Understanding Traction Control and Electronic Stability Control
Traction Control & Electronic Stability Control indicator lights
There are two electronic functions on a Jeep (and some other 4x4’s) that we normally want to disable for offroading: Traction Control (TC), and Electronic Stability Control (ESC). In this article, we will explain what these functions do and why we choose to NOT USE them during our offroad jaunts.
First, let’s back up a little bit and talk about how our Differentials work in our 2wd as well as 4wd vehicles. If a car drives straight, it is possible for all four tires to rotate at the same speed. But when that same vehicle turns, the outside tires need to rotate at a higher rate than the tires on the inside of the turn. It gets even more complicated, because the rear axle is pointing straight while the front axle is angled into the turn – creating a slight speed difference between the front tires versus the rear tires. Bottom line: during a turn in either direction, all four of your wheels are rotating at different speeds!
Differentials are mechanical gear arrangements that allow for slippage in rotational speed between wheels on either side. If the axle is connected to a driveshaft, it will send full power to one side, but reduced power to the other side, to allow for slippage.
However, things work counter-intuitively in terms of tire traction. Let’s say that both tires on either side of the axle are rotating, but one tire has good traction while the other tire is spinning freely without good traction. Logic would dictate that maximum power should be directed to the tire with traction, since that is the one that would propel the car. Unfortunately, the mechanical nature of the differential does just the opposite: maximum power gets directed to the freely spinning tire and is withheld from the tire that actually has traction. End result is that your car gets stuck.
This is where Traction Control comes into play. If we apply braking to the free spinning wheel, the differential is fooled into thinking that the other wheel (with traction) is the one “free spinning” since it is rotating faster than the wheel we have braked. Power is directed away from the wheel being braked and sent to the wheel spinning faster.
End result of this forced braking is that our engine power is now going to the “good” wheel that originally had traction and no longer to the originally free spinning wheel that had lost traction.
The problem is that the process of braking has reduced the total amount of engine force available to drive the wheel.
Modern Jeeps are equipped with a computer controlled automatic braking system tied in with your anti-lock braking system (ABS) to achieve this “Traction Control”.
“Note that on older Jeeps, experienced drivers will sometimes “two foot” the gas and brake pedals simultaneously on obstacles. This raises the overall engine RPM, prevents slipping backwards, and acts like Traction Control to direct more power to the wheels”
Electronic Stability Control is a bit different, in that it uses the computer sensors to detect and prevent fish-tailing & skidding at highway speeds. The computer responds by directing the ABS to brake one side or the other to stabilize proper direction, as dictated by the position of your steering wheel.
This explains why, if your steering wheel no longer physically points center up when driving straight, your ESC warning light will alert if a strong gust of wind pushes your jeep even a little sideways. It is a simple adjustment to center up your steering wheel, but not having the Jeep logo perfectly horizontal is more than cosmetic. It means that at some time on a trail, you took a knock from a rock and jiggled or loosened the steering linkage a few degrees off. It happens easily but is a simple fix for your mechanic to adjust.
When you engage 4wd-Low, your Jeep will automatically disable the Traction Control. That works for rock crawling and inching your way over obstacles.
Sand demands that you keep your speed up as much as possible. 4wd-Low is not conducive to exceeding 25mph. Therefore, we usually drive in 4wd-Hi in sand.
You should manually turn off your Traction Control while driving in sand, loose snow, or mud.
Traction Control button
Holding down the Traction Control button for more than 10 seconds will also disable the Electronic Stability Control function.
Instead, try “two footing” to raise your engine RPMs so as to increase your power, but allows you to ease off the braking so to “play” the differentials. Note that if you have lockers, you won’t see an advantage.
All Wheel Drive AWD
As long as you are here to learn, let’s bring up the topic of All Wheel Drive (AWD).
On a 2wd car, the engine power is 100% directed to the driving axle (usually Rear Wheel Drive, but it could be Front Wheel Drive). No power is directed to the non-driving axle. A differential directs the power to the (drivers or passenger) wheel with the least free spin when you turn.
On an AWD car, a differential directs engine power to the front or rear axle. The power is not evenly distributed but based on the comparative speeds of the axles. Similarly to the way a differential deals with left and right.
There are also differentials on both axles to distribute the resulting power to left and right.
The end result is that our AWD is not really “all” wheel drive but more realistically “any of the four” wheel drive.
That, of course, would really suck. So the automakers use a variation of Traction Control and Electonic Stability Control to distribute the engine power more evenly amongst the front/rear axles and the drivers/passenger side wheels. Computer sensors monitor all four wheels and emulate 4wd.
So then why can’t a Subaru keep up with a Jeep?
In a 4wd vehicle, the power to front/rear is evenly split by your transfer case, and NOT by a differential. No automatic braking is used to send power to both axles.
Lockers send equal power to both sides of a Jeep, and do not depend on braking to distribute to left/right. Limited slip differentials, available on some vehicles, achieve almost the same result, without braking.
AWD’s could, in theory, perform almost as well as a Jeep, if they were geared to deliver more torque to all of the wheels and if the drivetrains could withstand the extra stress created by using the computerized braking system to direct power.
However, in reality, the AWD cars give up too much power in their quest to even out the distribution, and risk overheating or overstressing the components when surmounting obstacles. The computers will limit or shut down the power in order to avoid catastrophic mechanical failure.
This is why my Subie just doesn’t cut it as an offroad rig. On the other hand, though, our Subie is superior to our Jeep when it comes to on-road driving in rain or light snow, because its computer system is reacting and correcting all four wheels extremely efficiently.
I believe that an AWD could be the offroad equal to a Jeep or Bronco if the engine and key components were super-sized. But for now, they are not, so we just have to stick with our tried and proven 4x4’s.
