Massey Ferguson 3515.04 – Engine Control Unit (ECU): DEF Tank Temp Sensor Voltage Low (ECU)

The system has detected that the voltage signal from the Diesel Exhaust Fluid (DEF) tank temperature sensor is below the normal operating range, indicating a "short circuit to ground" or a completely failed sensor. If the ECU receives a low voltage signal, it cannot accurately determine the DEF temperature, which will eventually trigger an engine derate or limp mode to prevent emissions non-compliance.

• 4710
• 7724S
• 5712SL
• 7722S
• 7722
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 4708
• 7726S
• 4707
• 4709
• 5711SL
• 7718S
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• 5608
• 5609
• 5610
• 5611
• 5612
• 5613
• 7614
• 7615
• 7616
• 7618
• 7619
• 7620
• 7622
• 7624
• 7626

1. 1

   Visual Inspection: Inspect the wiring harness leading to the top of the DEF tank. Look for chafed wires, melted insulation, or pinch points that could cause a short to ground.

2. 2

   Connector Check: Disconnect the electrical pigtail at the DEF header. Check for water intrusion, DEF crystallization (white crust), or corroded pins, which can bridge the connections and drop the voltage.

3. 3

   Voltage Test: With the key ON and the sensor disconnected, use a multimeter to probe the harness connector. Verify that the ECU is supplying the correct 5V reference signal to the sensor circuit. If the 5V reference is missing or grounded out on the harness side, the issue is in the wiring or the ECU, not the sensor.

4. 4

   Resistance Test: Measure the resistance across the temperature sensor pins on the DEF header itself. If the resistance reads as an open circuit (infinite) or a dead short (0 ohms), the internal sensor has failed.

1. 1

   Prep the Area

   Turn off the machine and disconnect the battery. Thoroughly clean the top of the DEF tank and the area around the header. Crucial: The SCR system is highly sensitive to contamination. Do not allow any dirt or debris to fall into the tank.

2. 2

   Relieve Pressure

   Wait at least 5-10 minutes after shutting off the tractor before disconnecting lines. The DEF pump runs a purge cycle at shutdown to empty the lines; interrupting this can leave lines pressurized.

3. 3

   Disconnect

   Unplug the electrical connector from the DEF header. Carefully disconnect the heated DEF supply and return lines (usually quick-disconnect fittings). Cap the lines immediately to prevent contamination.

4. 4

   Remove the Header

   Unscrew the locking ring or remove the mounting bolts securing the header to the tank. Carefully pull the header assembly straight up and out of the tank.

5. 5

   Install the New Part

   Insert the new DEF header assembly, ensuring the new rubber O-ring/gasket is properly seated to prevent leaks and contamination. Tighten the locking ring/bolts to OEM spec.

6. 6

   Reconnect

   Reattach the DEF fluid lines (listen for the "click" of the quick-connects) and plug in the electrical connector.

7. 7

   Clear Codes

   Reconnect the battery, turn the key to the ON position, and use an AGCO diagnostic tool (or compatible J1939 scanner) to clear the 003515.04 fault code. Monitor the live data to ensure the DEF temperature is now reading accurately.