Understanding Electronic Stability Control

Though the most important safety feature in any vehicle is a careful, skilled driver, technology is playing an increasingly important role in keeping coaches securely on the road. ESC is short for Electronic Stability Control, the latest advancement in vehicle stability. ESC is installed on coaches to reduce the risk of a spinout and "drift-out", conditions related to loss of directional stability and vehicle control. MCI has been supplying its D- and E- and J-Series coaches with standard-equipped ESC since 2009 (VIN-specific cut in), well ahead of DOT and market requirements.

ESC is not a substitute for normal safe-driving practices and will not prevent an accident. What ESC does is assist the driver in maintaining a stable operating condition by monitoring the vehicle's operating status. By calculating operating conditions such as speed, steering angle, direction of travel, throttle and brake application, along with engine torque load and wheel spin, ESC can sense and respond to an unstable condition faster than a driver can react to these conditions.

ESC is incorporated with the existing ABS (Anti-lock Braking System) and consists of:

  • Electronic stability control module or ECU (Electronic Control Unit)
  • Brake pressure sensor: Monitors if driver has brakes applied and how much
  • (ABS) modulator valves: Same as ABS/ATC, to apply or release service brakes
  • Steering angle sensor (SAS): Determines driver input to the vehicle steering
  • Yaw sensor: Determines vehicle "G Force" movement and vehicle direction of travel in relation to steering (wheel) angle (YAW ANGLE: angle of direction of vehicle travel relative to vehicle heading, aka side slip)

The ECU constantly monitors conditions such as road speed (wheel sensors), direction (yaw sensor), steering direction (SAS), and "G force" to determine if the vehicle has good traction with the road or is exceeding programmed safe limits for maneuvers such as lane changes or driving a curve.  As necessary, the system may reduce engine power, prevent a transmission shift, or even apply individual wheel brakes as needed to correct or prevent loss of directional stability.

While the effects of ESC may be noticeable to the operator as it reduces power or applies the brakes, its operation is invisible and requires no interaction from the operator. The ESC does not have control of steering, but only monitors the angle that the driver is turning the wheel.

An indicator lamp on the dash will let you know when ESC is activated. ATC and ESC share the same indicator lamp on the dash. If the ABS and ESC/ATC lamps come on when the ignition is turned on and the lamps turn off after three seconds, then your vehicle is equipped with ABS and ATC only.  If both lamps come on when the ignition is turned on and the ATC lamp stays on for a short period of time after the ABS lamp turns off, then your vehicle is equipped with ABS, ATC and ESC.


Note the two drawings below, with the red and green reference lines displaying steering-wheel angle in relation to time points "A", "B" and "C."

  1. At time point A, both vehicles make a lane change maneuver to avoid the obstacle. The red and green steering-angle lines are identical, both turned to the right. Note the red arrows on the green vehicle, indicating that the ESC has made brake applications.
  2. At time point B, the driver has already corrected the steering wheel back to the left and is beginning a third corrective maneuver. At this point the ECU has changed braking strategy to add one front wheel, as shown by the single red arrow. Note also that the steering-angle lines  now split apart, as the red vehicle continues to need more steering corrections.
  3. At time point C, the final correction has been applied by the ESC system, showing that the vehicle never left the crown of the road. The red vehicle is still making steering corrections and is on the shoulder of the road surface.

When ESC is activated, you should continue to drive normally and provide any corrections that you normally would to keep the vehicle on the correct path. The operator is always in control of the vehicle, so do not take your hands off the steering wheel. You should continue to steer and operate the throttle and brakes as you normally would. The ECU will recognize your actions and will adapt to them. If the vehicle exceeds the directional stability limits, the engine will decelerate and the ECU will apply the brakes individually to counteract directional instabilities.

TROUBLESHOOTING (NOTE: complete system diagnostics and component calibration can only be accomplished with the Meritor/Wabco "toolbox" software)

Since ESC is an extension of the ABS system, basic troubleshooting is the same as for ABS. For blink-code activation, press the ABS code switch one time, release, and monitor the action of the ABS fault light. If a fault is detected, the ECU will blink out that fault in its normal two-digit code format If more faults were detected during that period, the blink code shows the latest detected fault. If there are no faults found, the  indicator lamp will flash one time, then one time again.

Blink codes are displayed in a two-digit format, made of long ½-second "on" flashes for the first digit, and short 1/10th-second "on" flashes for the second digit.

Press and hold the ABS code switch for one second, then release to activate the code sequence:

First fault-code part (FC.a) Second fault-code part (FC.b)
1. No faults 1. No faults
2. ABS modulator
3. Sensor air gap
4. Sensor short / open
5. Sensor erratic / tire size
6. Sensor tone ring
1. Right front
2. Left front
3. Right rear
4. Left rear
5. Right third
6. Left third
7. System Function 1. Data link
2. ASR (ATC) valve
3. Endurance brake (third brake) relay
4. Warning lamp
5. ASR (ATC) configuration
6. ASR (ATC) prop / dif lock / stop valve/
7. Brake signal
8. ECU 1. Power supply low
2. Power supply high
3. Internal fault
4. Configuration error
5. Ground


Steering Angle Sensor SAS)
located at base of the steering column

NOTE: SYSTEM CALIBRATION WITH THE TOOLBOX SOFTWARE IS REQUIRED AFTER SERVICE TO THE SAS OR A FRONT END ALIGNMENT. For more information about obtaining this software and its use in ABS and ESC systems diagnostics and service, you may contact Arvin/Meritor directly.

The information received by the ECU from the SAS is critical to the ESC operation. Any change in its orientation to the coach thrust line requires a system calibration with the WABCO TOOLBOX software.

Electronic Control Unit (ECU)


J4500 Coach:
Rear junction box
#3 baggage bay, curb side
D-Series Coaches
Rear junction box
#3 baggage bay

YAW Sensor


J coach:
#3 baggage bay in center of gravity
D-series coaches:
#2 baggage bay in center of gravity

Brake Pressure Sensor


The brake pressure sensor, located in the left hand service compartment, is responsible for delivering driver application information to the ECU. With this information, the ECU can decide if it needs to apply or release a specific brake for stability.

Again, ESC is no match for safe driving practices and, under normal conditions, will never come into operation. But since life on the road isn't always normal, it provides an extra layer of safety for your driver, your passengers and your coach.

You can also visit your nearest MCI Service Center or call MCI Technical Support at 800-241-2947.

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