Antilock Brakes / Traction Control Systems: Description and Operation
ABS Description and Operation
BPMV and EBCM
BPMV and EBCM:
BPMV Hydraulic Flow
BPMV Hydraulic Flow:
1 - Master Cylinder
2 - Master Cylinder Reservoir
3 - Pump
4 - Left Front Master Cylinder Isolation Valve
5 - Left Front Prime Valve
6 - Brake Pressure Modulator Valve (BPMV)
7 - Damper
8 - Right Rear Inlet Valve
9 - Accumulator
10 - Right Rear Brake
11 - Right Rear Outlet Valve
12 - Left Front Outlet Valve
13 - Left Front Brake
14 - Left Front Inlet Valve
15 - Left Rear Inlet Valve
16 - Left Rear Brake
17 - Left Rear Outlet Valve
18 - Right Front Brake
19 - Right Front Outlet Valve
20 - Accumulator
21 - Right Front Inlet Valve
22 - Damper
23 - Right Front Master Cylinder Isolation Valve
24 - Right Front Prime Valve
This vehicle is equipped with the BOSCH 5.3 antilock braking system.
The vehicle is equipped with the following braking systems:
* Antilock Brake System (ABS)
* Dynamic Rear Proportioning (DRP)
* Traction Control System (ETS) (w/NW7)
* Traction Control System (TCS) (w/NW9)
* Vehicle stability enhancement system (VSES)
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-The system relay is energized when the ignition is ON and no ABS DTCs are present. It supplies battery positive voltage to the solenoid valves and pump motor.
- Vent tube-The vent tube, located in the EBCM, 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 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
- Master cylinder isolation valves (w/NW9), one per drive wheel
- Prime valves (w/NW9), one per drive wheel
* Wheel speed sensors (WSS)-As the wheel spins, the wheel speed sensor produces an AC signal. The EBCM uses this AC 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.
* Brake pedal position sensor (BPPS)-The body control module (BCM) uses the BPPS as an indication that the brake pedal is applied, and sends a brake apply class 2 signal to the EBCM.
* Lateral accelerometer sensor (w/JL4)-The EBCM uses the lateral accelerometer sensor as an indication of the lateral acceleration of the vehicle.
* Yaw rate sensor (w/JL4)-The EBCM uses the yaw rate sensor as an indication of the yaw rate of the vehicle.
* Steering wheel position sensor (SWPS) (w/JL4)-The EBCM uses the SWPS as an indication of the position and rotation of the steering wheel.
* Brake pressure sensor (w/JL4)-The EBCM uses the brake pressure sensor for more accurate control during a VSES event.
Initialization Sequence
The electronic brake control module (EBCM) performs one initialization test each ignition cycle. The initialization of the EBCM occurs when one set of the following conditions occur:
Both of the following conditions occur:
* The EBCM receives that there is a minimum of 500 RPM from the powertrain control module (PCM) via class 2 serial data message.
* The brake pedal is not applied.
OR
Both of the following conditions occur:
* The vehicle speed is greater than 16 km/h (10 mph).
* The brake pedal is applied.
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.
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 electronic brake control module (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 electronic brake control module (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 electronic brake control module (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 electronic brake control module (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 combination valves in the base brake system. The DRP control system is part of the operation software in the electronic brake control module (EBCM). The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.
Traction Control System (TCS) (NW7)
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 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 electronic brake control module (EBCM) sends a requested torque value to the powertrain control module (PCM) over the serial data link. The PCM 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 PCM 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 body control module (BCM) to turn the TCS OFF indicator ON. The driver can also disable TCS, if desired by pressing the TCS switch.
Traction Control System (TCS) (NW9)
The traction control system (TCS) is designed to limit wheel slip during acceleration when one or more of the drive wheels are accelerating too rapidly. Traction control will not have any effect on vehicle operation until the electronic brake control module (EBCM) detects one or both of the front wheels rotating faster than the rear wheels. The TCS accomplishes this by 2 methods, engine torque reduction and brake intervention. The 2 methods are used together to control wheel slip during acceleration. Both methods are described below.
Vehicle Stability Enhancement System (VSES)
The vehicle stability enhancement system (VSES) is also known as the Stabilitrak Plus system, and the instrument cluster indicator lamp is labeled Stability Off. The VSES consists of a yaw/lateral accelerometer sensor, a steering angle sensor, and a brake pressure sensor working in conjunction with the electronic brake control module (EBCM).
The yaw rate sensor and lateral accelerometer sensor are both in the same module and use a reference voltage of 5 volts. The lateral accelerometer sensor's operating range is 1.5 g resulting in an output range of 2.5-4.75 volts. The yaw rate sensor's range is 75 degrees/seconds, resulting in a signal proportional output of 0.25-4.75 volts. Zero yaw rate is 2.5 volts. The yaw/lateral accelerometer sensor is located inside the center console, between the automatic transaxle gear selector and the console storage compartment.
The steering angle sensor sends dual controller area network (CAN) signals to the EBCM. The EBCM determines steering wheel position and rotation using these signals. The steering angle sensor is located in the lower left hand side of the I/P, on the bottom of the steering column.
The brake pressure sensor uses a reference voltage of 5 volts. The sensor's operating range is 0.25-4.75 volts. The EBCM uses input from the brake pressure sensor for more accurate control during a VSES event.
Magnasteer
The variable effort steering system, Magnasteer, incorporates a controller into the electronic brake control module (EBCM). The EBCM receives inputs from wheel speed sensors. The EBCM processes this information and controls the amount of current to the Magnasteer actuator to vary the rate of input torque required at the steering gear. A DTC is stored in the EBCM for a Magnasteer malfunction. A Magnasteer DTC C0450 will not cause the ABS or TRAC OFF indicators to go ON. Refer to Variable Effort Steering System Description and Operation.
ABS Indicator
The instrument panel cluster (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 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.
* The IPC detects a loss of class 2 communications with the EBCM.
Service ABS System
The message center displays the service ABS system message when any one of many ABS-disabling DTCs is set. The electronic brake control module (EBCM) sends a serial data message to the driver information center (DIC) requesting this display.
Stability Off Indicator
The instrument panel cluster (IPC) illuminates the Stability Off indicator when the electronic brake control module (EBCM) detects a malfunction in the vehicle stability enhancement system (VSES). The IPC receives a class 2 message from the EBCM requesting illumination of both the VSES indicator and the TRAC OFF indicator. The EBCM disables both the vehicle dynamic system and the traction control system.
Brake Warning Indicator
The instrument panel cluster (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 class 2 message from the BCM requesting illumination. If the vehicle is shifted out of park with the park brake engaged, the BCM activates an audible warning.
* The IPC detects a low brake fluid condition (signal circuit is low).
* The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
Traction Control System Indicators
Service Traction System Message
The driver information center (DIC) displays the service traction system message when any one of many ABS-disabling DTCs is set. The electronic brake control module (EBCM) sends a serial data message to the DIC requesting this display.
Traction Control Active Message
The DIC the traction active message when engine torque reduction or brake pressure application is required to maintain traction. The EBCM sends a serial data message to the DIC requesting this display.
Traction Control Off Message
The DIC traction control off indicator when the following occurs:
* The EBCM disables engine torque reduction due to a malfunction and sends a serial data message to the DIC requesting illumination.
* The driver manually disables the traction control system (TCS) and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the instrument panel cluster (IPC) requesting illumination.
Stability Control System Indicators
Stability Control Active Message
The message center displays the stability system active message when a traction control system (TCS) or vehicle stability enhancement system (VSES) event occurs. The electronic brake control module (EBCM) sends a serial data message to the instrument panel cluster (IPC) requesting this display.
Stability Control Off Message
The message center displays the service stability system message when any one of many VSES-disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.
Service Stability System Message
The message center displays the service stability system message when any one of many VSES-disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.