Understanding Vehicle Stability Control
The goal of all electronic stability control systems is simple: prevent side skidding and the subsequent loss of control that side skidding creates. The topic of vehicle stability control (VSC) is often misunderstood. This article sheds light on how the system works along with tips on diagnosing issues relating to this electronically controlled vehicle handling technology.
Just what is stability control? And why do we have it? My GMC pickup caught me totally off guard when I was going up a familiar hill on a damp morning recently. I was accelerating normally when the rear end started to break loose. Before I could react, however, the StabiliTrak light started to flash, the rpms dropped, and I could hear the anti-lock brake system (ABS) pump running and the solenoids cycling. I never hit the brakes but my immediate reaction was to let off the gas... but by then my truck had already returned to the direction that I’d always intended. Almost before I lost control, I had it back again.
This was my introduction to electronic stability control (ESC). ESC goes by many different names: Mitsubishi calls its system active skid and traction control (ASTC); Toyota, vehicle stability control (VSC); Volvo, dynamic stability and traction control (DSTC); Jeep, electronic stability control (ESP); Honda, vehicle stability assist (VSA); Infiniti, vehicle dynamic control (VDC). These are just a few of the vast number of names that the manufacturers have given their ESC systems. But the goal of all ESC systems is simple: prevent side skidding and the subsequent loss of control that side skidding creates. ABS and traction control systems control the vehicle along the longitudinal (front-to-back) axis, while ESC controls the vehicle along the lateral (side-to-side) axis.
Government data has shown that vehicles equipped with ESC were involved in approximately 35% fewer severe collisions that involved loss of vehicle control versus non-ESC equipped vehicles.
ESC anticipates dangerous situations even before the vehicle goes out of control and is so effective, in fact, that the government mandated ESC on new vehicles built after 2012.
ESC is not a modern-day technology. Developed by BMW, Mercedes-Benz, Bosch and Continental Automotive Systems in the late 1980s, it made its road debut in 1995 on the S-Class Mercedes-Benz. Other manufacturers followed quickly. GM/Delphi introduced StabiliTrak as an option on some Cadillacs in 1997. Ford launched AdvanceTrac in 2000. By the mid-2000s most manufacturers offered some form of ESC.
Many ESC-equipped vehicles have lots of miles on them now and technicians are being asked to repair the ESC system that may have illuminated the warning lights on their dashboards, even without any driveability concerns.
The basic components that all ESC systems rely on to make its decisions are similar. A controller that uses algorithms to make the system function, four individual wheel speed sensors (WSS), a steering angle sensor (SAS), yaw rate sensors and lateral accelerometers.
Other components may be incorporated but not by all manufacturers; brake pressure sensor, longitudinal accelerometers, roll rate accelerometers and throttle position can further enhanced an ESC system. The latter two sensors are primarily involved in larger top-heavy SUVs and help prevent rollovers.
ESC is a complex system that links together sensors typically already on the vehicle and makes safe driving decisions based on the data they provide. Understanding the basics of these sensors and their correlation to the decisions that the ESC will make is essential to diagnostics and repair.
Gathering the information from... the wheel speed sensors
The wheel speed sensors are an essential part that must accurately report each of the vehicle’s wheel speeds to the ESC module. The familiar passive WSS uses a magnet, and generates an AC current and the resulting sine wave. This was the most common kind of WSS, but the active WSS is more accurate and can measure wheel speed all the way down to a complete stop and can even recognize that wheel’s rotational direction (it is possible to know that the wheel is turning backwards). These features have made the active WSS a popular choice for engineers on many of today’s vehicles.
Active WSS requires special testing and usually involves a scan tool (although there are some tests that can be done with a DVOM).
When testing an active WSS it must be carefully back-probed (still connected) during testing and you will be looking for a supply voltage.
This voltage can be 5.5 volts to up to 20 volts depending on the system. Many vehicles use 12 volts or battery voltage, and when testing it is imperative to make sure it’s the same on all four sensors. Because an active WSS is a two-wire sensor, the control unit will measure the return signal, so it’s not possible to test for a reference voltage or resistance across the connector terminals. Some Hondas, Acuras, and BMWs shut off the power supply if the sensor is disconnected, but it will come back if the sensor is hooked up and the key cycled.
Yaw rate and lateral acceleration sensors are fancy names for G sensors or accelerometers. Essentially, they are just electronic gyroscopes. The purpose is to sense a variation in direction, either left or right or side-to-side. The yaw rate sensor will measure the rotation rate of the vehicle and determines how far off-axis the vehicle is “tilting” or “rolling” in a turn. The yaw rate sensor is usually mounted low and in the center of the vehicle (often under the center console to be as close to the vehicle’s center of gravity as possible) so that its measurement is more precise.
The lateral acceleration sensor measures the rate of change in side-to-side movement to calculate the vehicle’s actual position in the turn.
In many cases the lateral and yaw rate sensors are incorporated into one single unit or combined as a sensor cluster. These sensors can be directly wired to the ESC controller or on the CAN-bus system, sharing information with other controllers.
Transverse and roll-rate sensors can also be incorporated into the system to help detect and prevent vehicle rollovers. These sensors can also be an all-in-one design or they can be mounted individually on the chassis.
Most of these sensors can only be tested with a scan tool, but like an SAS sensor you can test for communication activity on the output line with a meter or scope.
The steering angle sensor
The steering angle sensor (SAS) reports the position of the steering wheel (where the driver wants to go) and how fast the steering wheel is being turned. The ESC module needs to know this information to compare it with yaw and lateral accelerometer info to decide what the driver really intended to do.
There are analog, digital and controller area network (CAN) bus designed SAS sensors with each containing a cluster of sensors for redundancy, precision and safety.
An analog SAS will have a power (5V reference), a ground, and signal outputs. Normal straight-ahead steering wheel position scanner values or back probing the connector will show 2.8V on one sensor and 0.4V on the other.
The values should not match, just like TPS sensors on drive-by-wire systems. And the values may go positive or negative as the wheel is turned.
The digital SAS or contactless sensors will use either a Hall Effect sensor or an optical sensor to produce a digital square wave output that will change frequency as the steering wheel is turned.
The CAN-bus SAS can be directly wired to the ESC controller or the information created by the SAS can be shared along the CAN network of the vehicle, and used by other modules (directional headlights, EPS).
This style is diagnosed using a scan tool. It is only possible to see if the SAS is communicating by checking the voltage output on the CAN wire, but you have no idea what is being said.
How the system works
The ESC controller, either a separate self-contained unit or integrated into another controller (ABS module e.g.) linked via the CAN system for continuously comparing the information from the SAS input, WSSs and accelerometer sensors to determine if the vehicle is reacting the way the driver wants or expects it to. If the vehicle isn’t reacting that way (side-skid condition) the ESC will make the needed corrections to get the vehicle back to the direction the driver intended if it isn’t.
Using WSS data the ESC controller is looking to see if the vehicle is under-steering (the front wheels will be turning slower than the rear wheels) or over-steering (the front wheels turn faster than the rear wheels) compared to the inputs from the driver.
When this WSS data is combined with the yaw sensor data and SAS inputs, the ESC will make calculations and start applying wheel brakes to correct the situation. This brake application could be a single wheel or multiple wheels, depending on what the ESC module feels is the best response using the available data.
If the vehicle is under-steering on a right turn, it may only apply the right-side brakes to assist in a right rotation. The opposite happens if the vehicle is over steering or fish-tailing on a right-hand turn. It will only apply the left or outside brakes to compensate and allow the driver to regain control. If the situation cannot be corrected using the brakes alone, the ESC may also cut the throttle, retard ignition timing, shut off injectors and even change transmission settings to intervene, all attempting to allow the driver to regain control.
When this skid condition is happening, the driver may be alerted with a flashing ESC light. The driver may also hear the ABS functioning, and may notice a decrease in power until the ESC has decided to return control.
Other features can be added to the base ESC programs to improve other stability issues that normal drivers can experience.
GM’s StabiliTrak and Ford’s AdvancTrac can control trailer sway by detecting the yaw induced into the pulling vehicle by the trailer. Both systems will automatically apply the needed individual brakes on the tow vehicle to get the trailer and tow vehicle back in line.
Corner braking can be incorporated into the ESC system, to improve the vehicle’s stability if the driver brakes during a slippery turn or where traction is different from right to left.
Because an uneven braking force will result at the wheels due to a weight shift to the outside of the turn, the ESC unit will observe the front WSS speed differences and can pulsate the braking on the outside wheels to create an equal braking force on all four wheels.
Stopping on a road surface that has a friction difference between left and right sides of the vehicle may need some ABS activation to provide balanced braking, but the driver will usually need to provide some form of steering input to keep the vehicle going straight.
In this situation, the ESC unit may command some steering input from the electric power steering (EPS) if equipped even before the driver has a change to recognize the need to do so.
Repairing and replacing parts on an ESC-equipped vehicle
An up-to-date information system is a must in diagnosing the ESC system and naturally a good scanner. Most ESC systems will provide trouble codes that will lead a tech into the area of concern, commonly involving ABS sensors, yaw or SAS issues. But even a misfire may turn on an ESC light due to the fact the ESC needs the engine to be running properly for it to function as designed.
ABS sensor diagnostics and their replacement is going to be vehicle dependent. Many WSS are now incorporated in the wheel hub design and are not available separately. Understanding the type of WSS is critical for proper diagnostics. The use of a scan tool is going to be typical and most good generic scanners provide the ABS information needed for diagnosis.
Most SAS require some sort of resetting or calibration after replacement, an alignment, steering wheel removal or service or whenever an ESC component is replaced, but don’t be surprised if you must perform this even after a battery replacement. Many new alignment racks will now properly recalibrate the SAS after an alignment using proprietary software and the OBD-ll plug.
Many newer SASs self-calibrate by requiring you to turn the steering wheel slowly lock to lock, and driving the vehicle over a certain mph and distance and will turn off the service light after an appropriate drive cycle. But frequently a scanner is going to be required. Again, I have found a good generic scanner will guide you through the various vehicle-specific SAS calibration procedures.
Almost all G sensors (yaw, lateral, or roll rate) will need to be zeroed or initialized after replacement, to allow the ESC to recognize their neutral position. This usually requires the vehicle to be on a flat, level surface and generally in an unloaded state, with the wheels pointed forward. Yet again, a scanner will be required and the proper information on the procedure.
Attention to fastener torque, proper sensor orientation and location mounting is imperative for proper operation of these sensors.
If the ESC unit is going to be replaced, a factory scan tool may be required to install the flash or programming, but some are simple and only need to be initialized. (NOTE: The ABS/ESC module, a common failure part on late model Dodge Grand Caravans, just requires initialization if it has to be replaced.)
Bringing this to a safe stop
It’s important to remember that an ESC system has limits. Tire tread, tire pressure, steering components, proper brake components and suspension parts all must be inspected and verified to be in working order. Parts must be replaced properly and initialized to function correctly.
ESC was not designed to allow for faster cornering speeds or aggressive driving on slippery roads. It was designed to increase safety and vehicle control.
But this system isn’t fool-proof. It can’t overcome the simple laws of physics, and if the base systems aren’t functioning at the appropriate level, this system won’t be able to work as designed. ■
Jeff Taylor boasts a 32-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
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