Case IH 3972 – Engine ECU: After Run Relay / Throttle Motor Circuit Open

Depending on the machine family, this fault code indicates one of two issues. On Tier 4 engines (combines and tractors), it means the Engine ECU detects an open circuit on the ground-control side of the after-run relay, which keeps the ECU powered after key-off for DEF purging. On skid steers and compact track loaders, it indicates an open, short, or out-of-range signal on the intake throttle motor driver circuit.

• 7240
• 8240
• 9240
• Magnum
• T8
• SR175
• SV250
• TR320
• L220

1. 1

   Verify the code definition on the diagnostic display to confirm whether the text reads After Run Relay or Intake Throttle Motor to ensure you are chasing the correct circuit.

2. 2

   If diagnosing the After Run Relay: Locate the relay in the main fuse/relay box, check the associated 5 Amp or 25 Amp auxiliary fuses, and swap the relay with a known good identical relay.

3. 3

   Inspect the wiring harness between the Engine ECU and the relay block (or throttle motor) for chafed wires, rodent damage, or loose connector pins.

4. 4

   Using a multimeter, check for continuity on the low-side wire from the relay socket back to the ECU pin. An open circuit fault usually means infinite resistance (a broken wire).

5. 5

   If diagnosing the Intake Throttle Motor: Check if the intake throttle valve is physically stuck shut with soot or carbon, which can trigger driver circuit faults.

1. 1

2. 2

   Disconnect power

   Turn off the battery disconnect switch to prevent shorting any ECU pins during repair.

3. 3

   Access the component

   For the relay: Open the power distribution box and remove the suspect After Run Relay. For the throttle motor: Remove the intake piping to access the throttle valve assembly on the engine intake manifold.

4. 4

   Repair the open circuit

   If the multimeter test revealed a broken wire on the low-side driver circuit, trace the harness to the break. Splice and heat-shrink the wire according to OEM wiring repair standards.

5. 5

   Replace the faulty component

   If the wiring is intact but the relay coil is open (tests infinite ohms across the control pins), install a new relay. If diagnosing the throttle motor and the driver is burnt out internally, replace the intake throttle valve assembly.

6. 6

   Reassemble and clear codes

   Reconnect the battery, cycle the key, and use the AFS Pro monitor or an electronic service tool (EST) to clear the inactive fault codes. Run the engine and verify the code does not return.