Case IH 191-04 - DEC – Transmission Control Module (TCM) or Engine Control Module (ECM): Transmission Output Shaft Speed (TOSS) or Vehicle Speed Sensor (VSS) Signal Voltage Below Normal, or Shorted to Low Source

The control module is detecting that the voltage reading from the transmission output speed sensor circuit has dropped below the valid operating range. This is most commonly caused by a short to ground in the sensor's wiring harness, a failed sensor, or a disconnected/corroded connector.

• MAGNUM AFS Connect
• Case IH Magnum series (e.g., Magnum 180)
• Case IH Maxxum series (e.g., Maxxum 120)
• Case IH 4200 series (e.g., 4220, 4230, 4240)
• Case IH / New Holland construction equipment (e.g., B115B)

1. 1

   Inspect the transmission output speed sensor and its wiring harness for physical damage, rubbing, or contamination

2. 2

   Disconnect the sensor and inspect the connector terminals for pushed-back pins, moisture, or corrosion

3. 3

   Using a multimeter, check the wiring harness from the sensor to the ECM/TCM for continuity and verify the signal wire is not shorted to ground

4. 4

   Verify that the sensor is receiving its proper reference voltage (5V or 8V supply) and has a solid ground connection

5. 5

   Connect an Electronic Service Tool (EST) and monitor live data for 'Transmission Output Speed' or 'Wheel-Based Vehicle Speed' to check for 0 RPM or erratic readings

1. 1

   Power Down

   Turn off the machine's ignition and disconnect the battery to prevent electrical shorts during repair.

2. 2

   Locate the Sensor

   Locate the Transmission Output Speed Sensor on the transmission housing/output shaft area.

3. 3

   Disconnect

   Unplug the electrical connector from the sensor.

4. 4

   Harness Repair (If Applicable)

   If the diagnostic checks revealed a short to ground in the wiring, repair or replace the damaged section of the harness.

5. 5

   Remove the Sensor

   If the wiring is intact but the sensor is faulty, remove the sensor mounting bolt and carefully pull the sensor out of the transmission housing.

6. 6

   Install the New Sensor

   Insert the replacement OEM transmission output speed sensor and tighten the mounting bolt to the manufacturer's specified torque.

7. 7

   Reconnect

   Plug the electrical connector back into the sensor, ensuring it clicks securely into place.

8. 8

   Clear Codes

   Reconnect the battery, turn the ignition on, and use a diagnostic scan tool to clear the active/inactive 191-04 (SPN 191 FMI 4) fault code.

9. 9

   Test Drive

   Operate the machine to verify that the transmission shifts correctly, the speedometer functions, and the fault code does not return.