Charging system question W126
 

1983 300SD. Charging warning light started flickering, brighter at idle, almost no illumination at 2500 rpms. Started losing headlight power, turn signal strength, etc. on the way home. Got home, battery dead. Charged battery overnight.
This morning: Battery volts with engine off - 12.5 volts. Battery voltage with engine at idle - 14 volts. Charging light still flickering. Cleaned battery posts and terminals, cleaned wiring junction block on frame near alternator. Light on dash still illuminated at idle.
Any thoughts?

Alternator or battery. Since you get higher voltage at idle I would imagine it is the battery. You can take it to Autozone (or the equivalent) and get it tested for free.

But it should be higher voltage when the engine is running. 12V systems charge at >13 volts.

Have someone start the car as you measure voltage.If it drops below 10.5 volts.You need a battery.

The flickering light indicates that there is something wrong with the charging system. IE, the alternator or battery. The simplest thing to eliminate first is the battery. Get it tested for free. Then if it is bad replace it.

Also another simple thing to check is the belts on the alternator. They may be slipping and or loose.

The most common cause of the light flickering is worn brushes in the alternator's voltage regulating system. Easy job to replace it - used to cost about $30 for the whole thing, may be more now. Some here have changed the brushes themselves, if that is the way you go, do a search to find out where to get the brushes and how to do the job. Jim