CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 221

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Overview

ISX – Barometric Pressure Sensor Circuit SMALL | MEDIUM | LARGE

ISM – Barometric Pressure Sensor Circuit SMALL | MEDIUM | LARGE

Circuit Description

The ambient air pressure sensor responds to pressure changes in the atmospheric pressure. The pressure changes occur based on elevation that the engine is presently operating at. The ambient air pressure sensor has a 5 volt supply circuit, a sensor signal voltage, and a return circuit.

The electronic control module (ECM) provides a 5 volt supply to the ambient air pressure sensor on the sensor supply 1 circuit. The ECM also provides a ground on the sensor return circuit. The ambient air pressure sensor provides a signal to the ECM on the ambient air pressure sensor signal circuit. This sensor signal voltage changes based on the pressure of the atmosphere. The ECM will detect a low signal voltage when the vehicle is operating at high altitudes. The ECM will detect a high signal voltage when the vehicle is operating at sea level altitudes.

Component Location

The barometric pressure sensor is mounted on the main branch of the engine harness near the ECM. 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 keyswitch is in th ON position.

Conditions for Setting the Fault Codes

The ECM detects that the ambient air pressure signal voltage is greater than 4.91-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 of the ambient pressure 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 barometric pressure sensor shares supply and return wires in the engine harness with other sensors. Opens and shorts in the engine harness can cause multiple fault codes to be active. Before troubleshooting Fault Code 221, check for multiple faults.

Possible causes of this fault code include:

• Open return circuit in the harness, connectors, or sensor.
• Signal circuit shorted to sensor supply or battery voltage.

Refer to Troubleshooting Fault Code t05-221