CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 242

Wheel-Based Vehicle Speed Sensor Circuit Tampering Has Been Detected – Abnormal Rate of Change


Fault Code: 242
PID: P084
SPN: 84
FMI: 10/10
LAMP: Amber

Wheel-Based Vehicle Speed Sensor Circuit Tampering Has Been Detected – Abnormal Rate of Change. Signal indicates an intermittent connection or VSS tampering.

Engine speed limited to maximum engine speed without VSS parameter value. Cruise Control, Gear-down Protection, and Road Speed Governor will not work.

ISX and ISM – Vehicle Speed Sensor Circuit

Circuit Description

The vehicle speed sensor senses the speed of the tailshaft gear on the vehicle’s transmission. This speed signal is then transmitted to the engine electronic control module (ECM) and converted into a vehicle speed.

Component Location

The vehicle speed sensor is located in the rear of the vehicle transmission.

Conditions for Running the Diagnostics

  • The adjustable parameter “VSS Anti-Tampering” must be enabled.
  • This diagnostic runs continuously when the vehicle is in motion if the feature is enabled.

Conditions for Setting the Fault Codes

The ECM detects a tampering event of the vehicle speed signal.

Action Taken When the Fault Code is Active

  • The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
  • The engine speed operating range is limited to the adjustable parameter “Maximum Engine Speed without VSS”.
  • Cruise Control, Gear-down Protection, and Road Speed Governor features will be disabled.

Conditions for Clearing the Fault Code

  • The ECM detects a valid engine speed signal from the vehicle speed sensor.
  • The keyswitch must be turned ON, and the vehicle speed and engine speed must be 0 for 30 seconds.

Shop Talk

Verify the electronic control module (ECM) calibration is correct. Check the calibration revision history found on QuickServe® Online for applicable fixes to the calibration stored in the ECM. If necessary, recalibrate the ECM. Refer to Procedure 019-032 (ECM Calibration Code) in Section 19 in the corresponding Troubleshooting and Repair Manual for the engine being serviced.

Verify that the feature settings for the vehicle speed sensor anti-tampering (Fault Code 242), application type and automatic transmission are set correctly. If any of these are set incorrectly for the specific vehicle and application, Fault Code 242 could occur erroneously.

Driving techniques such as driving for extended periods of time in lower gears can log Fault Code 242. Fault Code 242 can be logged if the driver attempts to defeat the road speed governor by repeatedly cycling the keyswitch.

Once the speed signal has been restored the fault code will go inactive once the keyswitch is on for 30 seconds and the ECM senses zero vehicle and engine speed.

Refer to Troubleshooting Fault Code t05-242

Last Modified:  09-Jul-2010