Case IH F78 – Transmission Controller (TCU): No Signal From The Transmission Mid Speed Sensor

The transmission control unit is failing to detect rotational speed data from the mid-speed sensor located inside or on the transmission housing. Without this signal, the transmission controller cannot accurately determine the speed of the internal shafts, which can lead to shifting issues or default to a limp mode.

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190
• TM115
• TM120
• TM125
• TM135
• TM155

1. 1

   Inspect the Wiring Harness: Locate the transmission mid-speed sensor (identified as sensor 'B4' in OEM electrical schematics). Check the wiring harness for chafing, breaks, or shorts to ground.

2. 2

   Check Connectors: Disconnect the sensor and inspect the pins for corrosion, moisture, or pushed-out terminals.

3. 3

   Verify Sensor Placement: Ensure the mid-speed sensor and the output speed sensor (B5) connectors have not been accidentally swapped during previous maintenance.

4. 4

   Test Sensor Resistance: Use a multimeter to check the sensor for an open circuit to confirm if the sensor has failed internally.

1. 1

   Locate the Sensor

   Identify the transmission mid-speed sensor (B4) on the side of the transmission housing.

2. 2

   Clean the Area

   Thoroughly clean the area around the sensor to prevent dirt or debris from falling into the transmission housing when the sensor is removed.

3. 3

   Disconnect the Harness

   Unplug the electrical connector from the sensor.

4. 4

   Remove the Sensor

   Remove the retaining bolt holding the sensor in place. Carefully pull the sensor straight out of the transmission housing. (Have a catch pan ready in case of minor fluid seepage).

5. 5

   Install the New Sensor

   Lubricate the O-ring on the new sensor with clean transmission fluid. Insert it into the port and secure it with the retaining bolt.

6. 6

   Reconnect and Clear Codes

   Plug the electrical harness back in. Turn the key on, clear the stored fault codes via the tractor's H-Menu/diagnostic interface, and test drive the machine to ensure the F78 code does not return and that the transmission shifts smoothly.