Antilock Braking Mode
When wheel slip is noted during a brake application, the ABS will enter
the antilock mode. During antilock braking, the hydraulic pressure in the
individual wheel circuits is controlled in order 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 the hydraulic pressure
to each wheel brake. However, the ABS cannot increase the hydraulic pressure
above the amount that the master cylinder transmits during braking. During
antilock braking, a series of rapid pulsations will be felt in the brake
pedal. The rapid changes in the position of the individual solenoid valves
as they respond to the desired wheel speed causes the pulsations. This pedal
pulsation is present during antilock braking and will 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 rapidly cycle. During antilock braking
on dry pavement, the tires may make intermittent chirping noises as they
approach slipping. These noises and pedal pulsations are normal during antilock
operation. Brake pedal operation during normal braking should be no different
than previous systems. Maintaining a constant force on the pedal provides
the shortest stopping distance while maintaining vehicle stability.
Pressure Hold
When the EBCM senses wheel slip, the EBCM closes the Inlet valve and
keeps the Outlet valve closed in the BPMV in order to isolate the system.
This holds the pressure steady on the brake so that the hydraulic pressure
does not increase or decrease.
Pressure Decrease
If during the pressure hold mode the EBCM still senses wheel slip, the
EBCM will decrease the pressure to the brake. The Inlet valve is left 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
If during the pressure hold or the pressure decrease mode the EBCM senses
that the wheel speed returns to normal or increases, the EBCM will increase
the pressure to the brake. The Inlet is opened and the Outlet valve is closed.
The increased pressure comes from the master cylinder and is related to the
pressure applied to the brake pedal.
Traction Control System (TCS)
The TCS is designed to limit wheel slip during acceleration when one
or more of the drive wheels are accelerating too rapidly and the brake switch
is off. The EBCM monitors wheel speed slip through the ABS wheel speed sensors
then processes wheel speed sensor data and sends a desired wheel torque
value to the Powertrain Control Module via a Class 2 Serial Data Line signal.
Traction control will not have any effect on vehicle operation until the control
module detects one or both of the front wheels rotating faster than the
rear wheels. At this time, the EBCM will request the PCM to reduce the amount
of torque applied to the drive wheels. The PCM retards the timing and turns
off the supercharger (if equipped) and the fuel injectors. The EBCM applies
the front brakes in order to reduce the torque to the front wheels. Once
the front wheels begin to rotate at the same speed as the rear wheels, the
system will return full control to the driver. During the traction control
mode, if the brake is applied to only one front wheel, most of the engine
torque will be directed to the other front wheel, thus improving the vehicle
traction. Closing the TCS master cylinder isolating valve isolates the master
cylinder from the rest of the system. The TCS prime valves open in order
to allow the pump to get the brake fluid to build the pressure for braking.
The drive wheel circuit solenoids are energized as needed in order to allow
for pressure hold, pressure increase, or pressure decrease. The driver can
deactivate the TCS, if desired. In order to deactivate the TCS with the
engine running, depress the Traction Control switch. The TRAC OFF Indicator
in the Driver Information Display will illuminate. The system remains deactivated
until the ignition switch is cycled, or the switch is pressed again. The
TCS will be disabled under the following conditions:
| • | The Traction Control switch is in the OFF position. |
| • | The EBCM senses a valid brake switch input. |
| • | The park brake is engaged. |
| • | Catalytic converter temperature rises above the normal range. |
| • | Engine coolant temperature is outside the normal operating range. |
| • | The PCM sets a DTC that turns on the CHECK ENGINE indicator. |
| • | The EBCM sets any DTC that effects the TCS operation. |
Tires with ABS/TIM/TCS (if equipped)
The compact spare tire rotates faster than the standard tires that are
original equipment. The EBCM compensates for this faster rotation but a DTC C1245
will set and Low Tire Indicator will illuminate until the tire is repaired
or replaced. The RESET switch must then be pushed to reset the Tire Inflation
Module (TIM) system that is part of the EBCM. Refer to
Tire and Wheel Specifications
in Tire and Wheels
for information on replacement tires.
BRAKE Warning Indicator
The red BRAKE Indicator in the instrument cluster 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
(closed is when the Brake fluid is low in the master cylinder reservoir).
When the Brake fluid level switch is closed (low condition) a DTC C1247
is set in the EBCM. The BRAKE warning Indicator will stay illuminated until
the condition has been repaired.
ABS Indicator
The ABS indicator is located in the instrument cluster and will illuminate
if a malfunction in the ABS is detected by the Electronic Brake Control Module
(EBCM). The ABS indicator informs the driver that a condition exists which
results in turning off the antilock brake and traction control (if equipped)
function. If only the ABS indicator is on, normal braking with full power
assist is available. If the BRAKE and ABS indicators are on, a problem may
exist in the hydraulic brake system. Refer to
Hydraulic Brake Diagnostic System Check
in Hydraulic Brakes. Conditions
for the ABS indicator to turn on are as follows:
| • | 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 and TCS (if equipped) have been disabled. |
| • | Instrument panel cluster bulb check. When the ignition is turned
to RUN, the ABS indicator will turn on for approximately 3 seconds
and then turn off. |
| - | Illumination of the BRAKE warning Indicator may indicate reduced
braking ability. |
| - | Illumination of the ABS indicator without the BRAKE warning Indicator
on indicates only that antilock braking is no longer available. |
| - | Power assisted braking without antilock control is still available. |
TRAC OFF Indicator (if equipped)
The TRAC OFF indicator located in the Driver Information Display will
illuminate to notify the driver of the following:
| • | TCS operation has been inhibited by the PCM. |
| • | The driver has switched off TCS by pressing the Traction Control
switch. |
| • | The EBCM sets a DTC that effects the operation of the TCS. |
As a bulb check procedure, the EBCM will command the TRAC OFF indicator
on for 3 seconds when the EBCM receives Ignition power.
LOW TRAC Indicator (if equipped)
The LOW TRAC indicator is located in the Driver Information Display.
The LOW TRAC indicator will illuminate to alert the driver of a low traction
situation and to inform the driver the TCS is operating. It does not operate
during ABS operation. The LOW TRAC indicator will remain on for approximately
3 to 4 seconds after the TCS event is complete.
As a bulb check procedure, the EBCM will command the LOW TRAC indicator
on for approximately 3 to 4 seconds when the EBCM receives Ignition power.
