New Holland DCU 19740 – DCU or ECM/PCM: Main Relay opens too early / too late | main relay short circuit | P0685 ECM/PCM Power Relay Control Circuit /Open

The main power relay that supplies voltage to the control module is malfunctioning. The system has detected that the relay is either opening prematurely, opening too late, or suffering from a short/open circuit in its control circuit. This can prevent the control module from powering up correctly or shutting down properly (which can interrupt the module's EEPROM save procedures).

• New Holland T7 series
• New Holland T8 series
• New Holland T8.380
• Tier 3 and Tier 4 New Holland heavy equipment and excavators

1. 1

   Inspect the main relay connectors for poor contact, loose pins, or terminal fretting caused by engine vibration

2. 2

   Perform a forced operation test on the main relay to verify if it clicks and successfully bridges power

3. 3

   Inspect the wiring harness between the ECM/DCU and the relay panel for an open circuit (broken wire) or a short circuit to ground/power

4. 4

   Verify that the ECU/DCU is sending the ground/activation signal to the relay when the key is turned ON

1. 1

   Locate the Main Relay

   Access the main relay panel (location varies by model, usually in the cab fuse box or engine bay electrical center).

2. 2

   Inspect and Swap

   Remove the main ECM/DCU power relay. Inspect the socket for melted plastic or corrosion. Swap the relay with a known-good relay of the exact same type (e.g., the horn or lighting relay) to see if the fault code clears.

3. 3

   Repair Wiring

   If the relay is good but the code persists, use a multimeter to trace the control circuit wires from the relay block to the DCU/ECM. Repair any broken insulation or broken wires.

4. 4

   Replace the Relay

   If the relay is stuck or failing to hold the circuit closed, install a new OEM main relay.

5. 5

   ECU Replacement

   If the wiring is 100% intact, the relay is brand new, and the module still cannot control the relay, the service manual dictates that the ECU/DCU itself has an internal fault and must be replaced or reprogrammed.