Case IH 12620 – FR Control or ABS/Transmission Control Module: Rear left wheel speed sensor - speed jump to zero.

The control module detected a sudden, illogical drop in the speed signal from the rear left wheel speed sensor while the machine was in motion. This indicates the sensor's signal dropped out intermittently or completely, typically caused by a faulty sensor, damaged wiring, excessive air gap, or a damaged/dirty tone ring.

• Magnum 180
• Magnum 200
• Magnum 225
• Magnum 250
• Magnum 340
• Magnum 380
• Puma 165
• Maxxum 115
• Maxxum 125
• MX

1. 1

   Visually inspect the rear left wheel speed sensor, its connector, and the wiring harness for cuts, pinching, corrosion, or exposed wires.

2. 2

   Ensure the sensor is securely mounted and pushed in properly. Inspect the tone ring (magnetic or toothed reluctor ring) for missing teeth, damage, or heavy debris buildup.

3. 3

   Perform a resistance test on the disconnected sensor using a multimeter. A healthy passive sensor typically reads between 1,200 and 2,700 ohms; if outside this range, the sensor is faulty.

4. 4

   Perform an AC voltage output test by safely raising the rear left wheel and manually spinning it at least ½ revolution per second. The sensor should output a minimum of 0.25 Volts AC. If lower, try repositioning closer to the exciter ring.

5. 5

   Perform a wiring overlay test by temporarily swapping the left and right sensor wiring at the ECU. If the fault code switches to the opposite wheel, the wiring harness has a short or open circuit.

1. 1

   Secure the machine

   Park the tractor on a level surface, turn off the ignition, apply the parking brake, and chock the wheels.

2. 2

   Locate the sensor

   Locate the rear left wheel speed sensor on the axle/hub assembly.

3. 3

   Disconnect and clean

   Disconnect the electrical connector from the sensor and clean the surrounding area to prevent dirt from falling into the mounting hole.

4. 4

   Remove retaining bolt

   Remove the retaining bolt securing the sensor.

5. 5

   Remove the sensor

   Carefully pull the sensor out of the hub. If it is stuck, gently twist it while pulling—do not pry aggressively to avoid breaking the sensor head off inside the housing.

6. 6

   Inspect and clean mounting area

   Inspect and clean the mounting surface and the tone ring visible through the hole.

7. 7

   Install new sensor

   Insert the new wheel speed sensor until it is fully seated.

8. 8

   Secure the sensor

   Install and tighten the retaining bolt to the manufacturer's torque specification.

9. 9

   Reconnect harness

   Reconnect the electrical harness, ensuring the connector clicks and locks into place.

10. 10

    Clear code and test drive

    Use a diagnostic scan tool to clear the 12620 fault code, then test drive the machine to verify the speed signal remains stable and the code does not return.