John Deere 521835.20 – RLC: Control Unit Ground Circuit Fault

The fault code indicates an issue with the earth/ground circuit for the Roof Lighting Control Unit (RLC). The RLC manages lighting systems such as headlights, work lights, turn signals, and beacon lights. A poor ground connection disrupts the module's ability to function correctly and can result in improper operation or communication failures on the CAN bus network.

• 9470RT
• 9520RT
• 9570RT

1. 1

   Access and verify active fault codes on the diagnostic display; clear the code, operate lighting functions, and re-check for active status

2. 2

   Inspect battery state of charge and alternator output to ensure stable system voltage

3. 3

   Visually inspect wiring harnesses connected to the RLC for chafing, pinching, corrosion, or rodent damage, especially where passing through the cab structure

4. 4

   Check main ground connections for the cab and the RLC unit to ensure they are clean, tight, and free of corrosion

5. 5

   Perform a 'wiggle test' on the wiring harness leading to the RLC while the machine is running to identify intermittent connections or broken wires

1. 1

   Disconnect battery

   Disconnect the battery's ground cable to prevent electrical shorts and ensure safety before performing repairs.

2. 2

   Clean and secure ground connections

   If a loose or corroded ground connection was found, disassemble it, thoroughly clean all contact surfaces with a wire brush or appropriate cleaner, and reassemble securely.

3. 3

   Repair damaged wiring

   Cut out any damaged wire sections, splice in a new piece of wire of the same gauge using high-quality connectors, and seal the repair with heat-shrink tubing.

4. 4

   Test for shorts to ground

   Use a multimeter to check for continuity between the ground circuit of the RLC and the chassis ground. Disconnect various connectors along the harness to isolate the section containing the short if necessary.

5. 5

   Replace control unit

   If all wiring and ground connections are verified to be in good condition and the fault persists, replace the RLC unit.