New Holland UCM 5142 – UCM: Aux Control Function: Pin A Short to Power

The control module has detected an abnormally high voltage signal from the auxiliary hydraulic control thumbwheel (the roller switch located on the joystick). The system registers this as a short circuit to a power source on Pin A of the thumbwheel's circuit, which disables the auxiliary hydraulics as a safety precaution.

• L213
• L215
• L218
• L220
• L223
• L225
• L230
• C227
• C232
• C238

1. 1

   Visual Inspection: Inspect the wiring harness and connectors running from the UCM to the right-hand control handle (joystick). Look for pinched wires, abrasion, or corrosion at the connectors.

2. 2

   Voltage Test: With the ignition ON, back-probe the auxiliary thumbwheel axis input voltage at the UCM. A healthy thumbwheel switch (which operates on a 5V reference) typically rests at ~2.5V at idle and sweeps from ~0.5V to ~4.5V when rolled. If the voltage is pegged at or above 5V (or system voltage), there is a short to power.

3. 3

   Harness Continuity: Disconnect the joystick and the UCM. Measure the resistance of the thumbwheel signal wire to chassis ground and to the power supply lines to check for a short in the harness itself.

1. 1

   Disconnect Power

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

2. 2

   Isolate the Harness

   Disconnect the harness connector at the base of the affected joystick (usually the right-hand joystick for auxiliary controls).

3. 3

   Inspect Pins

   Check the connector pins on both the joystick side and the harness side for bent pins, moisture, or corrosion. Clean with contact cleaner if necessary.

4. 4

   Test the Harness

   If the harness tested faulty during the diagnostic phase (shorted to a 12V or 5V power wire), trace the harness back to the UCM, locate the chafed/shorted section, and splice in a repair wire using weather-sealed connectors.

5. 5

   Replace the Switch/Joystick

   If the harness tests perfectly fine, the internal potentiometer/sensor inside the joystick's thumbwheel has failed (shorted internally). You must either replace the individual thumbwheel switch (if you are comfortable de-soldering/re-pinning) or replace the entire joystick hand control assembly.

6. 6

   Reassembly and Clearing

   Reconnect the joystick and battery. Turn the key ON and verify that the UCM 5142 code changes from "Active" to "Stored/Inactive". Clear the code and test the auxiliary hydraulic flow.