When ABS was first introduced back in the mid-1980s, it was strictly a braking system for preventing wheel lockup and skidding. Then traction control was added as the technology evolved to prevent wheel spin during acceleration. Then came an advancement that would take ABS to an entirely new level. Electronic Stability Control (ESC) allows ABS systems to automatically brake individual wheels as needed to improve handling and steering control under all driving conditions.
Electronic stability control essentially makes ABS a full-time expert back seat driver that's constantly monitoring how the vehicle is responding to the driver and road conditions. If a problem starts to develop, it springs into action and takes whatever measures are necessary to get things back under control. This includes reducing engine power by backing off the throttle and/or retarding spark timing, and simultaneously applying one or more brakes to counter the forces that are causing the vehicle to lose control and/or traction. The neat thing is that all this happens automatically without any driver input!
HOW ELECTRONIC STABILITY CONTROL WORKS
To better control vehicle dynamics under all driving conditions, the ABS system needs some additional inputs. This includes a steering angle sensor to monitor the driver's steering inputs, a yaw sensor to detect changes in vehicle momentum that might cause the vehicle to spin out, oversteer or understeer, and a lateral acceleration (g-force) sensor to monitor changes in deceleration.
When the driver steers the vehicle, the steering angle sensor keeps the ABS control module informed about where the driver is aiming the vehicle and the rate at which the steering wheel is being turned (fast or slow). At the same time, the ABS control module looks at the inputs from its wheel speed sensors to determine if there are any differences in the rotational speeds of the right and left front and rear wheels. Turning a corner causes the inside wheel to rotate at a somewhat slower rate than the outside wheel.
If a vehicle begins to oversteer in a turn and the rear end starts to come around (which would cause the car to spin out), the speed difference between the left and right front wheels increases. If the vehicle understeers (loses front traction and goes wider in a turn), the speed difference between the left and right front wheels decreases.
If the stability control software in the ABS control module detects a difference in the normal rotational speeds between the left and right wheels when turning, it immediately reduces engine power and applies counter braking at individual wheels as needed until steering control and vehicle stability are regained.
Click Here to download/view a video clip of a Chevy Tahoe rollover test. In this clip, which was taken at the Bosch proving grounds in Flat Rock, Michigan, the driver makes a sharp swerve at 35 mph with the stability control system disengaged. The SUV goes up up on two wheels and nearly rolls over. The test is then repeated with stability control engaged and the vehicle remains flat on the ground.
STABILITY CONTROL APPLICATIONS
The first vehicles to come factory-equipped with electronic stability control were the 1995 BMW 750iL and 850Ci models with a 5.4 liter V12 engine. The Bosch-built Dynamic Stability Control (DCS) system monitors individual wheel speeds 50 times per second (every 20 milliseconds), and is always active whether the driver is braking or not. If the system senses an understeer or oversteer condition developing, it takes one of two courses of action depending on the amount of cornering force or lateral acceleration that's being developed.
If the vehicle's lateral acceleration is greater than about 0.6g,and the driver brakes normally (not hard enough to bring antilock braking into action), the DSC system modulates brake pressure so that the outside wheels are braked more than the inside wheels. This counteracts the oversteer or yaw effect that might otherwise cause the vehicle to lose control and spin out. If the driver realizes he's going too fast and hits the brakes hard enough to kick in the normal antilock braking, DSC reverts to a normal ABS braking mode allowing the system to selectively modulate rear brake pressure as needed.
MERCEDES ESP STABILITY CONTROL
An even more sophisticated Electronic Stability Program (ESP) appeared in 1996 on V-12 powered Mercedes S600 models. Like BMW system, the Vehicle Dynamics Control (VDC) system is also made by Bosch and provides automatic engine torque reduction and braking if a car enters a corner too fast or makes a sudden steering maneuver. But unlike the BMW system, Mercedes also uses individual front and rear braking to help the vehicle regain control.
With the Mercedes ESP system, the front brakes may be applied separately to help correct the vehicle's attitude. If the car is going into a left turn, for example, and is starting to oversteer, the ESP system applies the right front brake to help bring it back under control. To correct an understeer condition when cornering left, the ESP system applies the left rear brake. A dash warning light also flashes to alert the driver that the stability control system is active (a glowing triangle with an exclamation mark inside).
The Mercedes system takes inputs from two additional sensors: a lateral acceleration sensor and a yaw velocity sensor. The latter tells the control module if the car is turning on its axis so the computer can compare the input to the steering angle and speeds of the individual wheels.
In 1997, Cadillac added Delphi's new StabiliTrak stability control system to its Seville STS, DeVille Concours and Eldorado ETC models. The StabiliTrak system compares what the driver wants to do with how the car is actually responding. If the car isn't responding the way the driver wants, or is in danger of spinning or skidding out of control, StabiliTrak slows and stabilizes the car to help the driver regain control.
Like the BMW and Mercedes stability control systems, Cadillac also uses a steering angle sensor, yaw sensor and lateral acceleration (g-force) sensor.
LEXUS VEHICLE STABILITY CONTROL
In 1998, Lexus added the Bosch Vehicle Stability Control (VSC) system to some of its cars. Operationally, it is the same as the Mercedes and Cadillac systems, but adds a brake pressure sensor to monitor driver braking inputs (or lack thereof), and a warning buzzer in addition to the dash light to warn the driver when the system is active.
STABILITY CONTROL SYSTEMS
Stability control is being integrated into ABS systems on more and more cars, light trucks and SUVs. It makes the most sense on SUVs because of their higher center of gravity and increased risk of roll-over if they get sideways or spin-out on a slick road or suffer a tire blowout.
In 2006, stability control was standard equipment on about 29 percent of all cars and trucks, including 57% of all SUVs. By 2010 it was used on 85 percent of all new cars. By 2012 it was standard on all new cars, SUVs and light trucks.
Stability Control System Names
Stability control systems are offered under the following names:
* Acura: Vehicle Stability Assist (VSA)
Stability Control Demonstration
The first video below shows what can happen when a vehicle with a high center of gravity makes a sudden steering maneuver at high speed. The vehicle is traveling at 45 mph on a test track with its stability control system turned off. The second video shows the same test at a slightly faster speed (50 mph) with stability control on. Both videos were taken at a Bosch test track in Farmington Hills MI. The vehicle in these videos was driven by a professional driver, and the SUV was equipped with safety bars to prevent it from rolling over. So DO NOT attempt a roll over test like this yourself!
Without Stability Control
With Stability Control
ELECTRONIC STABILITY CONTROL SAVES LIVES
Electronic stability control cannot idiot-proof a vehicle. Nor can it overcome the basic laws of physics. If a vehicle is driven too fast into a turn to recover, it will go off the road. But stability control can help a driver regain control at lesser speeds (especially on slick roads) or when making sudden accident avoidance maneuvers. It can also prevent rollovers in vehicles with a high center of gravity such as an SUV.
Studies have shown that electronic stability control is saving lives and reducing accidents. Back in 2004, the National Highway and Traffic Safety Administration (NHTSA) confirmed the validity of the studies with its own field report that concluded stability control has reduced crashes by 35%. The Insurance Institute for Highway Safety (IIHS) later issued their own study that concluded the widespread application of stability control could save 7,000 lives a year. To view the IIHS report, Click Here.
In June 2006, the IIHS updated the results of their 2004 study by stating that up to 10,000 fatal crashes could be avoided annually if all vehicles were equipped with stability control.
The following is a brief summary of the NHTSA requirements for stability ,control systems under Federal Motor Vehicle Safety Standard (FMVSS) No. 126:
* Vehicles must pass a dynamic test that would work effectively in oversteer and understeer situations.
* ESC systems must: augment directional stability by applying the vehicle's brakes individually to induce correction of yaw torques; be computer controlled; be able to determine vehicle yaw rate and velocity; be able to monitor driving steering input; and be operational within the full speed range of the vehicle (except when there is a below-speed threshold where loss of control is unlikely).
* The system must have a telltale mounted inside the occupant compartment in clear view of the driver, to alert the driver when the ESC system is not functioning properly. A driver-selectable off-switch would be permitted to address times when a vehicle is stuck in sand or gravel or is being run on a track.
* There is a three-year phase-in period for new systems. Compliance began on Sept. 1, 2008 with 30% of all MY 2009 vehicles, followed by 60% for MY 2010, 90% for MY 2011 and 100% for MY 2012. Small-volume manufacturers are excluded from the phase-in requirements but must be fully compliant on Sept. 1, 2011. Multistage manufacturers and alterers would have to be fully compliant by Sept. 1, 2012.
* NHTSA estimates that with mass production the average cost for installation of the ESC will be around $111 per vehicle on vehicles that already include ABS brakes. Currently the cost for optional equipment is around $300 to $800.
STABILITY CONTROL DIAGNOSIS and REPAIR
Because the system in integrated into the ABS system, faults are self-diagnosed and turn on a warning light. Depending on the nature of the fault, the stability control system and ABS system may be temporarily disabled. The vehicle can still be driven normally but the stability control/ABS system will remain off-line and cannot prevent vehicle skidding or loss of control in an emergency driving situation. For this reason, a stability control/ABS warning light should be investigated immediately and repairs made as soon as possible.
Diagnostics require the use of a scan tool on most vehicles (with software that can access the stability control/ABS system). On some vehicles, it may also be possible to read fault codes and other diagnostic information through the driver information display by putting the system into a special diagnostic mode. Refer to the vehicle service literature for the specific procedures for doing this. It typically involves pushing various buttons on the climate control or driver display panel.
Typical problems include loss of sensor signal due to sensor failures or wiring faults, problems in the ABS modulator, pump motor, pump relay or high pressure accumulator. Always check for vehicle manufacturer technical service bulletins any time a stability control or ABS problem is encountered. The TSB can help you diagnose and repair the fault.
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