Traction Control System

ABS prevents wheel lockup during braking. Traction control prevents wheel spin during acceleration. They are opposite sides of the same coin, and they use the same hardware to do it. If you understand how ABS modulates brake pressure to a locking wheel, you already understand how traction control modulates brake pressure to a spinning wheel. The concept is identical — just applied in the other direction.

The ABS module continuously compares the speed of all four wheels. During acceleration, if one or two drive wheels begin spinning significantly faster than the non-drive wheels, the module knows those drive wheels have lost traction. On a rear-wheel-drive vehicle accelerating on wet pavement, the rear wheels may spin faster than the fronts. On a front-wheel-drive vehicle on ice, the front wheels may spin while the rears — which are just rolling along — report a much lower speed. That speed difference triggers traction control intervention.

Traction control uses two strategies, often simultaneously. First, it applies the brake to the spinning wheel. The ABS hydraulic control unit can selectively apply brake pressure to any individual wheel, just like during ABS activation but in reverse — adding pressure instead of reducing it. Braking the spinning wheel transfers torque to the wheel with traction through the differential. Second, the module sends a signal to the PCM to reduce engine power — by retarding ignition timing, cutting fuel injectors, or partially closing the throttle. Reducing engine output reduces the torque that caused the wheel spin in the first place.

Traction control has an off button for a reason. When a vehicle is stuck in snow, mud, or sand, some wheel spin is actually necessary to dig through the surface and find traction underneath. Traction control prevents that wheel spin. Rocking a vehicle out of a rut requires alternating forward and reverse with some wheel spin — traction control fights you the entire time. Turn it off when stuck. Turn it back on when you are out. Also, some performance driving situations benefit from controlled wheel spin — but for street driving, leave it on.

Because traction control uses ABS hardware, an ABS fault disables traction control as well. A wheel speed sensor fault, HCU fault, or pump motor fault takes out both systems. The traction control light illuminates along with the ABS light. Diagnose the ABS fault first — fixing it restores traction control as well.