New Holland DCU 19739 – DCU: Main Relay opens too early / too late | main relay shut off too late | ECM/PCM Power Relay Control Circuit High

The main power relay for the control module is staying energized longer than expected after the ignition key is turned off, or there is a short-to-battery voltage in the relay's control circuit. The system detects that the relay contacts are stuck closed or the control voltage is unexpectedly high.

• T7 series tractors
• T8 series tractors
• Magnum series

1. 1

   Verify the battery voltage is within normal operating range (typically above 8V is required for the diagnostic to run accurately).

2. 2

   Locate the DCU main power relay in the machine's fuse/relay box.

3. 3

   Remove the relay and test it with a multimeter to see if the internal contacts are physically stuck or welded closed (check for continuity across the load pins when the relay is unpowered).

4. 4

   Inspect the wiring harness between the Dosing Control Unit and the main relay for a short circuit to battery voltage.

5. 5

   Inspect the DCU power fuses to ensure they are intact and the correct amperage.

1. 1

   Turn the ignition key to the OFF position and wait for the machine's electrical system to fully power down

   (the DCU normally goes through a purge and shutdown cycle that can take a few minutes).

2. 2

   Access the main fuse and relay panel and locate the DCU Main Relay.

3. 3

   Remove the relay and perform a bench test.

   If the relay is stuck closed, replace it with a new relay.

4. 4

   If the relay functions normally, use a multimeter to probe the relay control circuit in the wiring harness.

   Check for a short to power (voltage present when the key is off and the DCU is asleep).

5. 5

   Repair any damaged, chafed, or melted wiring in the harness causing the short circuit.

6. 6

   Reinstall the relay and verify all connections are secure.

7. 7

   Connect a diagnostic tool, clear the DCU 19739 / P0687 fault code, and cycle the ignition to verify the code does not return and the relay clicks on and off appropriately.