CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 1242

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Overview

ISX and ISM Accelerator Pedal Sensor SMALL | MEDIUM | LARGE

Circuit Description

The accelerator position sensor is a hall-effect sensor attached to the accelerator pedal. The accelerator position sensor varies the signal voltage to the electronic control module (ECM) as the accelerator pedal is depressed and released. Low signal voltage is received by the ECM when the accelerator pedal is at 0 percent. A high signal voltage is received by the ECM when the accelerator pedal is at 100 percent. The accelerator pedal position circuit contains an accelerator pedal position 5 volt supply, accelerator pedal position return, and accelerator pedal position signal.

The accelerator pedal contains two position sensors. These position sensors are used to measure the throttle position. Both position sensors receive a 5 volt supply from the ECM. A corresponding signal voltage based on the position of the accelerator pedal is then received from the ECM. The signal voltage for accelerator position 1 is twice as much as the signal voltage from accelerator position 2.

Note: The sensor connection pin numbers shown in the diagram above are OEM specific and those provided are for example only. Refer to the OEM service diagram for the correct pinouts.

Component Location

The accelerator pedal position sensor is located on the accelerator pedal. Refer to the OEM troubleshooting and repair manual.

Conditions for Running the Diagnostics

This diagnostic runs continuously when the keyswitch is in the ON position.

Conditions for Setting the Fault Codes

The ECM detects that the readings between accelerator position sensor 1 and accelerator position sensor 2 differ by more than 2 percent.

Action Taken When the Fault Code is Active

• The ECM illuminates the red STOP ENGINE light immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code

The ECM will turn off the red STOP ENGINE light immediately after the diagnostic runs and passes.

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.

Newer engines use two throttle position sensors to determine the throttle position. Older throttle pedals used a single throttle position sensor and an idle validation switch. If Fault Codes 132 and 1241 are active when the accelerator pedal is in the idle position and Fault Code 132 goes inactive and Fault Code 1239 goes active when the throttle is depressed, the incorrect throttle pedal has been installed in the vehicle. A throttle pedal with two acceleration position sensors should be installed.

This fault code becomes active when the accelerator position sensor number 1 and accelerator position sensor number 2 are not reading the same value.

Possible causes of the fault include:

• Damaged accelerator pedal position sensor number 1 or damaged accelerator pedal position sensor number 2
• Damaged accelerator pedal position sensor number 1 and damaged accelerator pedal position sensor number 2 signal wires are shorted together.

Refer to Troubleshooting Fault Code t05-1242