John Deere 158.04 – CAB (Cab Control Unit) or ACU (Armrest Control Unit): Switched Supply Voltage Low

Triggered when a control unit registers that its switched supply voltage has dropped below 9.0 volts. This low-voltage condition indicates a problem with the power being delivered to the module, which can arise from issues with the battery, alternator, relays, fuses, or wiring harness connections. In response to this error, functions controlled by the affected module are typically disabled to prevent erratic behavior or damage.

• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 7630
• 7730
• 7830
• 7930
• 8110
• 8210
• 8310
• 8410
• 9230
• 9330
• 9430
• 9530
• 9630
• 9540
• 9560
• 9570 STS
• 9580
• 9640
• 9660
• 9680

1. 1

   Inspect the area around the batteries and alternator for any visual signs of damage, corrosion, or accumulated debris.

2. 2

   Check the machine's charging system to ensure the alternator is outputting the correct voltage (typically ~13.5V - 14.2V while running).

3. 3

   Verify voltage at the relays and the fuse box (specifically the Cab Load Center or Engine Compartment Relay Panel).

4. 4

   Check the wiring harness for chafed wires, breaks, or loose pins, particularly on the circuits supplying the CAB or ACU.

5. 5

   Disconnect the harness at the affected module (CAB or ACU) and measure the voltage at the switched power pins with the key ON. It must be well above 9.0 volts (ideally matching battery voltage).

1. 1

   Secure the Machine

   Park the tractor on level ground, turn off the engine, and remove the key.

2. 2

   Clean and Tighten Battery Terminals

   Remove battery cables, clean the posts and terminals of any corrosion, and reinstall them tightly to ensure a strong ground and power supply.

3. 3

   Test/Replace Relays

   Locate the power relays for the Cab and Armrest modules. Swap the suspected relay with a known good relay of the same type (e.g., horn or light relay). If the code clears, replace the faulty relay.

4. 4

   Repair Wiring

   If the voltage at the module connector is below 9V but the battery/relays are good, trace the wiring harness back to the load center. Repair any chafed or broken wires using proper soldering and heat-shrink techniques.

5. 5

   Clear Codes

   After repairs are made, cycle the key switch or restart the tractor. Some codes will clear automatically upon restart if the communication messages reset and the voltage returns to normal.

6. 6

   Module Replacement (Last Resort)

   If the wiring, relays, and charging system are perfect (delivering 12V+ to the module), but the module still internally registers low voltage and fails to communicate, the CAB or ACU module itself may have an internal hardware failure and will require replacement and dealer calibration.