New Holland DCU 19741 – DCU (Dosing Control Unit): Main Relay opens too early / too late | main relay open circuit | P0687 ECM/PCM Power Relay Control Circuit High

The control module has detected an open circuit or a short to a high-voltage source on the main power relay control circuit. Because of this electrical fault, the main relay is failing to operate correctly—either opening too early, opening too late, or failing to close at all.

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1. 1

   Verify if the fault is currently active or stored in the history

2. 2

   Locate the DCU/ECM main power relay in the machine's fuse/relay distribution box and check for physical damage, corrosion, or a burnt smell

3. 3

   Inspect the wiring harness connecting the DCU to the main relay for chafed wires, disconnected pins, or an open circuit

4. 4

   Test the power stage of the relay control circuit to see if it is shorted to a high voltage source

5. 5

   Verify that the DCU is receiving proper power and ground, as an incorrect engine shutdown process or power failure can trigger relay plausibility faults

1. 1

   Disconnect Power

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

2. 2

   Test the Relay

   Remove the DCU main relay. Use a multimeter to test the internal coil resistance and ensure the switch contacts have continuity when energized.

3. 3

   Replace the Relay

   If the relay fails the bench test (or if it is sticking open/closed), replace it with a new OEM relay.

4. 4

   Repair the Harness

   If the relay tests good, use a multimeter to trace the control circuit wire from the relay socket back to the DCU. Locate the open circuit or the short-to-power. Repair the damaged section of the wiring harness or replace the harness if heavily corroded.

5. 5

   Check the DCU

   If both the relay and the wiring harness are in perfect condition, the fault may lie within the power stage of the DCU itself. (Note: DCU replacement should be a last resort).

6. 6

   Clear Codes

   Reconnect the battery, turn the key on, and use a diagnostic tool to clear the DCU 19741 / P0687 codes. Run the machine to verify the code does not return.