Case IH t69 – Powertrain Control Unit (PCU) / Transmission Controller, or Engine Control Unit (ECU): Main Relay Defect, Shorted To High Source

The control module has detected that the main power relay circuit is shorted to a high voltage source (battery positive). The controller is seeing voltage on the relay control circuit when it shouldn't be, which often indicates that the relay is internally shorted/stuck closed, or the wiring harness is shorted to power. In some systems, this results in the relay failing to switch off, leaving the engine or modules enabled when they should be powered down.

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190
• AFX8010
• TM115
• TM140

1. 1

   Locate the Main Relay in the fuse/relay panel (designated as K14 on AFX8010 combines and some tractor fuse blocks)

2. 2

   Remove the relay and visually inspect the socket pins for signs of melting, corrosion, or arcing

3. 3

   Bench-test the relay to see if the internal contacts are fused closed (stuck on) or if the coil is shorted

4. 4

   With the relay removed, use a multimeter to check the relay control circuit in the harness for a short to battery voltage (High Source)

5. 5

   Inspect the main maxi-fuses associated with the PCU/ECU power supply to ensure a short hasn't blown a protective circuit

1. 1

   Disconnect Power

   Turn the ignition key to the OFF position and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Access the Relay Panel

   Open the access panel to the main fuse and relay board (location varies by cab/model).

3. 3

   Remove the Defective Relay

   Pull the main relay (K14 or PCU Main Relay).

4. 4

   Repair Wiring (If Applicable)

   If your diagnostic checks revealed a short to power in the wiring harness, trace the wire from the relay block to the PCU/ECU and repair the chafed or shorted section.

5. 5

   Install New Relay

   Plug a new, OEM-spec relay into the socket. Ensure it is fully seated.

6. 6

   Restore Power & Clear Codes

   Reconnect the battery. Turn the key switch to the "ON" position. Navigate the H-Menu or service tool to clear the inactive fault codes.

7. 7

   Recalibration (If necessary)

   If the PCU lost volatile memory due to the power issue, you may need to perform a Transmission/EDC calibration (Mode HA).