I was reading my bonnie'* manual and under traction control, it says the traction control works to limit wheel spin by reducing engine power AND working the front brakes. In my alero, the traction control only limits the engine power. Does this mean that the traction control is more like StabiliTrak (which wasn't officially invented until 1997)? How exactly does it work? wheel speed sensors only or does it have yaw sensors too?

thanks for clarifying!

There'* no yaw or steering angle sensors in the traction control system. Most of the traction control functions are carried out by the Anti Lock Braking System and Computer. The same wheel speed sensors that the ABS uses are used for traction control. When the computer detects that on of the front wheels is moving faster than the rear wheels, it uses the ABS system to brake the spinning wheel while also controlling engine output by means of actually pulling the throttle shut or electronically.

This is the traction control system description for my 98 Regency. The systems should be similar in function.

Antilock Brakes / Traction Control Systems: Description and Operation

ANTILOCK BRAKING MODE

When wheel slip is noted during a brake application, the ABS will enter the antilock mode. During antilock braking, the hydraulic pressure in the individual wheel circuits is controlled in order 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 the hydraulic pressure to each wheel brake. However, the ABS cannot increase the hydraulic pressure above the amount that the master cylinder transmits during braking. During antilock braking, a series of rapid pulsations will be felt in the brake pedal. The rapid changes in the position of the individual solenoid valves as they respond to the desired wheel speed causes the pulsations. This pedal pulsation is present during antilock braking and will stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping nose may also be heard as the solenoid valves rapidly cycle. During antilock braking on dry pavement, the tires may make intermittent chirping noises as they approach slipping. These noises and pedal pulsations are normal during antilock operation. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal provides the shortest stopping distance while maintaining vehicle stability.

PRESSURE HOLD

When the Electronic Brake Control Module/Electronic Brake Traction Control Module (EBCM/EBTCM) senses the wheel slip, the EBCM/EBTCM closes the Inlet valve and keeps the Outlet valve closed in the Brake Pressure Modulator Valve (BPMV) in order to isolate the system. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.

PRESSURE DECREASE

If during the pressure hold mode the EBCM/EBTCM still senses wheel slip, the EBCM/EBTCM will decrease the pressure to the brake. The Inlet valve is left closed and the Outlet valve is opened to the BPMV. The excess fluid is stored in the accumulator until the return pump can return the fluid to the reservoir.

PRESSURE INCREASE

If during the pressure hold or the pressure decrease mode the EBCM/EBTCM senses that the wheel speed is too fast, the EBCM/EBTCM will increase the pressure to the brake. The hold Inlet is opened and the Outlet valve is closed in the BPMV. The increased pressure comes from the master cylinder and is related to the pressure applied to the brake pedal.

TRACTION CONTROL

Traction control will not have any effect on vehicle operation until the control module detects one or both of the front wheels rotating faster than the rear wheels. At this time, the Electronic Brake and Traction Control Module (EBTCM) will request the Powertrain Control Module (PCM) to reduce the amount of torque applied to the drive wheels. The PCM retards the timing and turns off the supercharger (if equipped) and the fuel injectors. The EBTCM applies the front brakes in order to reduce the torque to the front wheels. Once the front wheels begin to rotate at the same speed as the rear wheels, the system will return full control to the driver. During the traction control mode, if the brake is applied to only one front wheel, most of the engine torque will be directed to the other front wheel, thus improving the vehicle traction. Closing the Traction Control System (TCS) master cylinder isolating isolates the master cylinder from the rest of the system. The TCS prime valves open in order to allow the pump to get the brake fluid to build the pressure for braking. The drive wheel circuit solenoids are energized as needed in order to allow for pressure hold, pressure increase, or pressure decrease. The driver can deactivate the TCS, if desired. In order to deactivate the TCS with the engine running, depress the
traction control disable switch. The TRACTION OFF Indicator in the IPC will illuminate. The system remains deactivated until the ignition switch is cycled, or the switch is pressed again.

MAGNASTEER

The speed dependent steering system (MAGNASTEER) incorporates a controller into the EBCM/EBTCM. A MAGNASTEER DTC C1241 will not cause the ABS or TRACTION OFF indicators to go on.

TIRES WITH ABS/TCS

The compact spare tire rotates faster than the others. The EBCM/EBTCM compensates for this faster rotation. Refer to Tires and Wheels for information on replacement tires.

REPLACEMENT TIRES

The tire size is important for the performance of the ABS/TCS. The replacement tires should be the same size load range and construction as the original tires. Replace the tires in axle sets and only with tires of the same tire performance criteria specification number. Use of any other tire size or type may seriously affect ABS/TCS operation.

BRAKE WARNING INDICATOR

The red BRAKE Indicator in the instrument cluster will illuminate to warn the driver of conditions in the brake system which may result in reduced braking ability. The indicator will also illuminate when the parking brake is applied or not fully released, or if the brake fluid level switch is closed. The BRAKE warning Indicator will stay illuminated until the condition has been repaired. The ABS indicator is located in the IPC and will illuminate if a malfunction in the ABS is detected by the EBCM/EBTCM. The ABS indicator informs the driver that a condition exists which results in turning off the ABS and TCS function. If only the ABS and TCS indicators are on, normal braking with full power assist is available. If the BRAKE and ABS indicators are on, a problem may exist in the hydraulic brake system. Conditions for the ABS indicator to turn on are as follows:
^ ABS Malfunction detected. As previously described the ABS indicator will turn on when a problem has been detected by the EBCM/EBTCM
^ Instrument panel bulb check. When the ignition switch is turned to the RUN position the ABS indicator will turn on for approximately three seconds and then turn off.

Illumination of the TRACTION OFF indicator indicates that a malfunction has occurred and the Traction Control System has been disabled, or the
Traction Control switch has been pressed turning the system off.

Illumination of the TRACTION ACTIVE indicator indicates that Traction Control is active. This indicator does not indicate a malfunction has occurred or service is needed.