Case IH U28 – Transmission Control Unit (TCU) / Transmission Controller: Clutch B (High clutch) calibration value is too high

During the transmission calibration process, the system applies pressure to the clutch pack to pull down the engine RPM. Code U28 triggers when the maximum calibration test pressure is reached without causing the engine speed to drop (typically by 50 RPM), or the current value to the Clutch Pack B solenoid exceeds the 480 mA threshold. This condition generally points to an electrical fault in the solenoid valve or its circuit, a hydraulic issue where clutch pressure is not being controlled properly, or a mechanical failure breaking the torque path.

• JXC
• MXM120
• MXM130
• MXM140
• MXM155
• MXM175
• MXM190
• Maxxum 110
• Maxxum 115
• Maxxum 120
• Maxxum 125
• Maxxum 130
• Maxxum 135
• Maxxum 140
• Puma 160
• Puma 165
• Puma 170
• Puma 185
• Puma 200
• Puma 215
• CX Series
• MXU Series

1. 1

   Disconnect the electrical connection on the Clutch B solenoid valve and measure the electrical resistance of the coil (should be approximately 10 Ω)

2. 2

   Inspect the wiring harness for the solenoid valve to ensure there is no short to ground or open circuit between the transmission control panel/ECM and the valve

3. 3

   Test the reverse solenoid hydraulic pressure circuit. Actuate the valve and verify if the pressure reaches approximately 16 to 19 bar (operating pressure when clutches are fully engaged)

4. 4

   Ensure that the engine speed was not manually adjusted by the operator while the automated calibration was being completed, as this can falsely trigger the U28 code

1. 1

   Locate the Valve

   Locate the transmission control valve assembly on the side of the transmission housing and identify the Clutch B PWM (Pulse-Width Modulation) solenoid valve.

2. 2

   Remove the Solenoid

   Disconnect the wiring harness connector from the faulty solenoid. Remove the retaining bolts securing the solenoid valve to the control valve assembly.

3. 3

   Inspect for Mechanical Faults

   If the solenoid tests fine electrically but hydraulic pressure is absent, suspect a mechanical fault within the transmission (e.g., sticking piston or synchronizer failure). This will require removing the transmission top cover for internal inspection.

4. 4

   Prepare the New Part

   Coat the new solenoid valve components and O-rings with clean transmission oil (OEM recommends CASE IH AKCELA NEXPLORE™ FLUID) prior to assembly.

5. 5

   Install the Solenoid

   Insert the new solenoid valve into the assembly. Tighten the retaining bolts to a torque value of 27 Nm (20 ft. lbs).

6. 6

   Reassemble and Calibrate

   Reconnect the electrical harness. Start the tractor, bring the transmission fluid up to operating temperature, and perform a full transmission clutch calibration procedure to clear the code and allow the TCU to learn the new valve's fill times.