CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 778 (ISM)

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Overview

ISM – Engine Camshaft Speed/Position SMALL | MEDIUM | LARGE

Circuit Description

The electronic control module (ECM) provides a 5 VDC supply to the engine camshaft speed/position sensor on the sensor supply circuit. The ECM also provides a ground on the sensor return circuit. The camshaft speed/position sensor provides a signal to the ECM on the camshaft speed/position sensor signal circuit. This sensor generates a signal to the ECM as the camshaft speed indicator lobe moves past the sensor. The ECM interprets this signal into an engine speed reading and determines engine position. This sensor is used as a backup sensor if the primary engine camshaft speed/position signal is lost.

Component Location

Both engine position sensors are located on the back side of the front gear housing above the fuel pump drive. The Primary engine position sensor (Number 1) is located above the backup engine position sensor (Number 2). Refer to Procedure 100-002 (Engine Diagrams) in Section E for a detailed component location view.

Conditions for Running the Diagnostics

This diagnostic runs continuously when the engine is running.

Conditions for Setting the Fault Codes

The ECM detects a loss of the camshaft speed/position sensor for more than one second.

Action Taken When the Fault Code is Active

• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code

The ECM will turn off the amber CHECK ENGINE light immediately after the ECM detects a valid signal from the camshaft speed/position sensor.

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.

This fault is logged when the ECM does not receive a valid engine speed signal. Both primary and backup engine position sensors use identical connectors on the engine wiring harness. Be sure you are examining the proper connector on the engine harness.

Possible causes of this fault code include:

• Open or short circuit in the engine harness or connectors.
• Open or short circuit in the engine position sensor Number 2.
• Signal circuit short to voltage source in the engine harness.
• Damaged or failed engine position sensor.

Refer to Troubleshooting Fault Code t05-778