John Deere 158.18 – FCC: Instrument Unit, Supply Voltage (ELX) Too Low

The supply voltage to the module drops below 11.2 volts while the engine speed is operating in the mid-range (specifically between 500 and 1500 RPM), indicating that the module is not receiving adequate electrical power to function properly.

• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R

1. 1

   Test battery health to ensure it is not deeply discharged, has dead cells, or is failing to hold a charge

2. 2

   Test the alternator output to verify it is producing sufficient voltage (above 12V) under moderate engine loads (500-1500 RPM)

3. 3

   Inspect the wiring harness leading to the FCC (Instrument Unit) for loose, corroded, or damaged pins and connectors causing high resistance

4. 4

   Back-probe the ELX supply pin at the FCC module with a multimeter and monitor voltage while varying engine RPM between 500 and 1500 to check for dips below 11.2V

1. 1

   Disconnect Power

   Turn off the tractor and disconnect the negative battery cable to ensure safety while working on the electrical system.

2. 2

   Clean Power Supply Points

   Clean the battery terminals, battery cables, and main chassis ground points with a wire brush to eliminate resistance caused by corrosion. Reconnect and re-test.

3. 3

   Repair Wiring

   If the diagnostic load test revealed a voltage drop between the battery/alternator and the FCC module, locate the damaged section of the wiring harness. Splice in new wiring or replace the damaged connector pins leading into the back of the dash module.

4. 4

   Replace the Alternator

   If the alternator fails the load test at 500-1500 RPM, relieve tension on the serpentine belt, disconnect the alternator electrical connections, unbolt the unit, and install a replacement OEM alternator.

5. 5

   Clear Codes

   Reconnect the battery, cycle the key switch, and use a John Deere diagnostic tool (or the CommandCenter display) to clear the stored 000158.18 fault code. Start the engine and run it between 500 and 1500 RPM to verify the code does not return.