Chevrolet Equinox Service Manual: ABS Description and Operation Dynamics Control ABS Control

This vehicle is equipped with a Continental Teves Mk25E brake system. The electronicbrake control module and the brake pressure modulator are serviced separately. Thebrake pressure modulator uses a four circuit configuration to control hydraulic pressureto each wheel independently.

Depending on options, the following vehicle performance enhancement systems are provided.

ABSTraction controlStability controlDynamic rear proportioningHill start assistHydraulic brake assistTrailer stability assistOptimized hydraulic braking system

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

Electronic brake control—The electronic brake control module controls the system functionsand detects failures. It supplies voltage to the solenoid valves and pump motor.Brake pressure modulator—The brake pressure modulator contains the following components:Hydraulic pump with pump motorFour isolation valvesFour dump valvesTwo traction control/stability control supply valvesTwo traction control/stability control isolation valvesPressure sensorHigh pressure accumulatorLow pressure accumulatorMulti-axis acceleration sensor—The yaw rate, lateral acceleration and longitudinalacceleration sensors are combined into one multi-axis acceleration sensor, externalto the electronic brake control module. The electronic brake control module receivesserial data message inputs from the yaw rate, lateral acceleration and longitudinalacceleration sensor and activates stability control and hill start assist functiondepending onmulti-axis acceleration sensor input.Steering wheel angle sensor—The electronic brake control module receives serial datamessage inputs from the steering wheel angle sensor. The steering wheel angle sensorsignal is used to calculate the desired yaw rate.Traction control switch—Traction control and stability control are manually disabledor enabled by pressing the traction control switch.Wheel speed sensors—The electronic brake control module sends a 12-volt referencevoltage to each wheel speed sensor. As the wheel spins, the wheel speed sensor producesan alternating current square wave signal. The electronic brake control module usesthe frequency of the square wave signal to calculate the wheel speed.Initialization Sequence

The electronic brake control module performs one initialization test each ignitioncycle. The initialization of the electronic brake control module occurs when one setof the following conditions listed below occur:

The brake pedal is not applied.The electronic brake control module detects that there is a minimum of 500-RPM fromthe engine control module via a serial data message.

OR

The vehicle speed is greater than 16-km/h (10-MPH).The brake pedal was applied.

The initialization sequence cycles each solenoid valve and the pump motor, as wellas the necessary relays, for approximately 1.8-seconds to check component operation.The electronic brake control module sets a DTC if any error is detected. The initializationsequence may be heard and felt while it is taking place, and is considered part ofnormal system operation.

ABS

When wheel slip is detected during a brake application, an ABS event occurs. DuringABS braking, hydraulic pressure in the individual wheel circuits is controlled toprevent any wheel from slipping. A separate hydraulic line and specific solenoid valvesare provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressureto each wheel. The ABS does not, however, increase hydraulic pressure above the amountwhich istransmitted by the master cylinder during braking.

During ABS braking, a series of rapid pulsations is felt in the brake pedal. Thesepulsations are caused by the rapid changes in position of the individual solenoidvalves as the electronic brake control module responds to wheel speed sensor inputsand attempts to prevent wheel slip. These pedal pulsations are present only duringABS braking and stop when normal braking is resumed or when the vehicle comes to astop. A ticking or poppingnoise may also be heard as the solenoid valves cycle rapidly. During ABS braking ondry pavement, intermittent chirping noises may be heard as the tires approach slipping.These noises and pedal pulsations are considered normal during ABS 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 previousnon ABS systems. Maintaining a constant force on the brake pedal provides the shorteststopping distance while maintaining vehicle stability. The typical ABS activationsequence is as follows.

Pressure Hold

The electronic brake control module closes the isolation valve and keeps the dumpvalve closed in order to isolate the slipping wheel when wheel slip occurs. This holdsthe pressure steady on the brake so that the hydraulic pressure does not increaseor decrease.

Pressure Decrease

If a pressure hold does not correct the wheel slip condition, a pressure decreaseoccurs. The electronic brake control module decreases the pressure to individual wheelsduring deceleration when wheel slip occurs. The isolation valve is closed and thedump valve is opened. The excess fluid is stored in the accumulator until the pumpcan return the fluid to the master cylinder or fluid reservoir.

Pressure Increase

After the wheel slip is corrected, a pressure increase occurs. The electronic brakecontrol module increases the pressure to individual wheels during deceleration inorder to reduce the speed of the wheel. The isolation valve is opened and the dumpvalve is closed. The increased pressure is delivered from the master cylinder.

Traction Control

When drive wheel slip is noted, the electronic brake control module will enter tractioncontrol mode.

First, the electronic brake control module requests the engine control module (ECM)to reduce the amount of torque to the drive wheels via a serial data message. TheECM reduces torque to the drive wheels and reports the amount of delivered torque.

If the engine torque reduction does not reduce drive wheel slip, the electronic brakecontrol module will actively apply the brakes on the slipping drive wheel. Duringtraction control braking, hydraulic pressure in each drive wheel circuit is controlledto prevent the drive wheels from slipping. The electronic brake control module commandsthe pump motor and appropriate solenoid valves ON and OFF to apply brake pressureto the slippingwheel.

Traction control can be manually disabled or enabled by pressing the traction controlswitch.

Stability Control

Stability control provides added stability during aggressive maneuvers. Yaw rate isthe rate of rotation about the vehicle's vertical axis. The stability control is activatedwhen the electronic brake control module determines that the desired yaw rate doesnot match the actual yaw rate as measured by the yaw rate sensor.

The desired yaw rate is calculated by the electronic brake control module using thefollowing inputs:

Vehicle speedLateral accelerationSteering wheel position

The difference between the desired yaw rate and the actual yaw rate is the yaw rateerror, which is a measurement of over steer or under steer. When a yaw rate erroris detected, the electronic brake control module attempts to correct the vehicle'syaw motion by applying brake pressure to one or more of the wheels. The amount ofbrake pressure which is applied varies, depending on the correction required. Theengine torque may be reducedalso, if it is necessary to slow the vehicle while maintaining stability.

Stability control activations generally occur in turns during aggressive driving.When braking during stability control activation, the brake pedal may pulsate.

Stability control can be manually disabled or enabled by pressing the traction controlswitch for 5-seconds.

Dynamic Rear Proportioning

The dynamic rear proportioning is a control system that replaces the mechanical proportioningvalve. Under certain driving conditions the electronic brake control module will reducethe rear wheel brake pressure by commanding the appropriate solenoid valves ON andOFF.

Hill Start Assist

The hill start assist allows the driver to launch the vehicle without a roll backwhile moving the foot from the brake pedal to the accelerator pedal. The electronicbrake control module calculates the brake pressure, which is needed to hold the vehicleon an incline and locks that pressure for a certain time by commanding the appropriatesolenoid valves ON and OFF when the brake pedal is released. Hill start assist isactivated whenthe electronic brake control module determines that the driver wishes to move thevehicle up-hill, either backwards or forwards.

The following inputs are used for hill start assist feature:

Accelerator pedal positionBrake switchBrake pressureClutch switchEngine torqueLongitudinal accelerationReverse gear informationVehicle speedHydraulic Brake Assist

The hydraulic brake assist function is designed to support the driver in emergencybraking situations.

The electronic brake control module receives inputs from the brake pressure sensor.When the electronic brake control module senses an emergency braking situation, itwill actively increase the hydraulic brake pressure to a specific maximum by turningthe pump motor ON.

Trailer Stability Assist

The trailer stability assist will detect any vehicle yaw instability, caused by anattached trailer. When instability is detected, the electronic brake control moduleattempts to correct the vehicle's yaw motion by applying brake pressure to one ormore of the wheels. The engine torque may be reduced also, if it is necessary to slowdown the vehicle.

Optimized Hydraulic Braking System

With some engines the electronic brake control module monitors the vacuum in the brakebooster with a vacuum sensor and controls a brake booster vacuum pump depending onvacuum sensor input. It also has a hydraulic brake boost feature which supplementsthe brake system to maintain consistent brake performance under conditions of lowbrake booster vacuum. Low brake booster vacuum conditions can include initial startup after the vehiclehas been parked for several hours, very frequent brake stops, or high altitude driving.The hydraulic brake boost system activates only during a brake apply under low vacuumconditions. In this case the electronic brake control module will actively increaseand control the hydraulic brake pressure by turning the pump motor ON and the appropriatesolenoid valves ON and OFF. When hydraulic brake boost is active, a series of rapidpulsations isfelt in the brake pedal.

Driver Information Indicators

Brake Warning Indicator

The instrument cluster turns the brake warning indicator ON when the following occurs:

The instrument cluster performs the bulb check.The electronic brake control module detects a fault and sends a serial data messageto the instrument cluster requesting illumination.The body control module (BCM) detects that the park brake is engaged. The instrumentcluster receives a serial data message from the BCM requesting illumination.

ABS Indicator

The instrument cluster turns the ABS indicator ON when the following occurs:

The instrument cluster performs the bulb check.The electronic brake control module detects a malfunction which disables the ABS andsends a serial data message to the instrument cluster requesting illumination.

Traction/Stability Control Indicator

The instrument cluster turns the traction/stability control indicator ON when thefollowing occurs:

The instrument cluster performs the bulb check.The ABS system is in traction control or stability control mode.The electronic brake control module detects a malfunction which disables the tractioncontrol or stability control and sends a serial data message to the instrument clusterrequesting illumination.

Traction Control Off Indicator

The instrument cluster turns the traction control off indicator ON when the followingoccurs:

The instrument cluster performs the bulb check.The driver manually disables the traction control by pressing the traction controlswitch. The electronic brake control module sends a serial data message to the instrumentcluster requesting illumination.

Stability Control Off Indicator

The instrument cluster turns the stability control off indicator ON when the followingoccurs:

The instrument cluster performs the bulb check.The driver manually disables the stability control by pressing and holding the tractioncontrol switch. The electronic brake control module sends a serial data message tothe instrument cluster requesting illumination.