My ML350 won't start. I have to jump the battery. Is alt or battery bad?

Before you do anything .. Strongly suggest checking out the link below: http://www.troubleshooters.com/dont_disconnect_battery.htm

Answer is Easy to Check. The Battery is only used to Start the Engine. Then the Alternator Actually runs the car's electrical systems Unless it is bad Then the car will drain power from the Battery to run the Electrical systems. TEST By: START the Car with a Battery that you know is good or a Fully Charged battery Booster. NOW: With the Car Running, Remove the Negative Lead from your Battery. If the Car stops running then your Alternator is BAD. If it Keeps running like nothing happened when you removed the Negative lead from the battery then the alternator is Good, and your Battery is Bad.

that would have been a mess! SO many computers on that car that could/would have fried and the car probably would have never been right again! i'm glad you posted that link,and hope the OWNER saw it and copied and pasted it before doing that! i have done that before, but it was in "1973", better NOT do it on todays vehicles...