Case IH 2621 – Transmission Control Unit (TCU): Transmission Output Shaft Speed Sensor (N_Ab1) Short Circuit to Ground

The signal wire running from the speed sensor to the control module has found a path to ground (showing 0 ohms or abnormally low resistance, typically < 10 ohms). Because the voltage is pulled to ground, the controller cannot read the magnetic induction/Hall effect pulses generated by the rotation of the transmission output shaft, resulting in a loss of ground-speed telemetry and erratic or inhibited gear shifts.

• Steiger T4B
• Puma 150
• Puma 165
• Puma 185
• Puma 200
• Puma 220
• Puma 240
• Optum 250
• Optum 270
• Optum 300
• Maxxum 110
• Maxxum 115
• Maxxum 120
• Maxxum 125
• Maxxum 130
• Maxxum 140
• Magnum 250
• Magnum 280
• Magnum 310
• Magnum 340
• Magnum 380

1. 1

   Connect the diagnostic service tool, navigate to the monitor screen, and select 'inst trans output speed' to see if the sensor output is reading 0 RPM while moving.

2. 2

   Visually inspect the wiring harness from the transmission controller to the output speed sensor for abnormal routing, wire damage, or chafing against the chassis.

3. 3

   Disconnect the sensor and the TCU connectors to inspect the terminals for pushed-back pins, corrosion, or moisture.

4. 4

   With the sensor disconnected from the harness, measure the resistance of the signal wire to ground. If it shows continuity (< 10 Ohms), the harness is shorted. If OK, test the sensor pins directly for an internal short to the casing.

1. 1

   Power Down

   Turn off the ignition and disconnect the battery to prevent electrical spikes during repair.

2. 2

   Locate the Sensor

   Locate the N_Ab1 transmission output speed sensor on the transmission casing (typically a right-angle 90-degree connector head profile).

3. 3

   Isolate the Fault

   Disconnect the sensor. If your diagnostic tests proved the wiring harness is shorted to ground, trace the harness back to the TCU and repair the damaged wire insulation.

4. 4

   Remove the Sensor

   If the sensor is internally shorted, remove the retaining bolt securing the sensor to the transmission housing and pull the sensor out.

5. 5

   Install the New Sensor

   Lubricate the pre-installed heavy-duty synthetic rubber O-ring on the new sensor with clean transmission fluid. Insert it into the transmission casing port to guarantee a leak-free seal.

6. 6

   Reconnect and Clear

   Secure the retaining bolt, plug in the weather-pack connector, and reconnect the battery.

7. 7

   Calibration/Testing

   Use the service tool to clear the fault code from the TCU memory. Perform a test drive to verify that the transmission output speed is registering correctly and that gear shifts/clutch modulation have returned to normal.