John Deere 3976.03 – Cab Roof Module (RLC): Cab Dome Light, Voltage Too High

The controller has registered a voltage on the dome light circuit that is higher than 16 volts.

• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 859M Tracked Feller Buncher

1. 1

   Test the overall system voltage at the battery with the engine running to check for overcharging (above 16 volts).

2. 2

   Inspect the wiring harness running from the Cab Roof Module (RLC) to the dome light for shorts to a higher voltage source.

3. 3

   Disconnect the dome light and test the voltage at the connector to determine if the dome light assembly has an internal short.

4. 4

   Check the RLC module connectors for corrosion, water intrusion, or bent pins that could be bridging circuits.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the negative battery cable to ensure safety while working on the cab's electrical system.

2. 2

   Address Overcharging

   If your diagnostic checks revealed system-wide voltage over 16V, replace the alternator/voltage regulator before proceeding, as this is the root cause.

3. 3

   Repair Wiring

   If a short-to-power was found in the cab roof harness, splice and repair the damaged section of the wire using heat-shrink connectors, or replace the affected harness section.

4. 4

   Replace the Dome Light

   If the dome light assembly is internally shorted or melted: Remove the mounting screws or pry the retaining clips holding the dome light to the cab headliner. Disconnect the electrical pigtail. Plug the new OEM dome light assembly into the harness and snap/screw it back into the headliner.

5. 5

   Clear Codes and Retest

   Reconnect the battery, start the machine, and turn on the dome light. Use the in-cab diagnostic monitor or a connected service tool to clear code 003976.03 and verify it does not return.