New Holland ENG 3991 – Engine Control Unit (ECU) / ISM-IDI ENGINE: Terminal B4 High Current (Relay and lamp terminal)

The control module has detected an abnormally high electrical current draw on the circuit connected to Terminal B4. This condition is typically caused by a short circuit to ground in the wiring harness, or an internal short within the relay or lamp that the terminal controls.

• L213
• L215
• L218
• L220
• L223
• L225
• L230
• L234
• SR175
• SV185
• SV300
• TV380
• C227
• C232
• C234
• C237
• C238
• C245
• B90B
• B100B
• B115B
• B95B

1. 1

   Turn the ignition switch ON and measure the voltage from the ECU connector Terminal B4 to the chassis ground to verify proper baseline voltage (typically 12V).

2. 2

   Inspect the wiring harness running from ECU Terminal B4 to the associated relay and lamp. Check the insulation of each wire for chafing, melting, or a short circuit to ground.

3. 3

   Disconnect the relay and lamp on the B4 circuit and test them for internal shorts that would cause an excessive amperage draw.

4. 4

   Verify the ECU connector at Terminal B4 for damaged pins, corrosion, or moisture intrusion.

1. 1

   Power down the machine and disconnect the battery

   This prevents further electrical damage.

2. 2

   Locate the Engine ECU and disconnect the harness connector containing Terminal B4.

3. 3

   Repair or replace the wiring harness.

   If the wiring harness is damaged or shorted to ground, repair the affected wire or replace the engine harness section.

4. 4

   Replace the relay or lamp.

   If the relay or lamp is internally shorted, replace the faulty component.

5. 5

   Reconnect all components, restore battery power, and clear the fault codes.

6. 6

   Run the machine to verify the repair.

   If code ENG 3991 returns immediately and the wiring/relays are confirmed to be in perfect condition, the Engine ECU has likely suffered internal driver damage on Terminal B4 and will require replacement or remanufacturing.