Case IH 603-03 - UCM – UCM: Top of brake switch left voltage high

The control module is detecting an abnormally high voltage signal on the left brake pedal switch circuit. This typically indicates a short circuit to a high voltage source (such as a 12V supply) in the wiring harness, or an internal short within the left brake switch itself.

• MAGNUM AFS Connect
• Magnum series
• Puma series
• Maxxum series

1. 1

   Inspect the left brake pedal switch for physical damage, sticking, or improper adjustment. Ensure the pedal returns fully and the switch plunger moves freely.

2. 2

   Inspect the wiring harness and connectors between the left brake switch and the UCM. Look for signs of wire chafing, pinched wires, or corrosion that could cause a short to power.

3. 3

   Disconnect the switch and use a multimeter to measure the voltage at the harness connector. Verify if there is a short to a 12V power source on the signal wire.

4. 4

   Test the brake switch continuity across its terminals when the pedal is depressed and released. If the switch shows continuity when it shouldn't (internally shorted), it must be replaced.

1. 1

   Power Down

   Turn off the tractor and disconnect the battery or turn off the master disconnect switch to prevent electrical spikes.

2. 2

   Access the Switch

   Locate the brake pedal assembly inside the cab and identify the left brake switch.

3. 3

   Disconnect

   Unplug the electrical connector from the left brake switch.

4. 4

   Removal

   Remove the retaining hardware (usually a lock nut or retaining clip) and extract the faulty switch from its mounting bracket.

5. 5

   Installation

   Install the new brake switch. Adjust the switch position/air gap according to the OEM service manual specifications so that it properly engages and disengages when the pedal is pressed.

6. 6

   Reconnect

   Secure the retaining nuts and plug the electrical connector back into the switch.

7. 7

   Clear and Test

   Reconnect the battery, turn the key on, and clear the fault code using the Electronic Service Tool (EST) or the in-cab display monitor. Depress the left brake pedal several times to ensure the code does not return and that the brake lights/interlocks function correctly.