John Deere 1638.03 – CCU: Transmission Hydraulic Oil Temperature Sensor Circuit Voltage High

The control module has detected an open circuit in the hydraulic oil temperature sensor circuit, which results in a high voltage reading. This alarm can also trigger if the engine speed is above 400 rpm, the hydraulic oil temperature is below –20°C (–4°F), and the engine coolant temperature is above 50°C (122°F) for longer than 7 seconds.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5045E
• 5055E
• 5065E
• 5075E
• 5065M
• 5075M
• 5085M
• 5100M
• 5100MH
• 5100ML
• 5115ML
• 5095M
• 5095MH
• 5105M
• 5015ML
• 5090M
• 5115M
• 7720
• 7820
• 7920
• 750J
• 850J
• 9630

1. 1

   Inspect the transmission/hydraulic oil cooler and the front grill for debris that could be causing actual temperature spikes or interfering with sensor wiring

2. 2

   Verify the hydraulic oil level in the sight glass on the rear differential

3. 3

   Perform a system reset by restarting the engine and operating the vehicle at rated speed for 1 to 2 minutes to see if communication messages reset

4. 4

   Check the hydraulic oil temperature sensor circuit for any signs of physical damage, corrosion, or loose connections that could cause an open circuit

1. 1

   Locate the Sensor

   Identify the Transmission/Hydraulic Oil Temperature Sensor on your specific machine (usually located on the transmission housing or hydraulic reservoir).

2. 2

   Inspect the Wiring Harness

   Disconnect the sensor and inspect the connector pins for corrosion, moisture, or poor terminal tension.

3. 3

   Test for an Open Circuit

   Use a multimeter to check the wiring harness between the sensor connector and the CCU for continuity. Repair any broken wires or shorts to power.

4. 4

   Replace the Sensor

   If the wiring harness is intact and the circuit is open internally at the sensor, the sensor has failed and must be replaced.

5. 5

   Clear Codes

   After replacing the sensor or repairing the wire, restart the tractor to clear the CCU communication messages and verify that full hydraulic function is restored.