Traction Control: How Does it Work?

Traction control. For most drivers, it’s great in the snow, great in the rain, and great on the pavement. For enthusiasts, it’s terrible pretty much everywhere. Or at least it used to be until automakers started taking it seriously.

Back in the old days, traction control meant a sensitive right foot. But there was only so much that could do for many drivers. So in 1971, Buick launched MaxTrac. Designed by GM’s AC division, MaxTrac was the very first traction control system and it debuted on the beautiful 1971 Riviera.

It works almost exactly the same as a modern traction control system, but with a little less modern technology. Make that a lot less.

The MaxTrac system used a transmission speed sensor, a front wheel speed sensor, an electric controller and an on-off switch. The transmission sensor measured rear wheel speed through the speedometer cable connection. Quick and easy. It turned rear wheel speed into an electrical signal, refreshing the speed at 25 times a second per mile per hour of speed. So at 60 mph, it pulsed 1,500 times a second. The front speed sensor did the same, but at just 6.25 Hz/mph.

The job of the electric controller was to compare the differences between the two signals. When the difference grew more than the car expected, it meant that the rear wheels were turning. The reason for the faster signal refresh in the rear was because the rear wheels changed speed more quickly than the front wheels. After all, it’s hard to spin the front tires on a rear-drive car. The system could respond more quickly to the rear wheels starting to spin. Because spinning tires at 60 mph is a bigger deal than the same at 5.

The computer sent a signal to the ignition, cutting spark if the rear tires were spinning faster than the fronts. Crude, simple, effective.

But cutting the ignition suddenly ran contrary to the EPA. Sudden ignition cuts on a carburetted car dump raw fuel into the exhaust. That gas ignited in the catalytic converter instead. So your Riviera could shoot flames. Briefly. So it was dropped in 1973 and pretty much nobody ever heard of it again.

Traction control returned in 1979 with a Cadillac system called the Traction Monitoring System. Allegedly. Because the only references to it I can find are a single line from a Wikipedia page that’s been copied far and wide.

By the mid-1980s it was starting to become common, and today it’s on everything. And it still largely works the same way.

Over the years, the sensors have gotten better. Now they use the ABS sensors instead of primitive speed sensors, which saves part costs. The method of cutting power has changed over time, too. From cutting ignition to cutting fuel AND ignition. Ever accidentally spin the rear wheels in a mid 90’s car when you needed to pull away quickly? And the car just stopped directly in front of that oncoming big rig? That’s what happens when primitive computers just cut fuel and spark.

Some vehicles, like mid 90’s BMWs added a second throttle plate that was just for traction and stability control. It opened and closed independently from the main butterfly to keep you from marking down the perfect set of elevens but it also didn’t make you come to a complete halt.

Even today, it’s mostly the same. Now, though, modern computers can just ease up on the digital throttle to stop your wheels spinning without bringing you to a sudden stop.

Better yet, automakers can add special modes thanks to better computing power and more control of the throttle. Like the Mustang’s burnout mode that lets you spin as long as the front wheels are turned straight. Or snow modes that recognize you need a little bit of wheel spin (but just a bit) when the snow gets deep. Even modes like McLaren’s Drift Mode that lets you choose how much wheelspin the traction control allows and how much sideways the stability control allows.

That’s because the tech that makes traction control possible is the same that makes stability control possible. It’s also why every car now has drive by wire instead of a cable throttle. Cables and the 2012 addition of mandatory stability control didn’t work well together.

Stability control uses those same wheel speed sensors, plus a few that measure steering angle and accelerometers that can sense what direction the car is actually moving in. It uses all of those to calculate if you’re experiencing understeer or oversteer.

So if the wheels are turned hard to the left, but the car is going straight, you’re understeering. If the wheels are turned to the right, but the back of the car is moving counterclockwise, you’re oversteering.

What does the car do then? Well, if you’re understeering like in that first example, the car knows you need to pivot left. Cutting throttle probably won’t help, and the ABS makes sure your wheels aren’t locked up, so what can the car do? Add some left rear brake thanks to some pressure from the ABS pump, and ease up on the others if you’ve got the whoa pedal floored. Like throwing out an anchor, the car will start to pivot around that left rear wheel. Hopefully getting you around the corner. The laws of physics still apply, though.

If you’re oversteering, it brakes the inside rear wheel. The right one, in that example. It should pull the car back straight, sending you happily on your way. Unless you were pulling a sweet drift, in which case consider this its digital apology.

So Traction Control uses speed sensors and your car’s throttle plate to detect and stop wheelspin. Stability Control uses those same speed sensors, plus a couple more to detect a loss of control and then uses the anti-lock system to help save you. And that’s how they do it.

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