During normal braking, pressure is applied through the brake pedal. Fluid travels from the master cylinder (2) into the BPMV (14). Once in the BPMV, the fluid travels through the normally-open isolation valves (3, 5, 13), through the normally-closed dump valves (6, 9, 12) and out into the brakes (7, 10).

During normal braking, the pump (4, 15) is not turned on. The low pressure accumulators (8, 11) are empty. Only residual pressure is stored in these accumulators.

The EBCM constantly monitors wheel speed sensor inputs for rapid deceleration. If the ABS becomes disabled for any reason, the driver will always have base brakes. The normally-open isolation valves (3, 5, 13) and normally-closed dump valves (6, 9, 12) will remain in these positions in order to allow normal fluid pressure to the wheels (7, 10).

ABS will not operate without wheel slip. The vehicle must be going at least 13 km/h (8 mph) in order to begin ABS operation.

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:

1. Pressure isolation/maintain
2. Pressure decrease
3. Pressure increase
4. Brake release (fluid return)

1. With the vehicle at 13 km/h (8 mph) or greater, the driver depresses the brake pedal.
2. The wheel speed begins to decrease as the master cylinder pressure and brake pressure increase.
3. As the wheel speed continues to decrease from vehicle speed, the normally-open isolation valve (3, 5, 13) for the affected channel closes to stop additional pressure to the wheel. The master cylinder pressure continues to increase as the driver depresses the pedal, but the wheel brake pressure is now limited to the ABS system pressure.
4. When the EBCM determines that the wheel is about to lock-up, the normally-closed dump valve opens (6, 9, 12). This bleeds off some of the pressure at the wheel cylinder (or caliper) in order to allow the wheel to return to a speed closer to the speed of the vehicle.
5. The dump valve is again closed and the isolation valve remains closed in order to allow the wheel speed to completely recover from the lock-up.
6. Once the vehicle has recovered from the lock-up tendency, the isolation valve is momentarily pulsed open in order to allow the master cylinder pressure and pump pressure to reach the brakes. This controlled pressure rise continues until the wheel is at optimum brake output or until the brake pressure is brought up to master cylinder output pressure. The ABS allows the brake fluid to flow to the wheel, build pressure and try to force another departure, repeating Step 3 through Step 6.

The following paragraphs describe the various modes in detail.

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 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.

At the end of the antilock stop, when the driver releases the brake pedal, the pump (4, 15) will remain on for a short time in order to help drain any fluid left in the LPA (8, 11). As the fluid drains back into the system, the spring force in the LPA pushes the piston to the home position. The isolation valve (3, 5, 13) is turned off and fluid returns through the isolation orifice.

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 can not 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. For further discussion of DRP malfunction modes, refer to Self-Diagnostics .

Correct tire size, proper inflation, accurate alignment and even wear are needed for good brake performance. These items are essential for proper ABS performance.

Use of the spare tire supplied with the vehicle will not affect the performance of the system.

If the replacement tires are not the same size as the original tires, you must change the tire size calibration within the EBCM using a Scan Tool. Refer to Tire Size Calibration portion of ABS System Description. Failure to change the tire calibration when replacing the original tires with a different size tire can affect the performance of the ABS.