ABS Operation. Chevrolet Export Express 1998

For 1990-2009 cars

### ABS Indicator Lamp Operation

The system uses an amber ABS indicator lamp in the instrument cluster in order to show system operation and malfunctions.

### Normal Lamp Operation

A bulb check occurs each time the ignition switch is turned to the RUN position. The ANTILOCK and BRAKE lamps should turn on, remain on for about two seconds, then turn off. The ABS indicator lamp also indicates system malfunctions. When the EBCM detects a malfunction in the system, the EBCM turns the ANTILOCK and sometimes the BRAKE lamp on. The lamp may remain on or turn off depending on the malfunction. In order to determine the specific cause of the malfunction, refer to the appropriate diagnostic system check.

#### ABS Safeguard Indicator Lamp Operation

This vehicle is equipped with an Electronic Brake Control Lamp Relay which acts as a safeguard indication whenever the ignition switch is in the RUN position and the 10-way EBCM harness connector is seated improperly, or disconnected. A ground is normally supplied to the relay from the EBCM through terminal K in the 10-way connector. Power is supplied from the BRAKE fuse 18, whenever the ignition is in the RUN position. This action energizes the Electronic Brake Control Lamp Relay, and switches a constant ground away from the ABS indicator lamp circuit. If the 10-way harness connector is removed while the ignition is in the RUN position, the relay deenergizes and the constant ground is connected through the contacts of the relay to CKT 867, which turns the ABS indicator lamp on.

### Tires and ABS

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

### Spare Tire

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

### Replacement Tires

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 . Failure to change the tire calibration when replacing the original tires with a different size tire can affect the performance of the ABS.

### Self-Tests

The ABS performs the following two system self-tests:

• | The first self-test is performed when the ignition is turned to RUN. Both the ABS indicator lamp and the BRAKE warning lamp will turn on for 2 seconds, then they will turn off. This test confirms correct operation of the EBCM and the lamps. If one of the lamps remains on, either the ABS or the base brake system will require service.

• | The second self-test is performed when the vehicle reaches a speed of greater than 4.8 km/h (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.

### Normal Braking Mode

Refer to Normal Braking Mode in BPMV Hydraulic Flow Chart .

During normal braking, pressure is applied through the brake pedal. Fluid travels from the master cylinder (2), through the combination valve (3) and into the BPMV (15). Once in the BPMV, the fluid travels through the normally-open isolation valves (4, 6 and 14), through the normally-closed dump valves (7, 10 and 13) and out into the brakes 8 and 11).

During normal braking, the pumps (5 and 16) are not turned on. The low pressure accumulators (9) 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 and normally-closed dump valves will remain in these positions in order to allow normal fluid pressure to the wheels.

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.

### ABS Braking Mode

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)

### Sequence Of Events

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 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. 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 Mode (Pressure Maintain)

Refer to Isolation Mode in BPMV Hydraulic Flow Chart .

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 (4). This will prevent any additional brake pressure applied by the driver from reaching the wheel. With the isolation valve closed, further increases in brake pressure from the driver will be prohibited.

### Dump Mode (Pressure Decrease)

Refer to Dump Mode in BPMV Hydraulic Flow Chart .

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).

The EBCM energizes the dump valve coil(s) in order to open the dump valve (5), allowing fluid from the wheels to be dumped into the LPA (6). 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 at approximately 1034 kPa (150 psi). A portion of the fluid also primes the pump (3) so it can begin building reapply pressure. The dump valves are opened independently in order to control the deceleration of the wheel.

### Reapply Mode (Pressure Increase)

Refer to Reapply Mode in BPMV Hydraulic Flow Chart .

The reapply sequence is initiated in order to obtain optimum braking at each wheel. The isolation valve (4) is momentarily pulsed open in order to allow the master cylinder (2) and pump (3) 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 the master cylinder output pressure.

If more pressure is required, more fluid is drawn from the master cylinder and applied to the brakes. 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.

### Brake Release

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