New Holland NH 3115 – Engine Control Unit (ECU): Main relay defect - NOT PLAUSIBLE

The engine controller is detecting an illogical or unexpected state from the main power relay. This typically means the relay is opening too early or too late during the shutdown sequence, sticking closed, or failing to supply consistent voltage when the ignition is turned on.

• T-Series tractors
• Boomer compact tractors
• C-Series skid steer loaders
• crawler excavators

1. 1

   Ensure the battery is fully charged and that the main chassis and ECU grounds are clean and tight, as voltage drops can cause erratic relay behavior.

2. 2

   Locate the ECU Main Relay in the primary fuse/relay panel and inspect the socket cavities for corrosion, pushed-out pins, or signs of heat damage/melting.

3. 3

   Bench test the main relay to see if the internal contacts are physically sticking closed when power is removed.

4. 4

   Check the continuity of the wiring harness between the key switch, the main relay control circuit, and the ECU pins to ensure there is no short to ground or open circuit.

1. 1

   Power Down

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

2. 2

   Access the Relay Panel

   Open the main electrical distribution box or relay panel (location varies by model, often in the cab console or engine bay).

3. 3

   Remove the Defective Relay

   Pull the ECU Main Relay from its socket.

4. 4

   Clean Contacts

   Spray the socket terminals with a high-quality electrical contact cleaner to remove any oxidation or debris.

5. 5

   Install Replacement

   Plug the new OEM main relay firmly into the socket, ensuring it seats completely.

6. 6

   Reassemble and Reconnect

   Close the relay panel and reconnect the negative battery cable.

7. 7

   Clear Codes

   Turn the ignition on. Use the New Holland Electronic Service Tool (EST) or a compatible diagnostic scanner to clear the inactive NH 3115 fault code from the Engine ECU.

8. 8

   Verify Repair

   Start the engine, let it idle, and perform a normal key-off shutdown sequence. Verify that the engine shuts down correctly and the code does not return.