John Deere 1638.04 – CCU: Hydraulic Oil Temperature Sensor Circuit Voltage Low

The controller detects that the hydraulic/transmission oil temperature sensor circuit output voltage has dropped below the acceptable threshold, indicating a short to ground. It can also occur if the engine speed is above 400 RPM, the hydraulic oil temperature reads above 115°C (239°F), and the engine coolant temperature reads below 30°C (86°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
• 7810
• 8230T
• 8330T
• 8430T
• 9230
• 9330
• 9430
• 9530
• 9630
• 444J
• 544J
• 624J
• 750J
• 850J
• 330LC

1. 1

   Check the hydraulic/transmission oil level in the sight glass on the rear differential to ensure low oil is not causing erratic readings.

2. 2

   Inspect the transmission/hydraulic oil cooler and front grill for accumulated debris, dirt, or blockages that could cause actual overheating.

3. 3

   Perform a system reset test by restarting the vehicle and operating it at rated speed for 1 to 2 minutes to see if the code clears.

4. 4

   Visually inspect the wiring harness leading to the hydraulic oil temperature sensor for chafing, melting, or pinched wires that would cause a short to ground.

1. 1

   Preparation

   Park the machine on a level surface, engage the parking brake, and turn off the engine. Relieve any hydraulic pressure if required by your specific model's manual.

2. 2

   Locate the Sensor

   Locate the Transmission/Hydraulic Oil Temperature Sensor (location varies by model, typically on the transmission housing or hydraulic valve block).

3. 3

   Disconnect and Inspect

   Unplug the sensor connector. Inspect the pins for corrosion, moisture, or damage.

4. 4

   Test the Harness

   Using a multimeter, test the sensor signal wire for continuity to ground. If the wire is shorted to the chassis, trace the harness and repair the damaged section of wiring.

5. 5

   Test/Replace the Sensor

   If the wiring harness tests good (no short to ground), the internal thermistor of the sensor has likely failed and shorted. Remove the old sensor (be prepared to catch a small amount of hydraulic fluid) and install the replacement sensor.

6. 6

   Reassembly and Clearing Codes

   Reconnect the wiring harness. Top off the hydraulic fluid if any was lost. Start the tractor, navigate to the CommandCenter or diagnostic display, and clear the CCU codes. Run the machine at rated speed to verify the code does not return.