Massey Ferguson 521001.03 – Engine ECU (AGCO SISU POWER EEM4): High Voltage Issue With DEF Heater Relay Latching Supply Voltage

The engine control system has detected that the control circuit for the Diesel Exhaust Fluid (DEF) heating main relay is short-circuited to a high source (battery voltage / +12V).

• 7616
• 5612
• 7622
• Interim Tier 4 (2012-2015)
• 7614
• 5611
• 7615
• 5610
• 5613
• 7620
• 5609
• 7618
• 5608
• 7619
• 7626
• 7624
• MF 7600 Series
• MF 5600 Series
• MF 8600 Series

1. 1

   Stop the engine and let it cool completely before starting work.

2. 2

   Locate the DEF Heating Main Relay in the machine's fuse/relay panel and check for signs of melting, burning, or an internal short.

3. 3

   Inspect the wiring harness between the Engine ECU and the relay panel for chafing, pinched wires, or damaged insulation that could cause a short to +12V.

4. 4

   Check for damaged, bent, or loose connector pins at the ECU and the relay block, and ensure all machine grounds are tight and free of corrosion.

5. 5

   Look for debris or water inside the connectors, which can artificially bridge the power and control circuits.

1. 1

   Disconnect Power

   Turn off the ignition and disconnect the battery negative cable to prevent accidental shorts during testing.

2. 2

   Test the Relay

   Remove the DEF heating main relay. Using a digital multimeter, test for continuity between the relay's control circuit pins and the high-current power pins. If there is continuity where there shouldn't be, the relay is internally shorted and must be replaced.

3. 3

   Test the Circuit

   If the relay tests good, probe the relay socket's control terminal (the wire going back to the Engine ECU). Reconnect the battery (ignition OFF) and check for +12V on this wire. If voltage is present when the ECU is not commanding the heater on, there is a short-to-power in the wiring harness.

4. 4

   Repair Wiring

   Trace the harness back to the Engine ECU. Locate the area where the wire is chafed and touching a power source. Repair the damaged wire using OEM-approved heat-shrink connectors and re-loom the harness.

5. 5

   Clean and Reassemble

   Clean all connector pins with electrical contact cleaner. Reinstall the relay (or install a new one) and ensure it is seated firmly.

6. 6

   Clear Codes

   Reconnect the battery, turn the key to the ON position, and use an AGCO Electronic Diagnostic Tool (EDT) or equivalent scanner to clear the active 521001.03 fault code.

7. 7

   Verify Operation

   Start the tractor and monitor the aftertreatment system data to ensure the DEF heater relay operates normally without throwing a high-voltage code.