General Description

ANTILOCK BRAKES SYSTEM (ABS)
Three braking systems are available for Saturn vehicles. The conventional (base) brake system consists of front disc brakes and rear drum brakes, with power assist. Optional on Saturn models is an antilock brake system (ABS), equipped with front disc brakes and rear drum brakes and power assist.
The brake pressure modulator valve (BPMV) mounted on left side of engine compartment, provides brake fluid modulation for each individual wheel circuit as required during antilock braking. During antilock mode, the BPMV can maintain or reduce brake fluid pressure independent of pressure generated in master cylinder. The BPMV does not provide more pressure than is applied by master cylinder during braking. The BPMV should never be disassembled.
The BPMV uses a four circuit configuration with a diagonal split. Individual circuits are provided for left front and right rear wheels, and left rear and right front wheels. The diagonal circuits are hydraulically isolated so that a leak or malfunction in one circuit will allow continued braking ability on the other. The BPMV consist of several other components which are described as follows:

PUMP MOTOR
The BPMV contains a motor driven recirculation pump. The pump serves one purpose.
During ABS reduce pressure events, it transfers fluid from brake calipers back to master cylinder.

ABS VALVES
The ABS valves decrease or maintain brake fluid pressure at individual wheel circuits. There are four apply, and four release solenoid valves. The solenoid valves maintain, increase, or decrease brake fluid pressure to individual wheel circuits. The EBTCM commands valves to their correct position during an antilock event. During antilock mode, the pressure in each hydraulic circuit can be held or released by activating the appropriate valves. The normal state of outlet valves is closed. This allows direct master cylinder pressure to the brakes during normal braking. The ABS valves are located within the BPMV and are not serviced separately.

ELECTRONIC BRAKE AND TRACTION CONTROL MODULE (EBTCM)
The EBTCM performs the following primary functions:
- Detects wheel slip tendencies
- Detects wheel speed differences
- Controls the brake system while in the antilock mode
- Monitors the system for proper electrical operation
- Provides dynamic proportioning for rear wheels
The EBTCM also controls the display of the ABS and traction control DTCs; while in diagnostic mode. The EBTCM continuously checks the speed of each wheel in order to determine if any wheel is beginning to slip. If a wheel slip tendency is detected, the EBTCM commands appropriate valve positions to modulate brake fluid pressure in some or all hydraulic circuits. This action prevents wheel slip and provides optimum braking. The EBTCM continues to control pressure in individual hydraulic circuits until a slipping tendency is no longer present. The EBTCM continuously monitors ABS/ETS for proper operation. If an error is detected, the EBTCM can disable ABS/ETS and turn on the ABS telltale in the I/P, or turn off the traction switch LED.

PUMP MOTOR RELAY AND SOLENOID VALVE RELAY
The pump motor relay provides power to the pump motor during antilock braking and traction control. The solenoid valve relay provides power to the solenoid valve relay provides power to the solenoid valves in the EBTCM. Both of these relays are an integral part of the EBTCM and cannot be serviced separately.

TIRES WITH ABS/TCS
The compact spare tire rotates faster than the others. The EBTCM compensates for this faster rotation.

BRAKE WARNING INDICATOR
The red BRAKE telltale in the instrument panel will illuminate to warn the driver of conditions in the brake system which may result in reduced braking ability. The indicator will also illuminate when the parking brake is applied or not fully released, or if the brake fluid level switch is closed. The BRAKE warning indicator will stay illuminated until the condition has been repaired.

ABS INDICATOR
The ABS telltale is located in the instrument panel and will illuminate if a malfunction in the ABS is detected by the EBTCM. The ABS indicator informs the driver that a condition exists which results in turning off the antilock brake function. If only the ABS indicator is On, normal braking with full power assist is available. If the RED BRAKE and ABS indicators are on, a problem may exist in the hydraulic brake system. Conditions for the ABS indicator to turn On are as follows:
1. ABS malfunction detected. As previously described, the ABS indicator turns On when a problem has been found in the ABS. The ABS indicator indicates that the ABS has been disabled.
2. Instrument panel panel bulb check. When the ignition is turned to RUN, the ABS indicator will turn on for approximately three seconds and then turn off.
- Illumination of the RED BRAKE warning indicator may indicate reduced braking ability.
- Illumination of the ABS indicator without the RED BRAKE warning indicator on indicates only that antilock braking is no longer available. Power assisted braking without antilock control and dynamic rear proportioning are still available.

LOW TRACTION INDICATOR
The LOW Traction indicator is located in the instrument cluster. The LOW Traction indicator will illuminate to alert the driver of a low traction situation and to inform the driver the ETS is operating. It does not operate during ABS operation.
As a bulb check procedure, the EBTCM will command the LOW Traction indicator on for three seconds when the EBTCM receives ignition power.

TRACTION CONTROL SWITCH
The traction control switch is a momentary contact switch. Each actuation of the switch alternates an internal latching which turns the traction control on or off. When the LED on the switch is On, the traction system is enable and when the LED is Off, the traction system is disabled. At engine start, (no malfunctions present) the traction control system is enabled regardless of previous switch state and the ABS controller illuminates the Traction Control LED. The ABS controller continues to monitor the traction switch for a change in state. If traction control is disabled for any reason other than driver input (turning switch Off), the LED will be turned off. If the malfunction goes away, the traction control led will light and the traction control system will be enabled.