• 2017

Pulling Out All the Stops

The Story on ABS, Traction Control, Wheel Speed Sensors and More

The anti-lock braking system (ABS) and traction control enable the driver to maintain control of their vehicle in adverse conditions. These two features are also closely related in the manner in which each operates. In fact, both systems share sensors (and the braking system) to perform their tasks.

Anti-Lock Braking System (ABS)

The purpose of the anti-lock braking system (ABS) is to prevent wheel lockup (wheel speed is zero) under braking conditions on virtually any type of road surface. A vehicle that stops without locking the wheels retains directional stability and some degree of steering capability, allowing the driver to retain greater control of the vehicle during all braking situations.

Wheel lockup, in which one or more wheels stops rotating (wheel speed is zero), can occur under hard braking, but especially under adverse road conditions such as rain, ice and snow. What feels like normal braking to the driver on ice and snow can cause one or more wheels to stop rotating, or to lock up. When such an event occurs, the brake system enters ABS mode.

During ABS braking, the hydraulic pressure to each road wheel is regulated by the integrated control unit (ICU). When the driver manually pumps the brakes when stopping on snow or ice, the hydraulic pressure is being regulated, but to all four wheels equally. With ABS, the hydraulic pressure is regulated to each road wheel independently. If only one wheel has locked up, only the hydraulic pressure to that wheel is regulated.

Hydraulic pressure at each front wheel is controlled, relative to the amount of slip at each wheel, to maximize the braking force by the front brakes. The rear wheels are controlled so that the hydraulic pressure does not exceed that of the highest slip rear wheel, all in order to maintain vehicle stability.

The ABS can build and release hydraulic pressure at each wheel based on the signals generated by the wheel speed sensors (one at each wheel to measure the speed at that wheel) and received at the anti-lock brake module (ABM). ABS operation is available at vehicle speeds above 3-5 mph. It turns off when the vehicle speed is slowed to 3-4 mph.

All-Speed Traction Control

Traction control can be viewed as the opposite of anti-lock braking. Instead of preventing wheel lockup (wheel speed is zero), traction control turns on when the road wheel begins to slip (wheel speed is uncontrolled). Traction control systems sense impending wheel speed based on the model of the rate of change of wheel speed under normal traction conditions. The wheel speed signals used by the traction control system come from the same wheel speed sensors used by the ABS.

Using the wheel speed signals, the traction control system (1) applies the brakes to one or more wheels and (2) reduces engine torque output using the electronic throttle control (ETC) to prevent wheel slip during acceleration. Throttle control makes the vehicle less dependent on brake application alone to maintain traction. While the driver can reduce loss of traction (tire slip) by easing up on the gas pedal, the electronic task of throttle control can be performed much more quickly and efficiently than doing it manually. Using both of these control methods (reducing torque and applying the brakes) makes it possible to achieve almost seamless torque application at all wheels. This is why the system is called all-speed traction control.

Wheel Speed Sensors and Tone Wheels

Wheel speed is the required parameter for the ABS/traction control systems. Without knowing the wheel speeds, these two systems simply do not work. Wheel speed is determined by a wheel speed sensor (see Figure 1). Each sensor is paired with a tone wheel to convert wheel speed into a small digital signal. At each wheel of the vehicle, there is one wheel speed sensor and one tone wheel.

The gear (tooth) type tone wheel serves as the trigger mechanism for each sensor. As the tone wheel rotates with the road wheel, a tooth, then an empty space, alternately pass the sensor. Each time the tooth passes the sensor, a digital signal is generated. As the speed of the road wheel increases, the frequency that the digital signal is generated by the sensor increases. This translates into a higher wheel speed. Likewise, as the speed of the road wheel decreases, the frequency that the digital signal is generated by the sensor decreases; thus, a slower wheel speed.

The tone wheels on the front of the vehicle are usually an integral part of the outboard constant velocity joints located in the front axle shaft. If the tone wheel is damaged, the entire axle shaft must be replaced as the tone wheel is not serviced separately. The rear tone wheels are usually an integral part of the rear wheel bearing. If the tone wheel is damaged, the bearing assembly must be replaced as the tone wheel is not serviced separately.

When the tone wheel is damaged, or if the sensor malfunctions, the ABS light will remain illuminated. This would indicate a problem with the ABS. These two components are the most likely to fail if there is a problem with the ABS or traction control system.

Wheel speed sensors are easily replaced. Disconnect the electrical connector, then remove the mounting bolt (see Figure 1). To install the new sensor, position it in place, then install and torque the mounting bolt. Then re-connect the electrical connector.

A quick note on tone wheels. Be sure the correct new part is installed on the vehicle. For example, the early PT Cruisers used two different rear wheel bearings and each bearing has a unique tone wheel. If the wrong bearing is installed, the ABS light will illuminate and the system will be disabled. Sometimes, a visual comparison of the new part and used part is necessary.

Anti-Lock Brake System Bleeding

If, for any reason, air enters the hydraulic base brake system, the system must be bled to purge the air from the system. The ABS must also be bled if air has entered the hydraulic control unit (HCU), or it is suspected that air has entered it. The base brake system and the ABS system must be bled as two separate systems.

After bleeding the base system, connect the appropriate scan tool to the OBD connector. Be sure that no Diagnostic Trouble Codes (DTCs) are stored in the ABM. Pressure bleeding is suggested for the anti-lock system to ensure that all air has been removed. Select the ECU VIEW on the scan tool, followed by ABS MISCELLANEOUS FUNCTIONS to access the bleeding procedures.

Follow the instructions displayed. When finished, disconnect the scan tool. Bleed the base system a second time, then fill the master cylinder reservoir to the MAX level.

As a final step, test drive the vehicle to check that the brakes are operating properly. There should be no spongy feel to the brake pedal.

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