New Holland DCU 19742 – DCU (Dosing Control Unit): Main Relay opens too early / too late _main relay shut off too early (before EEPROM update) _ P0685 ECM/PCM Power Relay Control Circuit /Open

This fault indicates that the main power relay supplying the control module is opening (shutting off) prematurely during the machine's shutdown sequence. When the ignition is turned off, the module must remain powered for a short duration to save data to its EEPROM memory and (in the case of emissions systems) purge the DEF/urea lines. If the relay drops out too early or there is an open circuit in the relay control wiring, this code is triggered.

• T6
• T7
• T7.210
• T7.220
• T7.270
• T8
• T8.325
• T9 Series
• Magnum Series
• 235-340 Series 4
• Steiger

1. 1

   Locate the DCU/ECM Main Power Relay in the fuse/relay distribution box. Swap it with a known good relay of the same type to see if the fault clears.

2. 2

   Check the wiring harness between the DCU and the main relay for open circuits, chafing, or corrosion. Pay special attention to the relay control circuit (ground/power triggers).

3. 3

   Verify that the ignition switch is not cutting power abruptly to the unswitched battery lines. The module requires constant battery power (Terminal 30) to hold the relay closed after the switched ignition power (Terminal 15) is turned off.

4. 4

   Check for weak batteries, loose battery terminals, or poor chassis grounds, which can cause voltage drops that force the relay to spring open prematurely.

1. 1

   Disconnect Battery

   Always disconnect the negative battery terminal before servicing electrical components.

2. 2

   Replace the Main Relay

   If the relay failed the diagnostic swap test, replace the faulty relay. Clean the relay socket terminals with contact cleaner if corrosion is present.

3. 3

   Repair Harness

   If an open circuit or high resistance was found in the control wiring, repair the damaged wire or replace the affected harness section. Ensure all pins in the DCU connector are fully seated and not pushed back.

4. 4

   Software Update

   In some cases, a software glitch can cause the module to command the relay off too quickly. Check with a New Holland dealer to see if an EEPROM/Software update is available for the DCU.

5. 5

   Module Replacement

   If the relay, wiring, and power supplies are perfectly intact, the DCU itself has an internal hardware failure in its relay control driver. The Dosing Control Unit must be replaced and reprogrammed by a dealer.