cylinder to reduce brake pressure during ABS operation. During ABS operation, modulator valve assembly does not apply pressure for pressure hold beyond that of driver input on brake pedal during braking. Modulator valve assembly transfers brake fluid from master cylinder to rear calipers during ASR operation. Two relays are mounted on top of modulator valve assembly. The pump motor relay controls pump motor, and solenoid relay controls power supply to solenoid valves.

With engine running, ABS ACTIVE light indicates ABS system is operating to prevent wheel lock-up during braking. SERVICE ABS light indicates ABS system has been disabled and requires service. ASR OFF light indicates driver has selected to turn ASR system off. ASR ACTIVE light indicates system is operating to prevent rear wheel slip during acceleration or severe cornering. SERVICE ASR light indicates ASR system requires service. BRAKE light indicates parking brake is applied, a problem with brake differential pressure is present, or brake fluid level is low. The Driver Information Center (DIC), located on right side of instrument panel, contains ABS ACTIVE, SERVICE ABS, ASR ACTIVE, ASR OFF and SERVICE ASR lights. BRAKE light is located on right side of instrument cluster.