Case IH 2634 – Transmission Control Unit (TCU): ZTC - Transmission output shaft speed sensor not plausible, number of teeth N_Ab2 is less than N_Ab1

The transmission controller is detecting that the number of gear teeth counted by sensor N_Ab2 is less than the number of teeth counted by sensor N_Ab1 over a given rotation. This indicates a sensor failure, a wiring issue, or physical damage to the sensor's target wheel.

• Steiger T4B
• Case IH CVX
• Puma CVX
• Magnum CVX

1. 1

   Power Cycle: Cycle the power to the machine to determine if the fault remains active or becomes inactive.

2. 2

   Electrical Circuit Testing: Check the N_Ab1 and N_Ab2 speed sensor circuits for short circuit to ground (minus), short circuit to power (plus), and open circuit / cable break.

3. 3

   Harness Inspection: Check for intermittent connections in the speed sensor wiring harness and the pins at the transmission controller.

4. 4

   Target Wheel Inspection: Check the speed sensor target wheel inside the transmission to ensure it is not damaged, warped, or missing teeth, which would cause the N_Ab2 sensor to undercount.

1. 1

   Park the machine

   Park the machine on a level surface, turn off the engine, and remove the key.

2. 2

   Disconnect the battery

   Disconnect the battery terminals (negative first, then positive) before working on the electrical system.

3. 3

   Locate the sensors

   Locate the N_Ab1 and N_Ab2 output shaft speed sensors on the transmission housing.

4. 4

   Inspect the wiring harness and connectors

   Disconnect the wiring harness from both sensors and inspect the connector pins for corrosion, moisture, or damage.

5. 5

   Test the wiring harness

   Use a multimeter to test the wiring harness between the sensor connectors and the ZTC module for continuity and shorts.

6. 6

   Remove the N_Ab2 speed sensor

   If the wiring is intact, remove the N_Ab2 speed sensor from the transmission housing.

7. 7

   Inspect the sensor tip

   Inspect the tip of the sensor for metallic debris, which can distort the magnetic field and cause miscounts.

8. 8

   Inspect the target wheel

   Look through the sensor mounting hole to inspect the target wheel for physical damage.

9. 9

   Replace the speed sensor

   If no physical damage is found on the target wheel and the wiring is good, replace the N_Ab2 speed sensor (and potentially N_Ab1 if cross-testing shows it is failing).

10. 10

    Reconnect and test

    Reconnect the battery, clear the fault codes via the Pro 1200 display or diagnostic tool, and test drive to verify the repair.