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Basics of Traction Control

by Larry Carley copyright

Mercedes EST traction control

Traction Control (TCS) is an option that is often found on vehicles equipped with antilock brake systems (ABS). Traction control is essentially an "add-on" feature to ABS that improves traction when the vehicle is accelerating on a wet or slick surface, or is accelerating too quickly for the tires to maintain their grip. Traction control prevents wheel spin by applying the brakes on the drive wheel that is losing traction, and/or momentarily reducing engine torque by various methods. The main difference between ABS and TCS, therefore, is that ABS only comes into play when braking while TCS only comes into play while accelerating.


Traction control shares many of the same components and sensor inputs with the ABS system:

* A common control module is often used with additional software and control circuits for TCS. In some vehicles, a separate TCS control module may be used.

* The same wheel speed sensors are used to monitor wheel speeds.

* The same pump and high pressure accumulator are used to generate and store hydraulic pressure for TCS braking.

* The same modulator (with a couple of extra solenoid valves) is used to control braking.


The primary input to the TCS control program comes from the wheel speed sensors. The sensors continually monitor the speed of the wheels anytime the vehicle is moving. The sensors generate a signal that is proportional to wheel speed, so by comparing wheel speeds the ABS/TCS system can detect changes that indicate a wheel is losing traction, skidding or spinning.

traction control offroad driving

When traction control is needed, it applies the brakes on the drive wheel(s) that is losing traction. Slowing the wheel allows it to regain traction. At the same time, torque is shifted through the open differential to the opposite wheel that still has traction.

Applying the brakes requires routing pressure from the ABS pump and high pressure accumulator through the ABS modulator. Traction control typically adds an extra solenoid valve in the ABS modulator for each drive wheel's brake circuit. This allows the system to apply pressure to slow the drive wheel if the wheel starts to spin. The ABS solenoids for the same brake circuit may also be called into play to hold, release and reapply pressure as needed until traction is regained.

Most traction control systems will discontinue braking after a certain length of time or after so many repeated braking applications to prevent the brakes from overheating (as when driving in mud or snow).

If both wheels are losing traction, traction control may brake both wheels equally to slow them down enough so that they can regain traction, and/or it may also send a request to the powertrain control module (PCM) to reduce engine torque until traction is regained. Depending on the vehicle application, any of a number of different torque reduction strategies may be used or combined:

* On some applications (Corvette, for example) a "throttle relaxer" is used. This device is attached to the throttle linkage and pulls against the linkage to reduce the throttle opening when traction control is needed. The driver can feel the relaxer pushing back against his foot telling him to ease up on the gas.

* On vehicles with throttle-by-wire (new Mustangs, for example), there is no cable or mechanical linkage between the gas pedal and throttle. The throttle is controlled electronically by the PCM based on sensor inputs from the gas pedal. Here, the traction control system may ask the PCM to momentarily reduce the throttle opening if a wheel is slipping.

* On other applications, TCS may ask the PCM to retard spark timing and/or disable one or more fuel injectors (up to four injectors on a V8) to reduce engine torque if braking alone is not enough to regain traction.

* On some applications, the traction control system may ask also the PCM or transmission control module (TCM) to shift the transmission to a higher gear momentarily to reduce torque until traction is regained.

* For additional feedback, most traction control systems illuminate or flash the TCS warning lamp on the dash when the system is active.


Some drivers may find the intervention of traction control annoying, especially if they drive a performance vehicle. So most systems have a button or switch that allows the driver to temporarily deactivate traction control. When the system is disabled, a warning light will illuminate to remind the driver traction control is not available. The system will remain disabled until the driver pushes the TCS button again, or until the start of the next ignition cycle (the default mode for TCS is usually ON).

Note: Disabling traction control does not disable or affect the operation of the ABS system. ABS remains on all the time unless it has disabled itself due to an internal fault.

Most traction control systems only operate at speeds below 30 MPH because traction control usually isn't necessary at higher speeds. Also, braking at higher speeds could have an adverse effect on vehicle handling and stability -- unless traction control is also part of a total stability control system that monitors vehicle stability and handling at all speeds.


The traction control system has its own warning light and shares its internal self-diagnostics with the ABS system. If a fault occurs in any of the components that affect the operation of either system, one or both warning lights will come on and a diagnostic trouble code (DTC) will be set in the control module that corresponds to the fault(s).

If the TCS or ABS warning lights are on, one or both systems are usually disabled and will remain offline until the fault is diagnosed and repaired.

The procedure for reading and clearing TCS/ABS fault codes will vary depending on the vehicle model year and system. On older vehicles (pre-OBDII) and some older imports (Acura, for example), the TCS/ABS module has manual flash codes and no scan tool is needed. On most newer applications, though, a scan tool is needed.

TCS/ABS diagnostics requires a scan tool with software that can talk to the TCS/ABS module or body control module (depending on how the vehicle is wired). The scan tool must also be CAN-compliant if the vehicle is a newer one with a Controller Area Network (CAN).

On most applications, the TCS/ABS system will have a number of self-tests that can be run through the scan tool. This includes tests that operate the pump and tests for the TCS/ABS solenoids. Some of these tests may only be available with an OEM factory scan tool.

If any of the hydraulic components in the ABS system or brake system are being replaced, a scan tool may also be needed to cycle the ABS solenoids so air can be bled out of the modulator and brake lines.

For specific service procedures, always refer to the vehicle manufacturer service literature. This includes bleeding sequences, wiring diagrams, wheel speed sensor resistance specifications and component test and replacement procedures.

Warning: The high pressure accumulator on TCS/ABS systems must be fully depressurized before working on the brakes or opening any brake lines. This can be done by pumping the brake pedal 30 to 40 times with the ignition off.

Most traction control problems that occur are related to the loss of a signal from a wheel speed sensor, a pump that fails to run, or a high pressure accumulator that leaks or can't hold pressure.

Wheel speed sensors can be checked by measuring their resistance with an ohmmeter and comparing the reading to specifications. Sensors are magnetic and may not produce a good signal if the tip is contaminated with metallic debris or the air gap between the sensor tip and tone ring is too large.

If a code indicates the high pressure pump isn't working, the underlying cause may be a faulty pump relay or a bad wiring connection. If the pump fails to run when the relay is bypassed with a fused jumper wire, the pump has failed and needs to be replaced.

A code that indicates the system is not holding pressure usually means the high pressure accumulator is leaking (check the seal between the accumulator and modulator/pump assembly, or that the rubber diaphragm inside has ruptured allowing the accumulator to lose its nitrogen gas charge (replace the accumulator after depressurizng the system).

Problems may also occur with any of the solenoid valves in the TCS/ABS modulator assembly. The problem may be mechanical (failure due to rust or corrosion in the valve) or electrical (failure of the solenoid). On most systems, the valves cannot be replaced separately so the entire modulator must be replaced as a unit.

If a code indicates an internal module fault, the TCS/ABS module can usually be replaced as a separate item. It may be attached to the modulator assembly or located elsewhere in the vehicle.

Communication errors between the TCS/ABS module and PCM or BCM (Body Control Module) may be the result of a wiring fault.

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