Case IH U91 – Transmission Control Unit (TCU): Synchronizer potentiometer signal is out of range for F1 position.

The sensor (potentiometer) responsible for monitoring the physical position of the transmission synchronizer for the F1 (Forward 1) gear is returning a voltage signal that falls outside the programmed acceptable limits. This typically occurs during gear engagement or during a transmission calibration routine.

• Puma 115
• Puma 125
• Puma 140
• Puma 155
• Puma 165
• Puma 180
• Puma 195
• Puma 210
• Puma 225
• Maxxum 110
• Maxxum 115
• Maxxum 120
• Maxxum 125
• Maxxum 130
• Maxxum 140
• MXU 100
• MXU 110
• MXU 115
• MXU 125
• JXC
• JXU

1. 1

   Monitor live data in the diagnostic/calibration menu for the F1 synchronizer potentiometer, watching for erratic voltage spikes or dead spots while the synchronizer moves

2. 2

   Inspect the wiring harness leading to the F1 synchronizer potentiometer and associated solenoids for chafed wires, melted insulation, or loose connections

3. 3

   Disconnect the potentiometer and check the connector for water ingress, corrosion, or pushed-back pins

4. 4

   Listen for abnormal transmission noises or harsh shifting, which could indicate the synchronizer ring or shift fork is mechanically binding

1. 1

   Preparation

   Park the tractor on a level surface, engage the parking brake, turn off the engine, and ensure all hydraulic pressure is relieved from the circuits.

2. 2

   Locate the Sensor

   Locate the F1 synchronizer potentiometer on the exterior of the transmission housing.

3. 3

   Electrical Testing

   Disconnect the 3-pin connector. Use a multimeter to verify that the sensor is receiving the correct reference voltage (usually 5V) and ground from the TCU. Test the continuity of the signal wire back to the TCU.

4. 4

   Replace the Potentiometer

   If the wiring tests good, the potentiometer is likely faulty. Remove the retaining bolts and carefully extract the old rotary position sensor.

5. 5

   Install the New Sensor

   Install the new potentiometer, ensuring the rotary tang aligns correctly with the internal shift fork mechanism. Tighten to OEM torque specs and reconnect the wiring harness.

6. 6

   Transmission Calibration

   After replacing a transmission potentiometer, you must perform a Full Power Shift (FPS) or CVT transmission calibration using the tractor's onboard display or the CNH Electronic Service Tool (EST). This allows the TCU to learn the new minimum and maximum voltage values for the F1 position.

7. 7

   Mechanical Escalation

   If the calibration fails and code U91 returns, the transmission will need to be split to inspect the internal F1 synchronizer, shift forks, and clutch packs for mechanical failure.