• Most medium and heavy-duty vehicles are required to have ESC systems to avoid lateral instability, wheel lift, and rollover.
• ESC systems may be temporarily disabled at low speeds (12 mph or less), must have a diagnostic system verifying it is working, and a dash indicator notifying the driver of a fault in the system.
• Portions of the ABS/traction control systems are located at the wheel ends, leaving them exposed to environmental hazards, and requiring extra attention during vehicle inspection.

The National Highway Traffic Safety Administration (NHTSA) regulations at 571.136 require most medium and heavy-duty vehicles (trucks and buses) to have electronic stability control (ESC).

The system uses yaw and roll sensors to sense if the vehicle is approaching the edge of its stability envelope. If the vehicle becomes unstable, either vertically (on the roll axis), or due to an understeer or oversteer condition (the yaw axis), the system communicates with the engine, telling it to reduce power to avoid a loss of control situation or rollover. The system will even make intelligent braking decisions (deciding which specific brakes to apply) to stabilize the vehicle, if necessary.

ESC performance standard

The performance standard for ESC systems require the vehicle to remain stable while driving through a 150-foot “J” turn too fast. To meet the performance standards, the system must activate when the driver enters the curve going over 30 mph. The system must then stabilize the vehicle and reduce its speed to less than 30 mph within 3 seconds and less than 28 mph within 4 seconds through automatic engine control and braking.

Basis for the standard

The decision to establish the requirement to adjust power and brake when 30 mph is exceeded in a 150-foot J turn is based on the lateral force that such a situation generates, which is 0.4 g. The reason 0.4 g was selected is 0.4 g represents the margin of lateral stability on a loaded tractor-trailer with a high center of gravity load. At 0.4 g of lateral force the vehicle is likely to suffer from lateral instability (“yaw” instability), wheel lift, and rollover.

Temporary disablement of ESC system at extremely low speeds

To allow drivers to operate and get moving in extreme low-speed situations without the system continually defueling and braking the vehicle, the system can be disabled (automatically or manually by the driver) if the vehicle is traveling under 12 mph. Once the vehicle exceeds 12 mph, it must be fully active. The ESC must also have a diagnostic system that verifies it is working correctly. The diagnostics must include a dash indicator to notify the driver if the diagnostics discover a fault in the system. For details on the system, see 571.136, and for details on the dash indicator, see 571.101. All that said, carriers should stress reducing speed and increasing following distance to avoid activating the ESC.

ESC systems are data intensive: Require good carrier maintenance

These systems are data intensive. The sensors will constantly be sampling the environment and reporting to the processor, which will be constantly making stability decisions. If an intervention is called for, the ESC unit must initiate communications with the engine through the vehicle’s electronic system. This is true even if the ESC system uses its own wiring for sensor-to-processor communications (or is one integrated unit).

The carrier’s maintenance team needs to be familiar with the ESC systems, including how they function, how to maintain them, and how to troubleshoot them.

Anti-lock braking system (ABS)

The antilock braking system (ABS), and the associated traction control system, are a combination electronic/air (or hydraulic) system. The electronic sensors and processor make decisions, and then influence the operation of the air or hydraulic system, if necessary. As well as being data-intensive (the wheel sensors are constantly checking and reporting wheel speed to the processor), the system also has an active portion (the modulator valve that releases/applies the brakes and the throttle control) that works off commands from the data system.

Cautionary note: One problem with ABS/traction control systems is that portions of the data system (the wheel sensors and the associated wiring) are in less-than-ideal positions (at the wheel ends). This location leaves them exposed to environmental hazards. While the components that are in these locations are designed to be there, they will still require extra attention during vehicle inspections.