Q: hi. 2002 mb c240. light came on for visit shop battery/alternator. Battery is new. how to know if alternator is bad?

Hello, thank you for writing in. The alternator can be tested by checking the voltage coming from it while it is running with a voltmeter. You should also check the voltage when revving the engine to 2000 RPMs. The alternator can also be removed and tested at most local automotive parts stores. There should also be a gauge on the dashboard that shows the battery voltage gauge to tell you what the alternator is putting out. You want the output to be 14 volts. The issue with the headlights likely revolves around the fact the vehicle will shut down certain systems to maximize what battery life it has. For more help with testing, contact out service department about scheduling an appointment.