Case IH t18 – Pump Control Unit (PCU): PCU Power Supply, Shorted To High Source

Failure in the power supply circuit to the PCU, where the system detects a voltage that is shorted to a high voltage source (such as battery voltage).

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190

1. 1

   Verify that no external power supply is improperly connected and test the tractor's alternator/charging system for overcharging faults.

2. 2

   Test the engine shut-down relay (Relay K5) for internal faults.

3. 3

   Disconnect the affected connectors and check for a short circuit or short to ground at connector C273. Test continuity between Pin 1 (wire 5420-G) and Pin 2 (wire 5400-Y), Pin 1 and ground, Pin 2 and ground, and Pin 5 (PC3-Y) and ground. Wiggle the harness and connectors during testing to check for an intermittent short.

4. 4

   Inspect the CAN BUS wires (Yellow and Green). Ensure they are properly twisted together and free from possible electrical interference.

5. 5

   Re-connect connector C316. Measure the resistance between connector C273 pin 1 (5420-G) and pin 2 (5400-Y). The resistance should be approximately 120 Ohms.

1. 1

   Preparation

   Turn the keyswitch to OFF and disconnect the battery (or pull the specified circuit fuse) to prevent false multimeter readings and protect the modules.

2. 2

   Harness Repair

   If a short circuit or short to ground was indicated during step 3 of the diagnostics, locate the damaged section of the wiring harness. Cut away the damaged wire and use a harness repair kit to splice in a new section. Wrap the repaired leads with vinyl insulation tape.

3. 3

   CAN BUS Routing

   If the CAN BUS wires are not twisted together or are suffering from interference, re-position, re-twist, or replace the harness as required.

4. 4

   Module Replacement (Based on Resistor Check)

   If the resistance measured in diagnostic step 5 is NOT approximately 120 Ohms, the CAN BUS terminating resistor in the engine control module has failed. You must remove and replace the Engine Control Module. If the resistance IS approximately 120 Ohms, the ECM is functioning correctly, but the fault lies within the pump's internal circuitry. You must remove and replace the Fuel Injection Pump.

5. 5

   Post-Repair

   Reconnect all harnesses, restore battery power, and clear the fault codes using the electronic service tool. Run the tractor to verify the t18 code does not return.