John Deere 000168.18 – BCU / BIF: System Voltage Too Low at Low RPM

The operating voltage of the primary control unit has dropped below 11.2 volts while the engine is operating between 512 and 1500 rpm. This triggers a CAUTION level alarm and indicates that the charging system is not maintaining adequate voltage, which could stem from a failing battery, a defective alternator, or poor electrical connections.

• 6020
• 6120
• 6220
• 6320
• 6420
• 6420S
• SE6020
• SE6120
• SE6220
• SE6320
• SE6420
• 6120L
• 6220L
• 6320L
• 6420L
• 6520L
• 6230
• 6330
• 6430
• 6145J
• 6165J
• 6180J
• 6205J
• 6185J
• 6210J
• 6M
• 5080R
• 5090R
• 5100R
• 1654
• 1854
• 2054
• 2104

1. 1

   Visually inspect battery terminals and cables for corrosion and ensure all connections are clean and tight.

2. 2

   Check the alternator drive belt for signs of wear, cracks, glazing, proper tension, and slipping.

3. 3

   Visually inspect the wiring harness connected to the alternator and starter for frayed wires, cracked insulation, or loose connectors (especially the three-pin connector on the alternator).

4. 4

   Perform a static voltage test on the battery with the engine off (should read approximately 12.6 volts).

5. 5

   Perform a battery load test by applying a load equal to half of its CCA rating for 15 seconds (voltage should not drop below 9.6 volts).

6. 6

   Test charging voltage at the battery with the engine running at around 1500 RPM (should be between 13.5 and 14.5 volts).

7. 7

   Test alternator output voltage directly at the main output terminal (BAT/B+) with the engine running (should be between 13.5 and 14.5 volts).

8. 8

   Perform an AC ripple test on the alternator output with the engine running (AC voltage should be under 0.5 volts; higher indicates damaged diodes).

1. 1

   Prepare machine for service

   Park the machine on a level surface, lower all implements to the ground, stop the engine, and remove the key. Disconnect the negative battery cable before beginning any repairs.

2. 2

   Address visual inspection findings

   Clean any corrosion from battery terminals and cable ends using a wire brush. Tighten any loose connections. Replace the alternator belt if it is worn, cracked, or cannot be properly tensioned.

3. 3

   Replace battery

   If the battery failed the load test (voltage dropped below 9.6 volts), replace it with a new one that meets the manufacturer's CCA and Ah specifications for your tractor model.

4. 4

   Replace alternator

   If diagnostic tests confirm the alternator is faulty (failed output or AC ripple test), loosen the belt tensioner and remove the drive belt. Disconnect electrical connectors, remove mounting bolts, and lift the old alternator out. Install the new alternator, secure mounting bolts, reconnect wiring, and properly tension the drive belt.

5. 5

   Verify repair

   Reconnect the negative battery cable. Start the engine and repeat the Charging System Test to confirm that the charging voltage is now within the correct range (13.5 to 14.5 volts). Clear the fault code from the control unit.