Case IH 3549 – Engine Control Unit (ECU) / EDC (Electronic Diesel Control): Humidity Sensor Signal Ratio failure - Signal Ratio above Limit

The signal ratio coming from the humidity sensor is reading higher than the acceptable programmed limit. This failure is frequently caused by the humidity sensor becoming physically saturated with water droplets.

• Magnum series (235–340, Series 4)
• Steiger
• Puma
• Maxxum

1. 1

   Inspect the engine air intake tract and the sensor element itself for water ingress, condensation, or heavy moisture (sensor saturation).

2. 2

   Inspect the humidity sensor's wiring harness and connector for corrosion, bent pins, or short circuits.

3. 3

   If water droplets are found, dry the sensor and the intake housing, clear the EDC fault codes using a diagnostic tool, and run the engine to see if the code returns.

1. 1

   Power down the machine and disconnect the battery

   This prevents electrical spikes to the EDC.

2. 2

   Locate the humidity sensor

   The sensor is mounted in the engine air intake system / air cleaner assembly.

3. 3

   Disconnect the electrical wiring harness

   Depress the locking tab and disconnect the electrical wiring harness from the sensor.

4. 4

   Remove the sensor

   Remove the mounting hardware securing the sensor to the intake piping and carefully pull the sensor out.

5. 5

   Clean the mounting port

   Wipe the mounting port clean, ensuring no debris or residual moisture falls into the engine air intake.

6. 6

   Install the new humidity sensor

   Install the new humidity sensor and secure the mounting hardware.

7. 7

   Reconnect the electrical harness

   Reconnect the electrical harness until it clicks into place.

8. 8

   Clear the fault code

   Reconnect the battery, turn the key to the ON position, and use a diagnostic tool to clear the 3549 fault code from the Engine ECU.