CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 249

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Overview

ISX and ISM – Ambient Air Temperature Sensor 1 Circuit SMALL | MEDIUM | LARGE

Circuit Description

The ambient air temperature sensor is used by the electronic control module (ECM) to monitor ambient air temperature. This can be used to sense the temperature inside the cab of a vehicle or the air temperature outside of the vehicle. The ambient air temperature signal is used by the ECM for the Idle Shutdown Ambient Air Temperature Override feature.

The ambient air temperature sensor is used by the ECM to monitor the ambient air temperature. The ambient air temperature sensor is a thermistor and changes resistance based on the temperature being measured. The ECM provides a 5 volt reference voltage to the sensor. The ECM monitors the change in signal voltage and converts this to a temperature value.

Component Location

The mounting location for the ambient air temperature sensor is OEM dependent. Refer to the appropriate 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 ambient air temperature signal voltage is greater than 4.8-VDC for more than 1 second.

Action Taken When the Fault Code is Active

• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
• A default value for the ambient air temperature reading is used.
• The Idle Shutdown Ambient Air Temperature Override feature will use the intake manifold air temperature sensor value to determine idle shutdown and availability of override.

Conditions for Clearing the Fault Code

The ECM will turn off the amber CHECK 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.

Possible causes of this fault code include:

• Open return circuit in the harness, connectors, or sensor.
• Open signal circuit or shorted to a voltage source.

Refer to Troubleshooting Fault Code t05-249