Brakes and Traction Control: Description and Operation
ABS Description and Operation
EBCM and BPMV
BPMV Hydraulic Flow
This vehicle is equipped with the DBC7.2 antilock braking system.
The vehicle is equipped with the following systems:
^ Antilock Brake System (ABS)
^ Dynamic Rear Proportioning (DRP)
^ Traction Control System (TCS) (w/NW7)
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 Relay - Supplies battery positive voltage to the solenoid valve relay and pump motor. The system relay is energized when the ignition is ON when no ABS DTCs are present that disable the ABS system.
- Solenoid Valve Relay - Supplies battery positive voltage to the solenoid valves. The solenoid Valve Relay is energized when the EBCM enters an ABS braking event.
- Vent Tube - The vent tube, located in the EBCM connector, is an opening to the internal cavity of the EBCM. It allows ventilation of the EBCM internals.
^ Brake Pressure Modulator Valve (BPMV) - The BPMV contains the solenoid valves and pump motor that are controlled by the EBCM to isolate or release brake pressure to individual wheels.
The BPMV contains the following components:
- Pump Motor
- Inlet Valves (one per wheel)
- Outlet Valves (one per wheel)
^ Wheel Speed Sensors (WSS) - EBCM sends a 12-volt reference voltage signal to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces a DC digital signal. The wheel speed sensor increases the signal frequency as the wheel speed increases, but does not increase the signal amplitude. The wheel speed sensors are replaceable only as part of the wheel hub and bearing assemblies.
^ Traction Control Switch (w/NW7) - The TCS is manually disabled or enabled using the traction control switch.
^ Brake Pedal Position Sensor (BPP) - The EBCM uses the BPP as an indication that the brake pedal is applied.
Initialization Sequence
The EBCM performs 1 initialization test each ignition cycle. The initialization of the EBCM occurs when both of the following conditions occur:
^ The EBCM detects that there is a minimum of 500 RPM from the PCM via a serial data message.
^ The brake pedal is not applied.
OR
The vehicle speed is greater than 16 km/h (10 mph).
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
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 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.
Traction Control System (TCS) (NW7)
When drive wheel slip is noted while the brake is not applied, the EBCM will enter traction control mode.
The EBCM requests the PCM to reduce the amount of torque to the drive wheels via serial data messages. The PCM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The PCM reports the amount torque delivered to the drive wheels via serial data messages.
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.
Brake System Indicator
The IPC illuminates the brake indicator when the following occurs:
^ The body control module (BCM) detects that the park brake is engaged. The IPC receives a serial data message from the BCM indicating the park brake status. If the vehicle speed is greater than 5 km/h (3.1 mph) with the park brake engaged, the BCM sends a serial data message to the radio in order to activate an audible warning.
^ The electronic brake control module (EBCM) detects a low brake fluid condition (signal circuit is low). The IPC receives a serial data message from the EBCM requesting illumination. The IPC sends a serial data message to the BCM indicating that the brake indicator is illuminated due to a brake malfunction. The BCM sends a serial data message to the radio in order to activate an audible warning.
^ The EBCM detects a malfunction in the braking system. The IPC receives a serial data message from the EBCM requesting illumination. The IPC sends a serial data message to the BCM indicating that the brake indicator is illuminated due to a brake malfunction. The BCM sends a serial data message to the radio in order to activate an audible warning.
^ The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
^ The IPC detects a loss of serial data communications with the EBCM.
ABS Indicator
The IPC 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 serial data message from the EBCM requesting illumination. The IPC sends a serial data message to the body control module (BCM) indicating that the ABS indicator is illuminated. The BCM sends a serial data message to the radio in order to activate an audible warning.
^ The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
^ The IPC detects a loss of serial data communications with the EBCM.
Traction Control System Indicator(s)
Low Traction
The IPC illuminates the low traction indicator when the following occurs:
^ The electronic brake control module (EBCM) detects a traction control event. The IPC receives a serial data 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.
Traction Off
The IPC illuminates the traction off indicator when the following occurs:
^ The electronic brake control module (EBCM) inhibits the traction control system. The IPC receives a serial data message from the EBCM requesting illumination. The traction control system is disabled when the following occurs:
- The body control module (BCM) detects that the traction control switch is pressed (signal circuit is low). The EBCM receives a serial data message from the BCM indicating the switch status.
- The BCM detects that the park brake is engaged (signal circuit is low) and that the engine is on. The EBCM receives a serial data message from the BCM indicating the park brake status.
- The PCM detects certain malfunctions and is unable to perform torque reduction requests from the EBCM. The EBCM receives a serial data message from the PCM indicating that traction control cannot be performed.
^ The EBCM detects a malfunction in the ABS system that disables traction. The IPC receives serial data messages from the EBCM requesting illumination of the traction off indicator.
^ The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.