The system uses an amber ABS indicator lamp in the instrument cluster in order to show system operation and malfunctions.
Whenever the key is turned to the RUN position, the instrument cluster starts a bulb check by illuminating the ANTILOCK and BRAKE indicators. The bulb check ends when the instrument cluster receives a Class 2 serial data message from the EBCM and BCM that no malfunctions were detected. If no malfunctions were detected, the instrument cluster will turn OFF the ANTILOCK indicator after two seconds, and the BRAKE indicator will turn OFF a second later. This confirms correct operation of the EBCM and the indicators.
If the EBCM detects a malfunction, either on power-up or during operation, it will notify the instrument cluster via a Class 2 message to illuminate the ANTILOCK indicator. If the malfunction can affect operation of the base brakes, the BRAKE indicator may be illuminated as well. In order to determine the specific cause of the malfunction, refer to Diagnostic Starting Point .
The ANTILOCK indicator can be illuminated when one of the following conditions occurs:
The BRAKE lamp can be illuminated when one of the following conditions occurs:
A self-test is performed when the vehicle reaches a speed of greater than 3 mph. At this time the internal EBCM relay, six solenoid coils and BPMV pump motor are cycled and checked for shorts/opens. The BPMV pump will make a slight sound when this function occurs.
The ABS will monitor the three-wheel speed sensors and control the hydraulic pressure changes at each wheel until the vehicle has come to a complete stop or until the driver has released the brake pedal. The system operates through the following process:
Isolation will occur when the driver applies excessive braking for the given road conditions, causing the wheels to decelerate at a rate which exceeds the vehicle's capability.
If the information from the wheel speed sensors indicate excessive wheel deceleration (imminent lock-up), the first step in the antilock sequence is to isolate the brake pressure being applied by the driver.
The EBCM applies a voltage to the isolation coil in order to close the isolation valve (3, 5, 13). This will prevent any additional brake pressure applied by the driver from reaching the wheel. With the isolation valve (3, 5, 13) closed, further increases in brake pressure from the driver will be prohibited.
Once the pressure is isolated, it must be reduced in order to get the wheels rolling once again. Reducing pressure is accomplished by dumping a portion of the brake fluid pressure into a low pressure accumulator (LPA) (8, 11).
The EBCM energizes the dump valve coil(s) in order to open the dump valve (6, 9, 12), allowing fluid from the wheels to be dumped into the LPA (8, 11). Very short activation pulses open and close the dump valve passageway in order to control this action. Brake pressure is lowered at the wheel and allows the affected wheel to begin rolling again.
The fluid taken from the wheels forces a spring back. The fluid is stored in the LPA (8, 11) at approximately 1034 kPa (150 psi). A portion of the fluid also primes the pump (4, 15) so it can begin building reapply pressure. The dump valves (6, 9, 12) are opened independently in order to control the deceleration of the wheel.
The reapply sequence is initiated in order to obtain optimum braking at each wheel. The isolation valve (3, 5, 13) is momentarily pulsed open in order to allow the master cylinder (2) and pump (4, 15) pressure to reach the brakes (7,10). This controlled pressure rise continues until the wheel is at optimum brake output or until the brake pressure is brought up to the master cylinder output pressure.
If more pressure is required, more fluid is drawn from the master cylinder (2) and applied to the brakes (7, 10). The driver will feel pedal pulsations or pedal drop. This is normal and expected when in the antilock mode.
As fluid is reapplied, the wheels begin to slow down at the optimum rate. If the wheels approach imminent lock-up again, the module will isolate, dump and reapply. These control cycles (isolation, dump and reapply) occur in millisecond intervals, allowing several cycles to occur each second.
Dynamic Rear Proportioning is a control system that replaces the hydraulic proportioning function of the combination valve in the base brake system of vehicles with front/rear disk brakes. The combination valve remains in the circuit to assist DRP in vehicles with front disk/rear drum brakes. The DRP control system is part of the operating software in the EBCM. It utilizes active control with the existing ABS to regulate the vehicle's rear brake pressure. The resulting benefits are:
DRP is active when the vehicle is decelerating above a programmed threshold and the rear wheels are at low levels of slip. This threshold is particular to each weight class of vehicle. DRP cannot activate if any of the following conditions exist
DRP operation is transparent to the driver. If the vehicle is braking under DRP control and the wheel speed sensors detect an ABS event, the ABS will take control over DRP until conditions warrant the return to normal braking/DRP operation.
