Operation CHARM: Car repair manuals for everyone.

Part 1




ABS Description and Operation

Definition of Longitudinal Wheel Slip

Longitudinal wheel slip is a loss of adhesion between the tires and the road surface. This occurs when the vehicle is moving in a straight ahead direction and the braking or acceleration forces applied to a tire exceeds the amount of adhesion available to that tire.





* At 0 percent slip, the front tire rolls freely (A).

* At 100 percent slip, the rear wheel locks-up (B) as the weight of the vehicle pushes the non-rotating tire along the road surface (C). When the wheels are locked-up, the vehicle's kinetic energy (forward motion) is converted into thermal energy (heat) between the tire and the road surface (1). This will result in an unstable and inefficient braking due to the effect of the following factors:

- Asphalt, cement, gravel or dirt road surfaces provide different degree of tire adhesion.

- Oil puddles, ice spots or other contaminants that cause a sudden change in the road surface condition.

- Wet, dry, smooth, rough road surface conditions affect tire adhesion.

When none of the wheels are locked during braking, the brakes work by converting kinetic energy (forward motion of the vehicle) into thermal energy (heat). The friction between the stationary brake pad and the rotating disc as it slides past the pad convert the motion of the wheel and tire into heat. The brake disc is designed to work like a heat sink, and absorbs as much as 80% of the heat generated during stopping. The brake disc is cooled as it spins through the air on the way to the next stop. The friction surfaces between the brake pads and the brake disc are designed to provide a stable and controlled braking action. Therefore, a vehicle that is braked without locking the wheel will stop in a shorter distance while maintaining directional stability and steering capability. Maximum braking efficiency is achieved when a wheel lock slip is prevented.

Definition of Lateral Wheel Slip

Lateral wheel slip is the loss of adhesion between the tires and the road surface, which occurs when the vehicle is cornering or when too much engine torque is applied to the vehicle and the following forces applied to the tires exceeds the amount of adhesion available to that tire:

* Cornering forces

* Acceleration force

* Braking force

In addition, steering control depends upon tire adhesion. A locked wheel in a 100 percent slip condition delivers poor braking and directional control.

The front tire direction (A) has minimal steering effect while the vehicle skids in direction (B). The tires must regain their adhesion before steering control is restored to the vehicle.






Definition of Understeer

When the vehicle is cornering (A) at high speed or when the vehicle encounters a slippery road surface, the vehicle understeers when the cornering, braking or acceleration forces applied to the tires exceeds the adhesion available between the tires and the road surface.





Under this condition, the vehicle spins (B.) with the front of the vehicle sliding in direction (C).

Definition of Oversteer

When the vehicle is cornering (A) at high speed or when the vehicle encounters a slippery road surface, the vehicle oversteers while cornering, braking or acceleration forces are applied to the tires exceeds the adhesion available between the tires and the road surface.





Under this condition, the vehicle spins (B.) with the rear of the vehicle sliding in direction (C).

Electronic Brake Control Module





The Electronic Brake Control Module (EBCM) (1) incorporates antilock braking, traction control and electronic stability program (ABS-TCS/ESP). The EBCM (1) is integrated with the Brake Modulator Assembly (2) to form one assembly.

Electronic Brake Control Module Inputs

The EBCM constantly monitors and processes input signals and data from the following:

* Battery positive voltage

* Brake fluid level switch

* Brake pedal position sensor (Data from the BCM)

* Brake vacuum sensor

* Chassis expansion data communication circuit

* Communication enable voltage

* G-MAN high speed data communication circuit

* Steering angle sensor

* Traction control switch (Data from the BCM)

* Wheel speed sensors

* Yaw rate sensor

Electronic Brake Control Module Outputs

Based on the input signals received, the EBCM activates the following actuators:

* Hydraulic modulator solenoid valves

* Hydraulic modulator pump motor

* Brake vacuum pump relay

The EBCM relays data to the following:

* Instrument panel cluster

* Diagnostic link connector

* G-MAN high speed data communication circuit

* Chassis expansion data communication circuit

Electronic Brake Control Module Self-test Initialization Sequence

When the ignition is switched on and the vehicle exceeds approximately 15 km/H. the EBCM performs a Self-test Initialization Sequence. After the Self-test is complete the EBCM constantly monitors the ABS-TCS/ESP system for faults.

Note:
The Self-test Initialization Sequence may be heard and felt while it is taking place, which is considered part of the normal system operation.

During the Self-test Initialization Sequence, the EBCM activates the hydraulic modulator cycling each of the solenoid valves as well as operate the pump motor to check for correct component operation. If the pump or any solenoid valves fail to operate, the EBCM sets the appropriate diagnostic trouble code (DTC).

After the Self-test Initialization Sequence is completed the EBCM continuously monitors inputs and outputs by comparing the logical sequence of input and output signals with the normal operating parameters stored in the EBCM. If any of the input or output signals are outside the normal operating parameters, the EBCM sets the appropriate DTC.

Brake Pressure Modulator Valve

The brake pressure modulator valve (BPMV) assembly modulates the brake fluid pressure based on the control signal sent by the electronic brake control module (EBCM).

To allow individual control of each wheel brake fluid circuit, a four-channel circuit configuration with a front/rear split is used. Each of the brake fluid circuits are hydraulically isolated, which enables continued braking ability if a leak develops in any of the brake fluid circuits. The BPMV components consist of the following:

* Two return pumps - Each pump draws excess brake fluid from the accumulators and brake calipers allowing the hydraulic modulator to return brake fluid to the brake master cylinder against brake fluid pressure during the ABS-TCS/ESP pressure reducing phase. In addition, the return pump applies pressure to the brake calipers during the ABS brake intervention phase.

* One electric motor - The electric motor drives the return pump.

* Two accumulators - The accumulators store the excess brake fluid during the ABS-TCS/ESP pressure reducing phase that enables the hydraulic modulator to apply instant pressure reduction.

* Four inlet valves - At rest position, each inlet valve allows brake fluid pressure to be applied to the brake callipers. When active, each inlet valve isolates a brake calliper from the brake master cylinder.

* Four outlet valves - At rest position, each outlet valve isolates a brake calliper from the accumulator and return pump. When active, each outlet valve directs excess brake fluid to the accumulator and return pump that allows pressure reduction.

* Two isolating solenoid valves - The isolating solenoid valves isolate the rear brake fluid circuits from the brake master cylinder which prevents the return of the brake fluid to the brake master cylinder during TCS operation.

* Two rear priming valves - Allow brake fluid to be drawn from the brake master cylinder into the hydraulic pump during TCS operation.

Wheel Speed Sensor

Active wheel speed sensors are fitted to this vehicle. Active wheel speed sensors are direction sensitive.

The wheel speed sensor receives a 12-volt power supply voltage from the electronic brake control module (EBCM). The wheel speed sensor contains an integrated circuit containing a magneto-resistive bridge. Wheel rotation changes the positioning of the tone wheel and the wheel speed sensor's magnetic field and varies the magneto-resistive bridge resistance. The wheel speed sensor's integrated circuit modifies and amplifies the varying resistance into a direct current (DC) square wave signal. Therefore as the wheel spins, the wheel speed sensor changes the voltage and current level output signal to the EBCM. The wheel speed sensor always has a current flow on the output signal circuit to complete the wheel speed sensor's ground path and for EBCM diagnostics. The EBCM uses the frequency of the square wave signal to calculate the wheel speed.

Brake Pedal Position Sensor





The brake pedal position sensor (1) is connected via three wires to the Body Control Module (BCM). The BCM supplies a 5 volt reference voltage and a reference ground to the brake pedal position sensor. Depending on the position of the brake pedal the brake pedal position sensor outputs a voltage to the BCM. The BCM converts the voltage to data and relays it to the EBCM via the Hi Speed GMLAN network.

Electronic Stability Program (ESP) Switch





The ESP switch incorporates traction control (TCS) which is part of a multi function center console switch assembly and is located in the console as shown above.

The ESP switch is a momentary contact switch that sends a ground output signal to the body control module (BCM). The BCM sends a signal to the EBCM via the serial data bus to determine the mode.

The ESP cannot be switched off, instead ESP will switch between On and Performance mode. The cluster will display 'Competitive Mode' or ‘Performance Mode'.

Yaw Rate Sensor

The yaw-rate sensor assembly (1) comprises of a yaw-rate sensor and a lateral acceleration sensor.





* The yaw-rate sensor produces a signal output voltage that corresponds to a vehicle rotation around its vertical axis.

* The lateral acceleration sensor produces a signal output voltage that corresponds to a vehicle lateral acceleration.

The EBCM uses the output signal voltage of the yaw-rate sensor and the acceleration sensor in conjunction with the wheel speed sensor signal output voltage and serial data output signal of the steering angle sensor, to support the calculation of actual vehicle behavior as compared to the driver intended direction.

Lateral Acceleration Sensor





Note:
This illustration represents the lateral acceleration sensor in symbolic form, which aims to show the operation of the acceleration sensor in a simplified manner.

The lateral acceleration sensor consists of the following components:

* Differential capacitors connected to the fixed side plates (1).

* Mass plate (2) suspended by springs (3) about its center of mass, which moves in response to vehicle lateral acceleration.

When the vehicle is stationary (View A), the distance between the mass plate and the 2 side plates are equal. Therefore, the capacitance between the 2 capacitors is the same and the acceleration sensor signal voltage is zero.

As the vehicle accelerates (View B), the side plates move with the vehicle (C) while the mass plate, which is suspended by springs tends to move in the opposite direction. Therefore, the distance between the side plates and the mass plates changes in proportion to the level of acceleration.

This changes the capacitance between the 2 capacitors causing the acceleration sensor to produce a signal voltage with an amplitude proportional to the movement of the mass plates.

Yaw Rate Sensor





Note:
This illustration represent the yaw rate sensor in symbolic form, which shows the operation of the yaw rate sensor in a simplified manner.

The yaw-rate sensor consists of the following components:

* Two oscillating plates (1) suspended by springs (2) around its center of mass.

* An acceleration sensor (3) incorporated within the oscillating plates.

* Permanent magnets, which project magnetic field to the oscillating plates.

While in the presence of a magnetic field, electric current is applied to the oscillating plates, which causes the plates to move at a constant amplitude (W).

When the vehicle is driving in a straight-ahead direction and there are not lateral forces applied to the acceleration sensors, the distance between the mass plates (4) and the side plates of the acceleration sensors are equal (5). Therefore, the capacitance between the capacitors are the same and the acceleration sensor signal voltage is zero.

When the vehicle rotates around its vertical axis, the following situation occurs.





* With the exception of the mass plates (1), the yaw-rate sensor (2), which is firmly attached to the vehicle rotates with the vehicle (Z).

* The mass plates, which are suspended by springs (3) and moves along with the oscillating plates (4), tend to float in its current position while the fixed side plates (5) rotate with the vehicle. Therefore, the distance between the fixed side plates and the mass plates of the acceleration sensor changes in proportion to the level of vehicle rotation around its vertical axis.

* This changes the capacitance between the capacitors causing the acceleration sensor to produce a signal voltage with an amplitude proportional to the movement of the mass plates.

The evaluation circuit of the yaw-rate sensor compares and evaluates the output signal of both acceleration sensors to calculate the level of vehicle rotation around its vertical axis.

Steering Angle Sensor

The steering angle sensor provides a signal output that represents the steering wheel degree of rotation. The EBCM uses this information to calculate the driver intended driving direction. The steering angle sensor contains:





* Gear Wheel (1)

* Measuring Gears (2)

* Measuring Gear Magnets (3)

* Evaluation Circuit (4)

* Anisotropic Magneto Resistive (AMR) Integrated Circuit (IC) (5)