Case IH 4243 - UCM – Transmission Control Module (TCM) or UCM: Open Circuit At Relay Reverse Warning Alarm

The controller detects a break in the electrical circuit (open circuit) leading to the reverse/backup warning alarm relay. Note: The description 'Left external fender switch open circuit/short to GND' actually corresponds to Fault Code 1074.

• MAXXUM T4E
• VESTRUM CVX
• Case IH Puma series
• Case IH Maxxum series
• Case IH Magnum / Optum series

1. 1

   For Reverse Alarm (Official 4243): Locate the reverse warning alarm relay (usually in the cab fuse/relay panel)

2. 2

   For Reverse Alarm: Test the relay for proper operation and check the relay socket for pushed-out pins or corrosion

3. 3

   For Reverse Alarm: Check the wiring running to the physical backup beeper on the rear of the machine

4. 4

   For Left Fender Switch (Code 1074 discrepancy): Inspect the left rear fender switch for physical damage, water ingress, mud, or strikes from debris

5. 5

   For Left Fender Switch: Follow the wiring pigtail from the back of the switch to the main chassis harness connector, looking for pinched wires

6. 6

   For Left Fender Switch: Disconnect the switch and check the chassis-side connector for a 5V or 12V reference signal and a good ground

7. 7

   For Left Fender Switch: Test the switch itself for continuity. If it reads 'open' (infinite resistance) even when pressed, the internal contacts have failed

1. 1

   Power Down

   Turn off the tractor, remove the key, and turn off the battery isolator switch.

2. 2

   Access the Switch

   Reach behind the left rear fender to locate the switch retaining mechanism.

3. 3

   Disconnect

   Unplug the electrical connector. If the connector is packed with dirt, clean it out with electrical contact cleaner and compressed air.

4. 4

   Remove the Switch

   Squeeze the retaining tabs or remove the locking nut (depending on the switch style) and push the switch out through the front of the fender housing.

5. 5

   Install the New Part

   Push the new switch into the fender receptacle until it clicks into place.

6. 6

   Reconnect and Seal

   Plug the harness back in. Apply a small amount of dielectric grease to the connector seal to prevent future moisture ingress.

7. 7

   Clear Codes

   Turn the battery isolator back on, power up the tractor, and use the instrument cluster diagnostic menu or the Electronic Service Tool (EST) to clear the inactive fault codes. Test the PTO/Hitch from the fender to verify operation.