Antilock Brakes / Traction Control Systems: Description and Operation
ABS Description and Operation
EBCM And BPMV:
1 - Electronic Brake Control Module (EBCM)
2 - Brake Pressure Modulator Valve (BPMV)
BPMV Hydraulic Flow
1 - Master Cylinder Reservoir
2 - Master Cylinder
3 - Pump
4 - Brake Pressure Modulator Valve (BPMV)
5 - Damper
6 - Right Rear Inlet Valve
7 - Accumulator
8 - Right Rear Brake
9 - Right Rear Outlet Valve
10 - Left Front Outlet Valve
11 - Left Front Brake
12 - Left Front Inlet Valve
13 - Left Rear Inlet Valve
14 - Left Rear Brake
15 - Left Rear Outlet Valve
16 - Right Front Outlet Valve
17 - Right Front Brake
18 - Accumulator
19 - Right Front Inlet Valve
20 - Damper
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/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 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/NW7) - 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.
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.
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) (NW7)
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 Driver Information Center will come On whenever the TCS is in operation. The message will remain On for 3-4 seconds after the TCS event is complete. The message does NOT operate during an ABS event.
Traction LED Test
This test allows the technician to flash the TCS LED On and Off with the Scan tool:
* Select BCM
* Select Scan tool's SPECIAL FUNCTION
* Select Traction LED test.
TCS Switch LED
Whenever the TCS switch LED is illuminated TCS is available. This will default ON every time the vehicle is started. The TCS can be disabled by pressing the switch. The TCS Switch and LED is controlled by the BCM.