Case IH 2626 – ZF Transmission Controller (ZTC): Transmission Output Shaft Speed Sensor (N_Ab1) Circuit Open or Shorted to Power

The diagnostic system has detected that the Transmission Output Shaft Speed Sensor (N_Ab1) electrical circuit is either open (a break in the wiring or a disconnected plug) or shorted to power (high voltage/short circuit to a high source). The N_Ab1 sensor reads the rotational speed of the transmission output shaft, which the controller uses to calculate vehicle speed, manage gear shifts, and control clutch engagement.

• Steiger T4B
• 521D
• 521E
• 621B
• 621C
• 721B
• 721C
• 821B
• 821C
• 921
• 921B
• Maxxum
• Puma CVX
• Magnum
• Farmall Pro EP

1. 1

   Visually inspect the wiring harness from the ZTC to the N_Ab1 sensor on the transmission housing for chafed wires, melted insulation, or disconnected plugs.

2. 2

   Disconnect the sensor and measure the resistance across the sensor pins. A reading of infinite resistance (0 ohms/out of limits) confirms an internal open circuit in the sensor.

3. 3

   Test the wiring harness between the sensor connector and the ZTC pins for continuity. Check for a short to a high voltage source (+) by measuring voltage on the signal wire with the ignition ON.

4. 4

   Verify that the ZTC is sending the correct reference voltage (typically 5V or 8V, depending on the specific Case IH model) to the sensor.

5. 5

   Abort the current diagnosis, cycle the ignition, and read the error memory again to see if the fault is an active hard fault or an intermittent stored code.

1. 1

   Power Down

   Turn off the ignition and disconnect the machine's battery to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the N_Ab1 Transmission Output Speed Sensor. It is mounted on the exterior of the transmission housing, typically near the rear output shaft.

3. 3

   Disconnect

   Unplug the electrical connector from the sensor. If the connector is corroded, clean it with electrical contact cleaner.

4. 4

   Remove the Sensor

   Remove the retaining bolt(s) holding the sensor in place. Carefully pull the sensor straight out of the transmission housing. (Have a catch pan ready, as a small amount of transmission fluid may weep from the port).

5. 5

   Install the New Sensor

   Lubricate the O-ring on the new sensor with clean transmission fluid. Insert the sensor into the housing and torque the retaining bolt to OEM specifications.

6. 6

   Reconnect

   Plug the wiring harness back into the sensor, ensuring the locking tab clicks into place.

7. 7

   Test and Clear

   Reconnect the battery, turn the ignition ON, and use a diagnostic tool to clear the 2626 fault code from the ZTC. Test drive the machine to ensure the transmission shifts correctly and the code does not return.