GMC chervolet camaro 1999 Service Manual page 740

Brakes
Vehicles equipped with ABS may be stopped by
applying normal force to the brake pedal. Brake pedal
operation during normal braking should be no
different than previous systems. Maintaining a constant
force on the pedal will provide the shortest stopping
distance while maintaining vehicle stability.
Pressure Hold
When the EBCM/EBTCM senses the wheel slip, the
EBCM/EBTCM 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/EBTCM
still senses wheel slip, the EBCM/EBTCM 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 cyclinder.
Pressure Increase
If during the pressure hold or the pressure decrease
mode the EBCM/EBTCM senses that the wheel speed
is too fast, the EBCM/EBTCM 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
Traction control will not have any effect on the
operation of the vehicle until the control module
detects one or both of the rear wheels rotating faster
than the front wheels. At this time the Electronic
Brake and Traction Control Module (EBTCM) will
request the Powertrain Control Module (PCM) to retard
timing. If the EBTCM continues to detect the rear
wheel are rotating faster than the front it will request
the ASM to reduce the throttle angle. The last measure
taken to control the rear wheel is for the EBTCM to
apply the rear brakes, thus reducing torque to the rear
wheels. Once the rear wheels begin to rotate at the
same speed as the front wheels, the system will return
full control to the driver. During Traction Control
mode, if the brake is applied to only one rear wheel,
most of the torque from the engine will be directed
to the other rear wheel which will improve the traction
of the vehicle.
The braking is accomplished by closing the TCS
Master Cylinder Isolating Valve, this isolates the
master cylinder from the rest of the system. The TCS
Prime valve opens to allow the pump to get brake
fluid to build pressure for braking. The drive wheel
circuit solenoids are energized as needed to allow for
pressure hold, pressure increase, or pressure
decrease.
The TCS may be deactivated by the driver if desired.
In order to deactivate the TCS with the engine
running, depress the TCS On/Off switch. The system
will remain deactivated until the ignition switch is
cycled, or the switch is pressed again.
Antilock Brake System 5-265
Tires with ABSITCS
The compact spare tire rotates faster than the others.
The EBCM/EBTCM compensates for this faster
rotation. Refer to Tires and Wheels in Suspension for
information on replacement tires.
Replacement Tires
The tire size is important for the performance of the
ABS/TCS. The replacement tires should be the
same size load range and construction as the original
tires. Replace the tires in axle sets and only with
tires of the same tire performance criteria specification
number. Use of any other tire size or type may
seriously affect ABS/TCS operation. For information
on replacement tires for this vehicle, Refer to Tires and
Wheels in Suspension.
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 red BRAKE indicator will also illuminate
when the park brake is applied. The BRAKE warning
Indicator will stay illuminated until the condition
has been repaired, or the park brake is released. Refer
to Brake System Testing in Hydraulic Brakes.
ABS Indicator
The ABS is located in the instrument cluster and will
illuminate if a malfunction in the ABS is detected by the
Electronic Brake and Traction Control Module
(EBCM/EBTCM). The ABS Indicator informs the driver
that a condition exists which results in turning off
the Antilock brake and traction control function
if equipped. If only the ABS Indicator is on, normal
braking with full power assist is available. If the BRAKE
and Antilock Indicators are on, a problem may exist
in the hydraulic brake system. Refer to Brake System
Testing in Hydraulic Brakes. Conditions for the
Antilock Indicator to turn on are as follows:
1. 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.
2. Instrument Panel Cluster bulb check. When the
ignition is turned to RUN, the ABS Indicator will
turn on for approximately three 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.
• Refer to Brake System Testing in Hydraulic
Brakes for diagnosis of any condition which
causes the BRAKE warning Indicator to
illuminate.