[Sid]

I had a alternater fitted in December 2024 by rac it failed and was diagnosed as faulty they replaced it 13th of December 2024 everything was fine until February 2025 now the alternater is only putting out 8volts instead of 14 and my vehicle won't start battery and starter motor all good, local mechanic said its alternater again, what could be causing this to happen again have had no previous electrical issues, any ideas anyone

[Rolebama]

Purely a thought. Alternators used to be a more common failure years ago, so if there was a faulty batch, usually regulators, then they would be sorted relatively quickly. Now alternators are far more reliable, in the event of a faulty batch, it is possible you could go through a few before finding a 'normal' one.

[NMNeil]

@Sid It depends on the car.
Most modern vehicles have the alternator controlled by the PCM, which makes sense, as the PCM can turn off the alternator when starting to reduce drag on the engine, same goes for turning it off under hard acceleration to put more power to the wheels for a while or just before the A/C compressor turns on taking the alternator load off the engine. The PCM can also detect the voltage, current and output waveform of the alternator, letting it inform the driver if anything is beginning to fail, rather that just failing at the most inopportune time.
Alternatively they are just fitting the car with a re manufactured (AKA junk) alternator each time.