Case IH 19739 – DCU or ECM / PCM: Main Relay opens too early / too late | main relay shut off too late | P0687 ECM/PCM Power Relay Control Circuit High

Malfunction in the ECM/PCM power relay control circuit. The system detects that the main power relay is opening too early or shutting off too late (sticking closed), indicating a high value in the power relay control loop.

• Tier 4a
• Case IH Magnum Series (e.g., 235-340 Series 4)
• Case IH STX Steiger Tractors (350 to 600 HP)
• Case IH Puma Series
• Case IH Maxxum and MX series

1. 1

   Verify that battery voltage to the ECM/DCU is within the normal operating range, as voltage spikes or drops can cause relay timing faults.

2. 2

   Swap the Main Power Relay with a known good relay of the exact same amperage and pin configuration (e.g., horn or work light relay) to see if the fault clears.

3. 3

   Check the control loop of the ECM/PCM power relay for a short to ground or short to battery.

4. 4

   Test the relay socket for proper ground and command voltage from the ECM/DCU when the key is cycled.

5. 5

   Inspect the wiring harness between the main relay panel and the ECM/DCU for chafing, corrosion, or open circuits.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Access the Relay Panel

   Open the main fuse and relay center (location varies by model, often in the cab headliner, right-hand console, or engine firewall).

3. 3

   Remove the Faulty Relay

   Pull the main ECM/DCU power relay. Inspect the spade terminals on the relay and the socket for signs of arcing, melting, or green corrosion.

4. 4

   Clean Contacts

   If mild corrosion is present, clean the socket terminals with electrical contact cleaner and a terminal brush.

5. 5

   Replace the Relay

   If the relay was sticking closed (causing the 'shut off too late' condition), install a new OEM replacement relay.

6. 6

   Repair Wiring

   If the socket tested bad for continuity during diagnostics, trace the wire back to the ECM/DCU and repair the open/shorted section using weather-sealed splice connectors.

7. 7

   Reassemble and Clear Codes

   Reconnect the battery, turn the key to the ON position, and use a diagnostic scan tool to clear the 19739 / P0687 / P0685 codes. Cycle the key to verify the code does not return.