Case IH 3974 – Engine Control Module (ECM): After run relay low side driver circuit short to battery failure

The engine control module has detected that the ground-side (low side) control circuit for the after-run relay is receiving unexpected battery voltage, indicating a short circuit to a power source.

• Steiger T4B
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Magnum Series Tractors (Magnum 340)

1. 1

   Remove the after-run relay and test it for internal short circuits

2. 2

   Check the wiring harness between the ECM and the after-run relay socket for chafed wires, melted insulation, or pinched harnesses

3. 3

   Inspect the relay block and ECM connectors for corrosion, water ingress, or bent pins that could bridge the driver circuit to a power circuit

1. 1

   Power Down

   Turn off the machine. Disconnect the battery to ensure safety while working on the electrical system.

2. 2

   Locate the Relay

   Access the engine fuse and relay distribution box. Locate the 5 Amp After Run Relay.

3. 3

   Inspect and Repair Wiring

   Trace the low-side driver wire from the relay block back to the ECM. If a short to battery voltage is found (e.g., wire insulation rubbed through against a hot wire), splice and repair the damaged section using weather-sealed connectors.

4. 4

   Replace the Relay

   If the wiring tests good, the internal coil or contacts of the relay are likely shorted. Replace the after-run relay with a new OEM unit.

5. 5

   Reassemble and Clear Codes

   Reconnect the battery, turn the key on, and use a diagnostic tool (or the machine's diagnostic display) to clear the inactive fault codes. Run the engine, shut it down, and verify that the after-run purge cycle completes without throwing DTC 6771 / 3974 again.