ABS Description and Operation. Chevrolet Evaluator 1985

For 1990-2009 cars

This vehicle is equipped with the DBC7.2 antilock braking system.

The vehicle is equipped with the following systems:

• | Antilock Brake System (ABS)

• | Dynamic Rear Proportioning (DRP)

• | Traction Control System (TCS) (w/NW7)

The following components are involved in the operation of the above systems:

• | Electronic Brake Control Module (EBCM) - The EBCM controls the system functions and detects failures.

| The EBCM contains the following components:

- | System Relay - Supplies battery positive voltage to the solenoid valve relay and pump motor. The system relay is energized when the ignition is ON when no ABS DTCs are present that disable the ABS system.

- | Solenoid Valve Relay - Supplies battery positive voltage to the solenoid valves. The solenoid Valve Relay is energized when the EBCM enters an ABS braking event.

- | Vent Tube - The vent tube, located in the EBCM connector, is an opening to the internal cavity of the EBCM. It allows ventilation of the EBCM internals.

• | Brake Pressure Modulator Valve (BPMV) - The BPMV contains the solenoid valves and pump motor that are controlled by the EBCM to isolate or release brake pressure to individual wheels.

| The BPMV contains the following components:

- | Pump Motor

- | Inlet Valves (one per wheel)

- | Outlet Valves (one per wheel)

• | Wheel Speed Sensors (WSS) - EBCM sends a 12-volt reference voltage signal to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces a DC digital signal. The wheel speed sensor increases the signal frequency as the wheel speed increases, but does not increase the signal amplitude. The wheel speed sensors are replaceable only as part of the wheel hub and bearing assemblies.

• | Traction Control Switch (w/NW7) - The TCS is manually disabled or enabled using the traction control switch.

• | Brake Pedal Position Sensor (BPP) - The EBCM uses the BPP as an indication that the brake pedal is applied.

### Initialization Sequence

The EBCM performs 1 initialization test each ignition cycle. The initialization of the EBCM occurs when both of the following conditions occur:

• | The EBCM detects that there is a minimum of 500 RPM from the PCM via a serial data message.

• | The brake pedal is not applied.

OR

The vehicle speed is greater than 16 km/h (10 mph).

The initialization sequence cycles each solenoid valve and the pump motor, as well as the necessary relays, for approximately 1.5 seconds to check component operation. The EBCM sets a DTC if any error is detected. The initialization sequence may be heard and felt while it is taking place, and is considered part of normal system operation.

The EBCM defines a drive cycle as the completion of the initialization sequence.

### Antilock Brake System

When wheel slip is detected during a brake application, the ABS enters antilock mode. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled 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 hydraulic pressure to each wheel brake. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.

During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the EBCM responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and 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 cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during antilock operation.

Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.

### Pressure Hold

The EBCM closes the inlet valve and keeps the outlet valve closed in order to isolate the system when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.

### Pressure Decrease

The EBCM decreases the pressure to individual wheels during a deceleration when wheel slip occurs. The inlet valve is 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

The EBCM increases the pressure to individual wheels during a deceleration in order to reduce the speed of the wheel. The inlet valve is opened and the outlet valve is closed. The increased pressure is delivered from the master cylinder.

### Traction Control System (TCS) (NW7)

When drive wheel slip is noted while the brake is not applied, the EBCM will enter traction control mode.

The EBCM requests the PCM to reduce the amount of torque to the drive wheels via serial data messages. The PCM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The PCM reports the amount torque delivered to the drive wheels via serial data messages.

### Dynamic Rear Proportioning (DRP)

The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the EBCM. The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.

The red brake warning indicator is illuminated when the dynamic rear proportioning function is disabled.

### Driver Information Indicators and Messages

The following indicators are used to inform the driver of several different factors.

#### Brake Warning Indicator

The instrument panel cluster (IPC) illuminates the brake warning indicator when the following occurs.

• | The body control module (BCM) detects that the park brake is engaged. The IPC receives a serial data message from the BCM requesting illumination. The brake warning indicator flashes at a rate of approximately twice per second when the park brake is engaged.

• | The EBCM detects a low brake fluid condition or a base brake pressure differential and sends a serial data message to the IPC requesting illumination.

• | The IPC performs the bulb check.

• | The EBCM detects an ABS-disabling malfunction which also disables dynamic rear proportioning (DRP) and sends a serial data message to the IPC requesting illumination.

#### ABS Indicator

The IPC illuminates the ABS indicator when the following occurs.

• | The electronic brake control module (EBCM) detects an ABS-disabling malfunction and sends a serial data message to the IPC requesting illumination.

• | The IPC performs the bulb check.

• | The IPC detects a loss of serial data communication with the EBCM.

• | The EBCM sends a serial data message to the IPC requesting illumination.

#### Low Traction Indicator

The IPC illuminates the low traction indicator when the following occurs:

• | The electronic brake control module (EBCM) detects a traction control event. The IPC receives a serial data message from the EBCM requesting illumination.

• | The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.

#### Traction Control Off Indicator

The IPC illuminates the traction off indicator when the following occurs.

• | The EBCM disables engine torque reduction due to a malfunction and sends a serial data message to the IPC requesting illumination.

• | The driver manually disables VSES and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the IPC requesting illumination.

The IPC flashes the traction off indicator if wheel slip is detected while engine torque reduction and brake pressure application are both disabled.

#### Service ABS System Message

The service ABS system message is displayed whenever the ABS warning indicator is illuminated.

#### Service Brake System Message

The service brake system message is displayed whenever the red brake warning indicator is illuminated.

#### Service Traction System Message

The drive information center displays the service traction system messages displayed whenever the traction control system is disabled or degraded because of a failure that affects traction control function.

#### Traction Control Active Message

The drive information center displays the traction active message when engine torque reduction or brake pressure application is required to maintain traction. The EBCM sends a serial data message to the IPC requesting this display.

#### Traction Control Off Message

The message center displays the traction control off message when one or more of the following conditions exists.

• | The driver manually disables the VSES and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the IPC requesting illumination.

• | A DTC is set that inhibits traction control.

• | The PCM inhibits engine torque management.