Massey Ferguson 520203.03 – Engine Control Unit (ECU): High Voltage Issue With Module Supply (ECU)

ECU Main Relay 1 (or Main Relay 2, depending on the specific model's wiring) has a short circuit to a HIGH SOURCE (+12V). The controller is detecting that the voltage on the main power relay circuit is higher than the acceptable threshold, which usually means the relay is internally shorted, stuck closed, or the wiring harness is shorted directly to battery positive.

• 4710
• 7724S
• 5712SL
• Interim Tier 4 (2012-2015)
• 7722
• 7722S
• 7615
• 5613
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 7616
• 7618
• 5608
• 4708
• 7619
• 7626
• 7726S
• 7624
• 4707
• 4709
• 5711SL
• 7614
• 7718S
• 5610
• 7620
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 5612
• 7622
• 7720
• Tier 4 (2015-2021)
• 5611
• 7715S
• 5609
• 7724
• 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 through 7726S

1. 1

   Check the overall system voltage with a multimeter while the machine is running to ensure the alternator voltage regulator hasn't failed and isn't overcharging the entire system.

2. 2

   Swap the ECU Main Relay with a known-good relay of the exact same type (e.g., a work light relay) to see if the fault code changes or clears.

3. 3

   Inspect the wiring harness between the ECU and the main relay box for rubbed insulation, pinched wires, or areas where the wire might be shorting to a +12V power source.

4. 4

   Inspect the relay socket for melted plastic, corrosion, or pushed-out pins, which can cause bridging between the power and control circuits.

1. 1

   Disconnect Power

   Disconnect the negative battery cable to prevent any further electrical damage or accidental shorts during the repair.

2. 2

   Access the Relay Panel

   Open the primary fuse and relay box (location varies by cab/model, often on the right-hand console or under the hood).

3. 3

   Remove the Faulty Relay

   Pull the ECU Main Relay. If the plastic housing is melted or smells burnt, the relay has failed internally.

4. 4

   Test the Circuit

   Using a multimeter, probe the relay socket's control circuit to ensure it is not receiving constant +12V when the key is off (which would indicate a harness short).

5. 5

   Repair Wiring (If Applicable)

   If a short to power is found in the harness, trace the wire back to the ECU, open the loom, and repair the damaged section of the wire using weather-sealed heat shrink connectors.

6. 6

   Install New Relay

   Plug the replacement OEM relay into the socket.

7. 7

   Clear Codes

   Reconnect the battery, turn the key to the ON position, and use a diagnostic tool to clear the inactive 520203.03 fault code.

8. 8

   Verify Repair

   Start the tractor and verify that the module supply voltage is within normal operating range and the code does not return.