Case IH 3973 – Engine ECU: After run relay low side ECU driver circuit over temperature failure.

The Engine Control Unit (ECU) has detected that the internal low-side driver (the ground-switching circuit) responsible for operating the 'After run relay' has exceeded its safe operating temperature. This is typically caused by excessive current draw on the circuit, which usually points to a short circuit in the wiring harness, a failing relay coil drawing too much amperage, or a damaged ECU.

• Steiger T4B
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Magnum tractors
• Tier 4B equipment

1. 1

   Locate the 'After Run Relay' in the machine's engine/auxiliary fuse and relay panel.

2. 2

   Remove the relay and inspect the terminals for signs of heat damage, melting, or corrosion. Test the relay coil for an internal short circuit that could be causing excessive current draw.

3. 3

   Inspect the wiring harness between the Engine ECU and the After Run Relay. Look for chafed wires, pinched harnesses, or exposed copper that could be causing a short to battery positive or ground.

4. 4

   Disconnect the Engine ECU harness and check for pushed, corroded, or burnt pins on the low-side driver circuit.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the battery to prevent further electrical damage during testing and repair.

2. 2

   Replace Faulty Components

   If the After Run Relay fails a bench test (e.g., shorted coil), replace the relay with a new OEM unit.

3. 3

   Repair Wiring

   If a short is found in the wiring harness between the ECU and the relay block, repair the damaged section of the wire using proper weather-sealed connectors, or replace the affected harness section.

4. 4

   Reassemble and Test

   Reinstall the relay and reconnect the ECU and battery.

5. 5

   Clear Codes

   Connect a diagnostic tool, clear the inactive fault codes, and run the engine. Shut the engine down to trigger the after-run sequence (which purges the DEF system) and verify that DTC 8671 / 3973 does not return. Note: If the wiring and relay are perfectly intact but the code persists, the Engine ECU's internal driver may be permanently damaged and require ECU replacement.