Case IH 3550 – Engine Control Unit (ECU) / EDC (Electronic Diesel Control): Humidity Sensor Signal Ratio failure - Signal Ratio below Limit

This code indicates that the signal ratio coming from the intake air humidity sensor has fallen below the minimum acceptable threshold programmed into the control module. When the signal drops below the limit, it typically points to a failing sensor, a wiring issue (high resistance or short), or a sensor that has become saturated with water.

• Case IH Magnum Series 235, 250, 280, 310, 340 (Series 4)
• Case IH Puma Series 240 and similar CVT models
• Case IH Patriot Sprayers 3250, 3550

1. 1

   Check for Accompanying Codes: Look for related informational codes such as 3557 or 3558, which indicate that the humidity sensor is 'possibly saturated with water droplets'. If these are present, the sensor might just be wet rather than permanently failed.

2. 2

   Inspect the Intake Tract: Inspect the air filter housing and intake piping for water ingress, excessive moisture, or snow packing.

3. 3

   Wiring and Connector Inspection: Disconnect the sensor and inspect the 5VDC connector pins for corrosion, pushed-back pins, or moisture. Check the wiring harness for chafing or shorts to ground that could pull the signal voltage below the limit.

4. 4

   Live Data Verification: Use the CNH EASy diagnostic tool to monitor the live signal voltage/ratio of the humidity sensor. If the reading is a static dead value despite varying conditions, the sensor is internally shorted or failed.

1. 1

   Power Down

   Turn off the engine, remove the key, and disconnect the battery to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the humidity sensor on the engine intake air piping, positioned between the main engine air filter housing and the intake manifold.

3. 3

   Disconnect

   Carefully depress the locking tab and unplug the electrical harness connector from the sensor.

4. 4

   Remove the Sensor

   Remove the retaining screws or mounting clip holding the sensor in place. Pull the sensor straight out of the intake pipe.

5. 5

   Clean the Port

   Wipe the mounting port on the intake pipe with a clean, lint-free rag to ensure no dirt or moisture enters the intake manifold.

6. 6

   Install the New Sensor

   Insert the new humidity sensor into the port, ensuring the O-ring/seal is properly seated to prevent unmetered air leaks.

7. 7

   Secure and Reconnect

   Fasten the mounting screws/clip and plug the electrical connector back in until it clicks.

8. 8

   Clear Codes

   Reconnect the battery, turn the key to the ON position, and use your diagnostic tool to clear fault code 3550. Start the engine and verify the code does not return.