Vehicle stability control systems: An overview of the integrated system that enhances braking, traction and skid control
Leon serves as one of the leading technical editors for Mitchell1. He is a graduate of Universal Technical Institute and has previously worked for Aamco Transmissions and as a mobile mechanic. He holds 609 Certification and specializes in automotive diagnostics.
Vehicle stability control systems (VSC) were created to help reduce the amount of wheel slip during acceleration and in harsh driving conditions. Typically for today’s high-torque engines, a form of wheel slip regulation or traction control can increase both safety and comfort, particularly on slippery roads. It allows for smooth starts and acceleration through all speed ranges without wheel-spin or fishtailing, it also reduces the amount of oversteer and understeer when cornering. So if your customer is driving like he stole it and happens to look at his dash and sees a stability control light suddenly illuminate for a second or two, he shouldn’t worry. This just means he has just taken advantage of the stability control system.
The way stability control systems work is by having a partnership between the live sensors and the anti-lock brake unit. It works by the sensors gathering information and processing it through the computer and sending it to the anti-lock brake system (ABS) unit. All of this information is processed in milliseconds. By applying hydraulic pressure through the anti-lock brake system, it can alter the traction of the wheels individually. The ABS system works by directing the amount of brake fluid to each individual wheel by the use of valves in the ABS system. Every manufacturer incorporates different sensors to make up their own version of a stability control system, but it all starts with the ABS system.
Steering angle sensor, vehicle speed sensor, wheel speed sensor, lateral acceleration sensor, brake pressure sensor and yaw rate sensor and computer or ECU are the basics for most stability control systems. Some of the more sophisticated systems adjust the throttle electronically, even limiting the number of cylinders in use while also using the ABS system to limit wheel slip.
Audi uses a system called Acceleration Slip Regulation (ASR). ASR works with the electronic accelerator and uses components of the ABS. If one wheel suddenly begins to rotate faster than the others (wheel slip), ASR intervenes in the engine management system and reduces power until the wheel stops spinning (this system also works with the electronic differential lock (EDL)). Citing another example, BMW uses a sub system called Dynamic Traction Control (DTC), which is a partner to Dynamic Stability Control (DSC). When driving on loose gravel or snow, activating DTC increases traction by allowing for more wheel slip while only enabling the DSC system when needed, and if desired DSC can be deactivated completely.
Drivers who desire a sportier driving style can activate DTC to allow for more wheel slip or drift.
Believe it or not, sound can also affect the way your customers drive. Mitsubishi is the manufacturer of the Lancer Evolution, a car known for its ability to handle just about any road in its path, by listening. The active stability control system (ASC) on the Evo operates differently from conventional brake systems. The difference is that the ASC system monitors sounds, sensations and vehicle performance. Noise monitoring was introduced as another method to “see” and in this case hear what is happening while the car is driven, it works with the other sensors to provide valuable live data.
For example let’s say your customer hears a loud clunk while driving. In that moment, the sound monitors would send a signal to the computer to check all systems to check for possible malfunctions.
Diagnostic trouble codes (DTCs) are set under different conditions, depending on the malfunction detected. Most DTCs will only be set during vehicle operation. Some DTCs will also be set during the self-check immediately after the engine is started. Most of the time when a warning light turns on for the traction control system it is due to a failed sensor. Some of the most common traction control system failures include:
Malfunction of wheel speed sensors, steering wheel angle sensor, lateral sensor, vehicle speed sensor.
Damaged wiring harness and connectors.
To further evaluate an illuminating Traction Control Light:
1. Gather information about the problem. (Ask your customer when does the problem occur? Only occurs in certain conditions?)
2. Verify that the condition described exists. (Check for visual signs.)
3. Check the vehicle for any DTCs.
4. If you can verify the condition but there are no DTCs, or the system cannot communicate with the scan tool, check that the basic brake system is operating properly.
A dynamic brake control system description example
The dynamic brake control system/vehicle stability control on the 2009 Toyota Sequoia is designed to enhance braking, traction and skid control in a comprehensive integrated system. We’ll briefly discuss each sub-system.
The ABS (anti-lock braking system) helps prevent the wheels from locking when the brakes are applied firmly or on a slippery surface. The skid control ECU (built into the VSC actuator) detects wheel lock conditions by receiving vehicle speed signals from each speed sensor, and sends control signals to the pump motor and solenoid valve to prevent the wheels from locking by controlling brake fluid pressure to each wheel cylinder. The ABS warning light illuminates when the ABS malfunctions.
The EBD (electronic brake force distribution) control utilizes the ABS, performing brake force distribution between the front and rear wheels according to driving conditions. When the brakes are applied during cornering, it also controls the braking forces of the right and left rear wheels, helping to maintain vehicle stability (to prevent oversteer).
The skid control ECU receives speed signals from each speed sensor to detect the slip conditions of the wheels, and sends control signals to the solenoid. The solenoid valve controls the brake fluid pressure of each wheel cylinder, and divides the control power properly between the front and rear wheels and the right rear and left rear wheels. The ABS and BRAKE warning lights and VSC OFF and SLIP indicator lights illuminate to indicate malfunctions in the EBD system.
The primary purpose of the BA (brake assist system) is to provide auxiliary brake force to assist drivers who cannot generate enough brake force during emergency braking. The skid control ECU receives speed signals from each speed sensor and the fluid pressure signal from the master cylinder pressure sensor to determine whether brake assist is necessary. If brake assist is deemed necessary, the skid control ECU sends control signals to the pump motor and solenoid. The pump and the solenoid valve then control the pressure applied to each wheel cylinder. The ABS warning light and VSC OFF and SLIP indicator lights illuminate to indicate malfunctions in the BA system.
The VSC OFF switch allows you to change to normal mode, TRAC OFF mode, AUTO LSD (limited slip differential) mode and VSC OFF mode. The TRAC (traction control) for 2WD or 4WD vehicles in H2 mode helps to prevent the drive wheels from slipping if the driver presses down on the accelerator pedal excessively when starting off or when accelerating on a slippery surface (prevents wheel spin). The skid control ECU detects the vehicle’s slip condition by receiving signals from each speed sensor and the ECM (via CAN communication). The skid control ECU controls engine torque with the ECM and brake fluid pressure through the pump and solenoid valve. The SLIP indicator light blinks when the system is operating. The VSC OFF indicator light blinks and the SLIP indicator light illuminates when the TRAC system malfunctions.
The VSC system helps to prevent the vehicle from slipping sideways when front or rear wheel skidding occurs during cornering.
The skid control ECU determines the vehicle condition by receiving signals from the speed sensor, yaw rate and deceleration sensor and steering angle sensor. The skid control ECU controls driving torque with the ECM, and brake fluid pressure with the solenoid valve.The SLIP indicator light blinks and the skid control buzzer sounds when the system is operating. When there is a malfunction in the VSC system, the VSC OFF indicator light blinks and the SLIP indicator light illuminates. NOTE: The VSC OFF switch stops VSC operation.
A-TRAC (active traction control) for a 4WD system controls the engine output and the brake fluid pressure that is applied to the slipping wheel and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve an LSD (limited slip differential) effect. The A-TRAC system operates when the 4WD control switch is set to 4L or 4H.
The SLIP indicator light blinks when the system is operating. The VSC OFF indicator light blinks and the SLIP indicator light illuminates to indicate a malfunction in the A-TRAC system. For 2WD vehicles (and 4WD vehicles in H2 mode), the AUTO LSD (auto limited slip differential) will operate with the VSC switch on (TRAC OFF or AUTO LSD mode) and the accelerator pedal is depressed.
The AUTO LSD system is used to restrain brake pressure and reduce differential movement, transmitting drive torque to the opposite drive wheel to ensure stability when driving off-the-road, and/or when drive wheels slip when starting on a slope with one wheel on snow/ice.
HINT: Depressing the brake pedal cancels control of the AUTO LSD system.
The SLIP indicator light blinks and the AUTO LSD indicator light remains on when the system is operating. The VSC OFF indicator light blinks and the SLIP indicator light illuminates to indicate a malfunction in the AUTO LSD system.
AUTO LSD operation becomes possible in TRAC OFF mode or AUTO LSD mode. TRAC OFF mode begins when the vehicle is in normal mode and the VSC OFF switch is pressed for a short amount of time.
AUTO LSD mode begins when the vehicle is in normal mode or TRAC OFF mode and the VSC OFF switch is pressed for three seconds or more. During TRAC OFF mode or AUTO OFF mode, the combination meter’s AUTO LSD indicator light and SLIP indicator light illuminate, and TRAC operation is disabled.
When the vehicle speed is more than 31 mph, the vehicle automatically returns to normal mode.
When vehicle speed is 31 mph or less, the vehicle returns to AUTO LSD mode.