John Deere 158.03 – CCU, CAB, PTQ, ASU, or EPC: Switched Supply Voltage High

The control module is receiving an ignition supply voltage that exceeds the normal operating threshold (over 16.0 volts). This is almost always a symptom of a charging system failure where the alternator is overcharging the electrical system.

• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 6230R
• 6250R
• 7720
• 7820
• 7920
• 8130
• 8230
• 8330
• 8430
• 8530
• 9430
• 9530
• 9630
• 331G
• 333G
• 204L
• 210L
• 304L
• StarFire 6000 Receiver
• John Deere Rate Controller 2000

1. 1

   Visually inspect around the batteries and the alternator for any signs of physical damage, loose connections, or accumulated debris.

2. 2

   Test the static battery voltage (engine off) to ensure the batteries are not damaged or boiling over from being overcharged.

3. 3

   Start the machine and use a multimeter to measure the voltage at the battery terminals. If the reading is above 15.5V - 16.0V, the alternator's internal voltage regulator has failed and is overcharging the system.

4. 4

   Check the machine's charging system wiring harness and connections to ensure there are no shorts to power causing the voltage spike.

1. 1

   Disconnect Power

   Disconnect the negative battery cable(s) to ensure safety before working on the charging system.

2. 2

   Remove the Alternator

   Relieve tension on the serpentine drive belt and remove it from the alternator pulley. Disconnect the electrical harness and the main power cable from the rear of the alternator. Remove the mounting bolts and extract the alternator.

3. 3

   Replace the Alternator / Voltage Regulator

   Because the voltage regulator is typically integrated into the alternator on these machines, the standard repair is to replace the entire alternator assembly.

4. 4

   Reinstall

   Mount the new alternator, torque the mounting bolts to OEM spec, reconnect the electrical connections, and reinstall the drive belt.

5. 5

   Test and Clear Codes

   Reconnect the batteries. Start the tractor and verify with a multimeter that the charging voltage is now stable (typically between 13.5V and 14.5V). Use a diagnostic tool to clear the 000158.03 fault code from the CCU and any other modules that logged it.