ABS Description and Operation with LAT

The antilock brake system (ABS) consists of the following:

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

The electronic brake control module (EBCM) performs one initialization test each ignition cycle. The initialization of the EBCM occurs when one set of the following conditions occur:

Both of the following conditions occur:

OR

Both of the following conditions occur:

The initialization sequence may also be commanded with a scan tool.

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.

When wheel slip is detected during a brake application, the antilock brake system (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 electronic brake control module (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.

The electronic brake control module (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.

The electronic brake control module (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.

The electronic brake control module (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.

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 electronic brake control module (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.

When a conventional vehicle is stopped on a hill, the brakes will hold the vehicle in place until the brake pedal is released. When a hybrid vehicle is completely stopped on a hill, the engine may shut off. When the driver releases the brake pedal, and applies the accelerator, the engine will restart. During the restart sequence, the vehicle may roll. Inputs and outputs from the following electronic control modules (ECUs) are required in order to accomplish a safe smooth transition during vehicle stops and starts:

The hill start assist feature minimizes vehicle rolling during the transition between brake release and engine restart. The brake pressure modulator valve (BPMV) maintains pressure in the brake lines, even though the pedal is released, until the engine starts. Brake pressure is then released following engine restart.

The BPMV is actuated through the EBCM by a duty cycle from the ECM. The ECM also sends a serial data message to the body control module (BCM) to control the brake light signal during the transition between engine off, and engine start. The EBCM and BPMV is a serviceable component.

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

First, the EBCM requests the engine control module (ECM) to reduce the amount of torque to the drive wheels via the requested torque signal circuit. The ECM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The ECM reports the amount torque delivered to the drive wheels via the delivered torque signal circuit.

If the engine torque reduction does not eliminate drive wheel slip, the EBCM will actively apply the drive wheel brakes. During traction control braking, hydraulic pressure in each drive wheel circuit is controlled to prevent the drive wheels from slipping. The master cylinder isolation valve closes in order to isolate the master cylinder from the rest of the hydraulic system. The prime valve then opens in order to allow the pump to accumulate brake fluid in order to build hydraulic pressure for braking. The drive wheel inlet and outlet solenoid valves then open and close in order to perform the following functions:

The vehicle stability enhancement system (VSES) adds an additional level of vehicle control to the electronic brake control module (EBCM).

Yaw rate is the rate of rotation about the vehicle's vertical axis. The lateral accelerometer measures the sideways acceleration of the vehicle. The VSES is activated when the EBCM determines that the desired yaw rate and lateral forces do not match the actual attitude of the vehicle.

The desired attitude of the vehicle are calculated from the following inputs:

The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. If the yaw rate error becomes too large, the EBCM attempts to correct the vehicle's yaw motion by applying differential braking to the appropriate wheel. The amount of differential braking applied to the left or right front wheel is based on both the yaw rate error and side slip rate error.

The VSES activations generally occur during aggressive driving, in turns, or on poor road conditions, without much use of the accelerator pedal. When braking during VSES activation, the pedal pulsations feel different than the ABS pedal pulsations. The brake pedal pulsates at a higher frequency during VSES activation.

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:

ABS Indicator

The IPC illuminates the ABS indicator when the following occurs:

Stability System Off Indicator

The IPC illuminates the stability control off message when one or more of the following conditions exists:

Traction Disabled Indicator

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

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

Service Hill Start Message

The driver information center (DIC) displays SRVC HILL START when it receives a serial data message from the ECM that a fault is detected that affects the hybrid or hill start assist system.