John Deere 158:

Test System Voltage: Start the tractor and use a multimeter to measure the voltage directly at the battery terminals. If the reading is consistently above 15.5V (often spiking to 16V–18V), the alternator is overcharging the system.

Inspect Alternator and Battery: Visually inspect the battery and alternator for physical damage, loose connections, or heavy accumulation of debris that could cause grounding issues or short circuits.

Check Grounds: Verify that the main chassis, engine, and cab ground cables are clean and tight. A floating ground can sometimes cause voltage spikes.

Check for Cascading Codes: High voltage often triggers secondary fault codes across multiple modules (e.g., CAN bus dropouts, sensor faults). If multiple codes share the same timestamp, prioritize fixing the voltage issue first.