Charging System Diagnosis and Repair
The charging system is an alternator, a rectifier bridge, a voltage regulator, and the wiring that connects them - plus, on modern vehicles, a battery current sensor and a PCM that manages charging strategy. The alternator produces AC in the stator, the rectifier converts it to DC, and the regulator controls field current to hold a target voltage. Normal regulated output is roughly 13.5 to 14.8 volts, varying with temperature and battery state; smart charging systems may command well below 13 volts during acceleration or above 15 volts briefly after a cold start, so do not condemn a system for a voltage that is merely unusual.
Tests
Measure battery voltage at the battery posts and at the alternator B+ terminal with accessories and a load applied. A difference over about 0.5 volt indicates a wiring or ground problem, not a failed alternator. Check AC ripple at the battery with the meter set to AC volts; excessive ripple points to a failed diode in the rectifier. Verify the field/control circuit and the voltage-sense wire - a bad sense connection causes overcharging because the regulator never sees the true system voltage. Confirm the belt, tensioner, and pulley decoupler before condemning the unit, and listen for bearing noise.
Traps
- Never disconnect a battery while the engine runs to 'test the alternator' - voltage spikes can destroy modules.
- Testing output with a deeply discharged battery gives misleading low readings.
- A glowing charge indicator can be a control-circuit fault rather than a bad alternator.
- Replacing an alternator without finding the cause of a repeated diode failure (often a bad battery or a shorted load).
Rule of thumb: if B+ and battery positive differ by more than 0.5 volt under load, fix the cable before the alternator.