Case IH 3991 – Engine ECU: ISM-IDI Engine ECU Relay Output Power Short Circuit

The Engine Control Unit (ECU) has detected excessive current draw or a short circuit to ground on the output side of the ECU power relay circuit.

• skid steer
• Skid Steer Loaders (SSL)
• Compact Track Loaders (CTL)
• SR175
• SR210
• SR220
• SV280
• SV300
• TR270
• TR310

1. 1

   Verify battery voltage is stable (around 12.6V resting) and check the main power strap fuse for corrosion or separation.

2. 2

   Swap the Engine ECU power relay with a known good relay of the same amperage to see if the fault clears.

3. 3

   Inspect the ECU relay socket cavities for melted plastic, pushed-out pins, or green corrosion.

4. 4

   Check the wiring harness between the ECU (Terminal B4) and the relay block for continuity and shorts to ground, especially near engine mounts or under the cab.

5. 5

   Check the fuses supplying the relay (often 25A or 7.5A) to see if the short circuit has blown the circuit protection.

1. 1

   Disconnect Power

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

2. 2

   Repair Wiring

   If your multimeter test revealed a short to ground in the harness between the relay and the ECU, open the wire loom at the chafed location. Cut out the damaged wire section, solder in a new piece of wire, and seal it with marine-grade heat shrink.

3. 3

   Replace the Relay

   If the wiring is intact but the relay failed internally (or failed the swap test), install a new OEM Engine ECU relay into the socket.

4. 4

   Clean Connections

   If corrosion was found at the main frame fuse or relay block, clean the terminals with electrical contact cleaner and a wire brush. Apply dielectric grease to prevent future moisture intrusion.

5. 5

   Reassemble and Clear Codes

   Reconnect the battery. Turn the ignition key to the ON position. Use the machine's display panel or an electronic service tool (EST) to clear the inactive fault codes.

6. 6

   Test Operation

   Start the engine and monitor the display. Verify that the red master telltale remains off and the machine operates without random shutdowns.