John Deere 158.18 – Roof Lighting Control Unit (RLC): Switched supply voltage (ELX) to the module has dropped below 11.2 volts

The switched supply voltage (ELX) to the module has dropped below 11.2 volts while the engine is running (typically at speeds between 500 and 1500 RPM). This low-voltage condition points to an inability of the electrical system to maintain adequate voltage under moderate engine load conditions.

• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 7200R
• 7215R
• 7230R
• 7250R
• 7270R
• 7290R
• 8245R through 8400R
• 8310RT
• 8335RT
• 8360RT
• 9470RT
• 9520RT
• 9570RT
• 7630
• 7730
• 7830
• 7930
• 9230
• 9330
• 9430
• 9530
• 9630

1. 1

   Attempt vehicle recovery by reducing electrical load (turn off customized accessories) or increasing engine RPM above 1500 to see if voltage recovers.

2. 2

   Inspect batteries and alternator for physical damage, loose connections, or accumulated debris.

3. 3

   Test battery resting voltage (should be >12V) and alternator charging voltage under load (should be ~13.5V - 14.5V).

4. 4

   Check the wiring harness and ground connections supplying the RLC/CLC for high resistance or shorts.

1. 1

   Clean and Tighten Connections

   Disconnect the batteries and clean all battery terminals, cable ends, and chassis ground connections. Reconnect and tighten securely.

2. 2

   Service the Alternator

   Clear any chaff or debris from the alternator cooling fins. Ensure the alternator drive belt is properly tensioned and not slipping.

3. 3

   Test and Replace Batteries

   Perform a load test on the batteries. If they fail to hold a charge under load, replace them.

4. 4

   Test and Replace Alternator

   Run the tractor and measure the alternator output. If it fails to maintain at least 12.5V under load at operating RPM, replace the alternator.

5. 5

   Inspect the Wiring Harness

   Trace the wiring harness from the power source to the Roof Lighting Control Unit / Cab Load Center. Look for chafing, corrosion, or pinched wires. Repair or replace the harness if damaged.

6. 6

   Replace the Control Module

   If the charging system and wiring are fully functional, providing proper voltage to the module's connector, but the code persists, the internal circuitry of the RLC/CLC is likely faulty and the module must be replaced.

7. 7

   Clear Codes

   Cycle the key switch or use the CommandCenter display to acknowledge and clear the diagnostic trouble code.