Massey Ferguson 520204.04 – Engine Control Unit (ECU): Low Voltage Issue With Module Supply (ECU) / ECU Main Relay 2 (or 3) short circuit to GROUND

The system logs this code when it detects a short circuit to ground (earth) on the circuit for ECU Main Relay 2 (or 3). When this short is detected, the main relays automatically switch off to protect the system, which can cause engine malfunctions or a complete shutdown.

• 7616
• 5612
• 7622
• Interim Tier 4 (2012-2015)
• 7614
• 5611
• 7615
• 5610
• 5613
• 7620
• 5609
• 7618
• 5608
• 7619
• 7626
• 7624
• MF 4707
• MF 4708
• MF 4709
• MF 4710
• MF 5608
• MF 5609
• MF 5610
• MF 5611
• MF 5612
• MF 5613
• MF 5710 SL
• MF 5711 SL
• MF 5712 SL
• MF 5713 SL
• MF 7614
• MF 7615
• MF 7616
• MF 7618
• MF 7619
• MF 7620
• MF 7622
• MF 7624
• MF 7626
• MF 7714
• MF 7715
• MF 7716
• MF 7718
• MF 7719
• MF 7720
• MF 7722
• MF 7724
• MF 7726

1. 1

   Battery & Supply Voltage Check: Verify the battery is fully charged and the terminals are clean and tight. Low system voltage can sometimes trigger false relay faults.

2. 2

   Relay Testing: Locate the ECU Main Relays in the primary fuse/relay panel. Swap the suspect relay (Main Relay 2 or 3) with a known good relay of the same type (e.g., a lighting relay) to see if the fault clears.

3. 3

   Wiring Harness Inspection: Inspect the wiring harness between the ECU and the relay panel. Look for chafing, melted insulation, or pinched wires that could be shorting directly to the tractor chassis (ground).

4. 4

   ECU Connector Inspection: Disconnect the EEM3/EEM4 controller and inspect the pins for corrosion, moisture ingress, or bent terminals, which can bridge circuits to ground.

1. 1

   Disconnect Power

   Turn off the ignition and disconnect the negative battery cable to prevent electrical spikes during repair.

2. 2

   Replace Faulty Relay

   If the diagnostic relay swap cleared the code, replace the faulty ECU Main Relay with a new OEM relay.

3. 3

   Repair Wiring (If Applicable)

   If the relay is good but the harness is shorted, trace the specific wire for Main Relay 2/3. Cut out the damaged section of the wire, solder in a replacement segment, and seal it with heat-shrink tubing. Secure the harness away from sharp chassis edges or hot engine components.

4. 4

   Reactivate the Relay

   According to AGCO EEM4 documentation, to determine if the short circuit to ground was temporary or permanent, the main relay must be reactivated (usually by cycling the key or clearing the code via a diagnostic tool).

5. 5

   ECU Replacement (Worst Case)

   If the external wiring and relays test perfectly, the short circuit to ground is likely internal to the EEM3/EEM4 controller's FET transistors. In this case, the Engine ECU must be replaced and reprogrammed by an authorized Massey Ferguson/AGCO dealer.