Very simple to diagnose an alternator. Use a DVM in VDC mode. Measure voltage across battery with engine off. It should be about 12 to 12.6 volts. Start engine and measure again. If the voltage is about 13.5 to 14.5 then the alternator is charging as it should. If not, check the brushes. They are very possibly worn to a point that one or both are no longer making contact.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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