Traction Control System Operation

SYSTEM OPERATION

The Antilock System and the Traction Control System (ABS VI/TCS) are both part of the same hydraulic and electric system. Both Systems use many of the same components and a problem in either system may disable the other system until it is repaired. Antilock braking controls wheel slip when braking; traction control controls wheel slip when accelerating.

Base Braking Mode

Under normal operating conditions, the brake system will operate using conventional braking by means of brake pedal force, the vacuum booster, and the compact master cylinder. For the front wheels, the brake fluid flows from the master cylinder, through the ABS hydraulic modulator, and then out to the front wheels. The fluid flow to the rear wheels flows first through the master cylinder, through the ABS hydraulic modulator, through the TCS hydraulic modulator, and then out to the rear wheels.

ABS Hydraulic Modulator Fluid Flow

Figure 1 ABS Front Base Braking Mode:

Each front ABS channel consists of a motor, solenoid, ESB, ball screw, piston, and check valve. Under normal operating conditions (base brakes), the piston is held in the up most (or "home") position and the solenoid is open (not energized), (refer to Figure 1). This is accomplished by turning the ball screw via the motor to drive the nut upwards. Once at the up most position, the piston is held by an Expansion Spring Brake (ESB). Two paths are available to transfer braking pressure to the front wheels: (1) through the ABS modulator, around the check valve, and out to the front wheels, (2) through the ABS modulator, past the normally open solenoid, and out to the front wheels. The solenoids in the front circuits provide an alternate brake pressure path to the front wheels. With this arrangement, if ABS were to lose power or malfunction with the piston not in its home position. a redundant brake fluid path is available, (see Figure 1). The rear ABS channels operate in a similar manner except no solenoid exists. The fluid flows from the master cylinder, through the ABS modulator, through the TCS modulator, and then out to the rear wheel brakes.

TCS Hydraulic Modulator Fluid Flow

Figure 2 Modulator Base Braking Mode:

Each TCS channel consists of a motor, piston, ball screw, spring, and poppet valve. Under normal operating conditions (base brakes) the piston is located in the downward (or "home") position and the poppet valve is unseated, (refer to Figure 2). This is accomplished by turning the ball screw via the motor to drive the nut downward. The brake fluid enters the inlet port of the TCS modulator, flows past the unseated poppet valve to the outlet port of the TCS modulator, and then out to the rear calipers. There are no solenoid valves or ESBs within the TCS modulator.

Antilock Braking Mode

ABS VI has been designed to improve the controllability and steerability of a vehicle during braking. ABS VI accomplishes this objective by controlling the hydraulic pressure applied to each wheel brake. Antilock braking occurs only when the brake switch is closed and a microprocessor, located in the EBTCM, determines one or more wheels is about to lose traction during braking. The EBTCM will then allow the hydraulic modulator to change the brake pressures several times each second to keep the wheel(s) from locking and provide the driver with maximum vehicle controllability. ABS VI CANNOT INCREASE THE BRAKE PRESSURE ABOVE THE MASTER CYLINDER PRESSURE APPLIED BY THE DRIVER, AND CANNOT APPLY THE BRAKES BY ITSELF.

Figure 5 - Front ABS Braking Mode:

The result is greatly improved braking, enabling the driver to more easily maintain steerability and to bring the vehicle to a controlled stop. ABS VI provides effective braking and directional control over a wide range of road surfaces and driving conditions.
If any wheel(s) begins to approach lock-up, the EBTCM will control the three (3) motors and two (2) solenoids, appropriately, to control brake pressure to the affected wheel(s). During front wheel ABS operation, the solenoids are turned "ON" to isolate that brake pressure path to the wheel (see Figure 5). The EBTCM then provides controlled current to the motors to regulate the speed and amount of movement. As the motors move backwards, the piston follows the nut downward, allowing the check valve to seat. The brake pressure to the wheel is now a function of the controlled volume within the piston chamber. To reduce brake pressure, the motor drives the nut further downward. To reapply or increase brake pressure, the motor drives the nut and piston upward. If ABS was entered during low brake pressure, such as on ice, and dry pavement is then encountered during reapply, the piston is driven all the way to the top. This results in the unseating of the check valve, and a return to base brakes until sufficient brake pressure exists to cause the wheel to approach lock-up again. At this point, the ABS cycle would start again. This process can occur in less than one second if the driver is pressing firmly on the brake pedal.
Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, anytime wheel brake pressure exceeds the brake pressure at the master cylinder (caused by reduced force on the brake pedal), the check valve unseats and a small volume of brake fluid is returned to the master cylinder. In this manner, ABS VI CANNOT INCREASE BRAKE PRESSURE ABOVE MASTER CYLINDER PRESSURE APPLIED BY THE DRIVER AND CANNOT APPLY THE BRAKES BY ITSELF. When ABS is no longer required, the pistons are returned to their up most (or "home") position and are held by the ESBs (refer to "ESB Operation," found in this article). The solenoids on the front channels are simultaneously opened to again provide a redundant base braking path.

Figure 6 Rear ABS Braking Mode:

The rear channel operates m a similar manner except no solenoid is used (refer to Figure 6). Both rear brake pressures are controlled by the same motor and both rear brake pressures are controlled together. If either rear wheel begins to lock, brake pressure to both rear wheels is reduced to maximize vehicle stability.
There is no rear solenoid due to the nature of the braking system. The vast majority of braking is accomplished with the front brakes. If an ABS failure were to occur that affected the operation of the rear base brakes, a diagnostic trouble code would be stored and the EBTCM would turn "ON" both the "ABS INOP" lamp and the "BRAKE" warning lamp.

Traction Control Mode

The Traction Control System (TCS) is an add on to the ABS VI Antilock brake system. The TCS provides the capability to control wheel spin at each driven wheel. This improves the ability to maintain vehicle stability and acceleration (drive traction) under changing road and vehicle load conditions.
Traction Control occurs when the EBTCM determines one or more of the drive wheels are accelerating too rapidly and the brake switch is "OFF." The EBTCM processes wheel speed sensor data and can control wheel slip by utilizing up to three different methods: 1. By commanding the PCM to retard ignition timing, thereby reducing engine torque; 2. by utilizing the throttle adjuster motor to reduce the throttle angle, thereby reducing engine torque and 3. by brake intervention through the use of the TCS hydraulic modulator.

Figure 3 Traction Control Mode:

The EBTCM can control one or both of the TCS motors simultaneously. The TCS motors are bi-directional and are used with the gear assemblies to drive the ball screws and pistons in the TCS modulator. The bi-directional motors allow pressure to be applied, held, or released from the wheels. During TCS operation, the ball screws and pistons are driven upward from their home position. This process seats the poppet valve (thus isolating the master cylinder). The brake pressure to the wheel is now a function of the controlled volume within the piston chamber (refer to Figure 3). As drive wheel speed is reduced, the pistons are driven downward to their home position. This unseats the poppet valve and releases fluid pressure from the rear brake calipers. This process of apply, hold, and release can repeat itself several times a second, until drive wheel speed is reduced a prescribed amount.
The EBTCM also controls the throttle adjuster motor and sends commands via a PWM signal to the PCM to retard ignition timing, thereby reducing engine torque and wheel slip.
TCS cannot be entered if the brake switch is "ON." Base brakes and ABS take precedence over TCS in all conditions. If TCS is being utilized and the brake switch is depressed, the EBTCM will immediately command the TCS motors to re-home the TCS ball screws and pistons. The applied brake pressure from the master cylinder can overcome the poppet valve spring to apply brake pressure immediately to the wheels during the re-home operation.

TCS THERMAL SHUTDOWN MODE

During prolonged TCS operation, brake lining temperatures may rise to a higher than desired level and the lining wear rate may increase. For this reason, the TCS system incorporates a brake thermal model which estimates brake temperatures during braking and TCS operation. The program calculates potential brake temperature based on time vs. amount of and duration of TCS brake applications. If the brakes or TCS modulator reach a potential over-temperature condition the TCS modulator is disabled but the PCM will still control the adjuster assembly and spark control. If the adjuster assembly reaches a potential over-temperature condition the TCS system will disable. The amber TCS warning lamp will turn "ON" and TCS will remain disabled until the brake linings are cooled to the proper temperature. The TCS system will automatically re-enable at this time. No DTC's will be stored.
Normally TCS will not disable during a traction event, but if the maximum temperature threshold is reached, it will disable.

ESB OPERATION

Figure 4 Expansion Spring Brake:

The Expansion Spring Brake (ESB) is used to hold the ABS piston in the upmost (or "home") position. ESBs are used only in the ABS modulator. An ESB is a spring that is retained in a housing at a close tolerance. One end of the spring is in contact with the motor drive dog and the other end is in contact with the pinion drive dog (refer to Figure 4). In normal braking, brake pressure is present on the top of the ABS piston, applying a downward force. The force applies a counterclockwise torque to the motor pinion which tries to rotate the spring counterclockwise. The counterclockwise torque expands the spring outward within the housing and prevents gear rotation.
When the motor is turned "ON" and tries to drive the ball screw nut, the end of the ESB in contact with the motor drive dog rotates inward causing the spring to contract in its housing allowing the motor to rotate the modulator gear (refer to Figure 4). The most common application of this principle is in window crank mechanisms, where a small amount of force on the crank handle allows the window to be lowered or raised, but the weight of the window or force on the window will not allow the window to move downward. For the ESB, brake pressure on the top of the pistons corresponds to the weight of the window and the motor corresponds to the window crank handle.