The purpose of the Antilock Brake System (ABS) is to minimize wheel slip during heavy braking. The ABS performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during an braking event. This allows the driver to maintain directional stability while minimizing stopping distance.
ABS Control Module (EBCM)
The ABS control module monitors the speed of each wheel to detect wheel slip. If wheel slip is detected, the module commands the appropriate valve positions in the valve assembly to modulate brake pressure in some or all of the hydraulic circuits. This action prevents wheel slip and provides optimum braking. In addition, the ABS control module performs a continuous diagnostic routine to detect malfunctions. If the module detects an electrical malfunction, it can disable ABS/TCS/DRP, illuminate the appropriate telltale and store a Diagnostic Trouble Code (DTC).
The ABS control module contains a solenoid for each ABS/TCS valve it operates and a relay for the solenoid circuit and pump motor circuit respectively. The relays and solenoids are not serviceable.
ABS Operation
During an ABS event, the ABS control module will control the hydraulic pressure in the individual wheel circuits to prevent any wheel from slipping. The control module can decrease or hold hydraulic pressure by energizing the appropriate valve solenoid. A rapid pulsation is felt in the brake pedal and a ticking or popping noise can be heard as control module commands valve solenoids in response to wheel speed changes.
When the ABS control module detects wheel slip, it holds pressure by closing both the inlet valve from the master cylinder and the outlet valve. If pressure hold is not enough to control wheel slip, the ABS control module releases pressure by opening the outlet valve and allowing some pressure bleed-off into the accumulator. The accumulator stores this fluid until the motor-driven pump returns fluid to the master cylinder.
The control module opens the inlet valve to allow master cylinder pressure to the wheel circuits again when no wheel slip is detected.
Traction Control System (TCS)
The Traction Control System (TCS) compares front wheel speeds to rear wheel speeds to determine if drive wheels lose traction. The TCS activates when drive wheel speed exceeds speed of non-drive wheels by a calibrated value. This allows the driver to maintain acceleration and directional stability while accelerating on low traction surfaces.
The TCS limits wheel slip during acceleration when one or more of the drive wheels lose traction. The brake switch must be off for TCS to operate.
The ABS control module monitors wheel speed output and compares drive wheel to non-drive wheel values to detect wheel slip. During a TCS event, the ABS control module sends a requested torque value to the ECM/TCM (L81) or PCM (L61). The ECM/TCM or PCM initiates an engine torque reduction routine to slow down the drive wheels. This routine consists of ignition timing reduction, fuel injector cut-off and transmission shift control. The ECM/PCM also sends a torque delivered value to the ABS control module. If the engine management routine is insufficient to achieve the desired wheel speed, the ABS control module will then use the TCS isolation valves and prime valves to slow down the drive wheels. The isolation valves close to isolate the wheel circuit from the master cylinder and the prime valves open to allow the motor-driven pump to access master cylinder fluid so it can build pressure to apply the desired brake.
If the ABS control module detects a malfunction, it will disable TCS and will command the BCM to turn the TRAC LED Off. The driver can also disable TCS, if desired, by depressing the TRAC switch. When the driver depresses the TRAC switch, the ABS control module will disable TCS and will command the BCM to turn the TRAC LED Off.
Valve Assembly (BPMV)
The valve assembly provides brake fluid pressure modulation for each of the individual wheel circuits, as required, during an ABS/TCS event. During an ABS event, the valve assembly can maintain or reduce brake fluid pressure that is applied by the master cylinder. The valve assembly cannot increase pressure beyond what is applied by the master cylinder (driver). The valve assembly contains a motor-driven pump, ABS inlet and outlet valves, as well as TCS prime and isolation valves. With exception of the ABS control module (EBTCM), the valve assembly is not serviceable and should never be disassembled.
The valve assembly is an addition to the four-circuit diagonally-split hydraulic system found on vehicles without ABS.
Dynamic Rear Proportioning (DRP)
Dynamic Rear Proportioning (DRP) is an electronic brake proportioning feature that replaces the mechanical proportioning valve in the base brake system. DRP uses existing ABS components to regulate the vehicle's rear brake pressure and provide optimum front-rear brake balance in all braking situations.
This vehicle is equipped with the BOSCH 8.0 antilock braking system.
The vehicle is equipped with the following braking systems:
| • | Antilock Brake System (ABS) |
| • | Dynamic Rear Proportioning (DRP) |
| • | Traction Control System (TCS) (w/NW9) |
The following components are involved in the operation of the above systems:
| • | Electronic Brake Control Module (EBCM) -- The EBCM controls the system functions and detects failures. |
| The EBCM contains the following components: |
| • | System Relays -- The inlet and outlet solenoid relay is energized when the ignition is ON and no ABS DTCs are present. It supplies battery positive voltage to the solenoid valves, the pump motor relay is energized and supplies battery positive voltage to the pump motor when ABS is activated. |
| • | Brake Pressure Modulator Valve (BPMV) -- The BPMV contains the hydraulic valves and pump motor that are controlled electrically by the EBCM. The BPMV uses a 4 circuit configuration with a diagonal split. The BPMV directs fluid from the reservoir of the master cylinder to the left front and right rear wheels and fluid from the other reservoir to the right front and left rear wheels. The diagonal circuits are hydraulically isolated so that a leak or malfunction in one circuit will allow continued braking ability on the other. |
| Important: There is a rubber isolator located under the BPMV and on the mounting studs. The rubber isolators protect the BPMV and the EBCM from vehicle vibrations. |
| • | The BPMV contains the following components: |
| - | Pump Motor |
| - | Inlet Valves (one per wheel) |
| - | Outlet Valves (one per wheel) |
| • | Wheel Speed Sensors (WSS) -- As the wheel spins, the wheel speed sensor produces an AC signal. The EBCM uses this alternating current signal to calculate wheel speed. The wheel speed sensors are replaceable only as part of the wheel hub and bearing assemblies. |
| • | Traction Control Switch (w/NW9) -- The TCS is manually disabled or enabled using the traction control switch. |
| • | Stop Lamp Switch - The EBCM uses the stop lamp switch as an indication that the brake pedal is applied. |
Initialization Sequence
The EBCM performs one initialization test each ignition cycle. The initialization of the EBCM occurs when the vehicle speed is greater than 16 km/h (10 mph).
The initialization sequence may also be commanded with a scan tool.
The initialization sequence cycles each solenoid valve and the pump motor, as well as the necessary relays for approximately 1.5 seconds to check component operation. The EBCM sets a DTC if any error is detected. The initialization sequence may be heard and felt while it is taking place, and is considered part of normal system operation.
The EBCM defines a drive cycle as the completion of the initialization sequence.
Pressure Hold
The EBCM closes the inlet valve and keeps the outlet valve closed in order to isolate the system when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
Pressure Decrease
The EBCM decreases the pressure to individual wheels during a deceleration when wheel slip occurs. The inlet valve is closed and the outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cylinder.
Pressure Increase
The EBCM increases the pressure to individual wheels during a deceleration in order to reduce the speed of the wheel. The inlet valve is opened and the outlet valve is closed. The increased pressure is delivered from the master cylinder.
Dynamic Rear Proportioning (DRP)
The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the EBCM. The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.
The red brake warning indicator is illuminated when the dynamic rear proportioning function is disabled.
Traction Control System (TCS) (NW9)
The tracking control system (TCS) compares front wheel speeds to rear wheel speeds to determine if drive wheels lose traction. The TCS activates when drive wheel speed exceeds speed of non-drive wheels by a calibrated value. This allows the driver to maintain acceleration and directional stability while accelerating on low tractions surfaces.
The TCS limits wheel slip during acceleration when one or more of the drive wheels lose traction. The brake switch must be off for TCS to operate.
During a traction event, the EBCM sends a requested torque value to the ECM/TCM over the serial data link. The ECM/TCS initiates an engine torque reduction routine to slow down the drive wheels. This routine consists of ignition spark timing reduction, fuel injector cut-off and transmission shift control. The ECM/TCM also sends a torque delivered value to the EBCM over the serial data link.
If the EBCM detects a malfunction, it will disable TCS and will command the BCM to turn the TCS LED Off. The driver can also disable TCS, if desired by pressing the TCS switch.
ABS Indicator
The instrument panel cluster illuminates the ABS indicator when the following occurs:
| • | The electronic brake control module (EBCM) detects a malfunction with the antilock brake system. The IPC receives a class 2 message from the EBCM requesting illumination. |
| • | The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds. |
| • | In the event of a Class 2 serial data loss, the ABS telltale will be On. |
Brake System Indicator(s)
Brake Warning Indicator
The IPC illuminates the brake indicator when the following occurs:
| • | The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds. |
| • | The body control module (BCM) detects that the park brake is engaged. The IPC receives a class 2 message from the BCM requesting illumination. |
| • | The body control module (BCM) detects that the master cylinder brake fluid reservoir level is low. The IPC receives a class 2 message for the BCM requesting illumination. |
| • | In the event of a Dynamic Rear Proportioning (DRP) failure. |
Low Brake Fluid Indicator
The IPC illuminates the low brake fluid indicator in the message center when the IPC receives a low signal from the low brake fluid switch.
Traction Control System Indicator(s)
Low Traction Message
The LOW TRACTION message on the instrument panel cluster (IPC) will come On whenever the TCS is in operation. The message will remain On for 3-4 seconds after the TCS event is complete.
TRACTION OFF Indicator
The EBCM illuminates the traction off indicator if any of the following conditions are present.
| • | The EBCM inhibits the traction control system. |
| • | The driver manually disables the traction control system by pressing the traction control switch. |
The EBCM inhibits the traction control system when a TCS-disabling malfunction occurs.
