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

A malfunction in the control circuit of the main power relay that supplies voltage to the control module. The module logs this code if the relay opens too early, shuts off too late, or has an open/high-resistance circuit, indicating its power supply was abruptly cut or improperly managed during operation or the afterrun shutdown sequence.

• Tier 4a
• Case IH Magnum Series (235, 260, 290, 315, 340 Series 4)
• Case IH Steiger Series (350 through 600)
• Case IH Puma Series
• Case IH Optum Series

1. 1

   Check battery and ground: Verify that the batteries are fully charged and the terminals are clean. A sudden voltage drop during the key-off sequence can cause the relay to drop out prematurely.

2. 2

   Inspect the DCU/ECM Main Relay: Locate it in the power distribution box. Pull the relay and inspect the spade terminals for corrosion, heat discoloration (bluing/melting), or loose fitment in the socket.

3. 3

   Test the wiring harness: Use a multimeter to check the control circuit wiring between the relay panel and the DCU/ECM. Look for an open circuit (broken wire) or a short to ground/power.

4. 4

   Observe the afterrun sequence: Turn the key ON, then OFF. Listen for the DEF pump purging and the audible click of the main relay shutting off. If it clicks off instantly with the key rather than delaying, the relay control circuit is failing.

1. 1

   Disconnect Power

   Disconnect the negative battery cable to prevent electrical spikes while working on the control circuits.

2. 2

   Access the Relay Panel

   Open the main power distribution center (location varies by model; often in the cab behind the seat or in the engine bay).

3. 3

   Replace the Relay

   Remove the suspect ECM/DCU Main Relay. Clean the socket terminals with electrical contact cleaner. Install a new, tested relay of the exact same amperage and pin configuration.

4. 4

   Harness Repair (If Applicable)

   If the relay was not the issue, trace the wiring harness from the relay block to the DCU. Repair any chafed wires or broken pins at the DCU connector.

5. 5

   Relearn/Clear Codes

   Reconnect the battery. Turn the ignition ON (do not start) and use a diagnostic scan tool to clear the 19741/19742 (P0685/P0687) fault codes.

6. 6

   Verify Repair

   Turn the key OFF and wait for the complete afterrun sequence to finish. The relay should hold power for several seconds to allow the EEPROM to update before clicking off.