Case IH 3968 – Engine Control Unit (ECU) / Engine Control Module (ECM): Post Drive Relay, Open circuit on the Low Side

An open circuit on the Low Side means the ECU is unable to detect electrical continuity on the ground/control side of the Post Drive (After Run) Relay circuit. If the circuit is broken, the ECU cannot keep itself powered down properly, triggering the fault.

• 240 Series Axial-Flow Combines
• Magnum Series (e.g., Magnum 250, 340)
• Skid Steer Loaders (SSL) & Compact Track Loaders (CTL) with ISM-DI engines

1. 1

   Verify battery voltage is sufficient. Low voltage or a weak battery can cause sporadic open-circuit relay faults during the after-run sequence.

2. 2

   Locate the After Run Relay in the auxiliary relay panel or engine fuse box. Check for physical damage, melted plastic, or corroded terminals.

3. 3

   Swap the After Run Relay with a known-good relay of the exact same amperage and pinout from another non-critical circuit to see if the fault clears.

4. 4

   Use a multimeter to check the wiring harness between the ECM and the relay block. Specifically, test the low-side control wire for continuity and ensure it is not shorted to ground or broken.

1. 1

   Disconnect Power

   Turn off the ignition and disconnect the negative battery terminal to prevent short circuits.

2. 2

   Access the Relay Panel

   Open the engine fuse/relay box or auxiliary relay panel (refer to the specific machine's operator manual for the exact cavity, e.g., Cavity 2 or 6 for the Engine Aux Relay).

3. 3

   Test the Circuit

   Remove the After Run Relay. Using a digital multimeter, verify that the high side has battery voltage. Then, check the low side (control circuit) wire from the relay socket back to the ECU harness connector for an open circuit (infinite resistance).

4. 4

   Repair the Harness

   If the wire is broken or chafed, repair the affected section of the wiring harness. If the harness is severely damaged, it may need to be replaced.

5. 5

   Replace the Relay

   If the wiring tests good, the internal coil of the relay has likely failed open. Install a new OEM After Run Relay.

6. 6

   Clear Codes and Test

   Reconnect the battery, turn the key to the ON position, and use the Electronic Service Tool (EST) or the in-cab diagnostic menu to clear the fault code. Start the engine, let it run briefly, and shut it down. Listen for the relay to click and verify the ECU completes its after-run cycle without throwing the code again.