Massey Ferguson U 1485 – Engine Control Unit (ECU): Main relay error

The engine controller detects an improper state or voltage on the main power relay circuit. FMI 3 indicates no shut off (short to BAT+), and FMI 6 indicates shut off too soon (short to ground).

• 8600 Series
• 8650
• 8660
• 8670
• 8680
• 8690

1. 1

   Locate the ECU main relay in the primary fuse/relay panel and inspect the spade terminals for corrosion, melting, or signs of arcing.

2. 2

   If code is U 1485 (3) 'No shut off': Test for welded contacts by checking continuity across the load pins while unpowered (there should be no continuity).

3. 3

   Inspect the wiring harness between the main relay socket and the Engine ECU for a short to battery voltage (BAT+) or a short to ground (EARTH).

4. 4

   Swap the main relay with a known good relay of the identical type (e.g., a work light relay) to see if the fault status changes from Active to Inactive.

1. 1

   Disconnect battery

   Turn off the ignition and disconnect the tractor's negative battery cable to prevent electrical spikes.

2. 2

   Access relay center

   Access the main electrical fuse and relay center (location varies slightly by cab configuration, usually under the right-hand console or side panel).

3. 3

   Extract relay

   Extract the faulty ECU main relay.

4. 4

   Repair wiring

   If the diagnostic checks revealed chafed or shorted wiring in the relay block pigtail, repair the wiring using heat-shrink butt connectors.

5. 5

   Apply dielectric grease

   Apply a small amount of dielectric grease to the terminals of the new relay to prevent moisture intrusion.

6. 6

   Install new relay

   Plug the new main relay firmly into the socket.

7. 7

   Reconnect battery

   Reconnect the negative battery cable.

8. 8

   Clear fault code

   Connect a diagnostic service tool (like AGCO EDT) to the machine, save the error log, and clear the U 1485 fault code from the Engine ECU memory.

9. 9

   Verify repair

   Cycle the key and run the engine. Turn the key off and verify the ECU shuts down properly without throwing the code again.