CM2150 Electronic Control System – FAULT CODE 256

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Overview

ISB – Barometric Pressure Sensor Circuit SMALL | MEDIUM | LARGE

ISC and ISL – Barometric Pressure Sensor Circuit SMALL | MEDIUM | LARGE

Circuit Description

The ambient air temperature sensor is a variable resistor sensor and is used to measure the temperature of the air entering the turbocharger compressor inlet of the engine. The electronic control module (ECM) supplies 5 volts to the ambient air temperature signal circuit. The ECM monitors the change in voltage caused by changes in the resistance of the sensor to determine the ambient air temperature.

Component Location

The ambient air temperature sensor is located in the air intake. Refer to Procedure 100-002 (Engine Diagrams) in Section F for a detailed component location view.

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 less than 0.12-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.

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.

The ambient air temperature sensor shares return wires in the engine harness with other sensors. Opens and shorts in the engine harness can cause multiple fault codes to be active. Check fault codes with multiple counts first.

Possible causes of this fault code include:

• Signal circuit shorted to ground in the harness
• Signal circuit shorted to return or ground in the sensor.

Refer to Troubleshooting Fault Code t05-256