Voltage regulator or alternator?
 

Hi
Charging light is on.Voltage at battery before starting is 11.8-12v, when car is started voltage drops as expected and slowly comes up to 11.8-12v once car has started. Voltage does not change even when engine is revved, how do i know if the voltage regulator or the alternator itself is the problem without swapping parts.
Thanks

If your alternator is like those in the later 123s (meaning my '85), then the regulator is part of the alternator. There are threads here somewhere (use the search functions) that will show you how to R&R the regulator, which includes the brushes, a wear part and common failure source. Two screws, as I recall, and the regulator will pop out of the alternator.

Jeremy

A cheap way to go may be the local auto parts store, you know the ones, check your system for free.
Or, just pull the voltage regulator and inspect the brushes. That would be that white thing held in with two screws. When mine went, the dash light would glow. When the alternator itself went, you could hear a sharp, dry bearing noise.
I found this, it looks the same as mine.
http://www.***************/search/product.aspx?sid=x4kxfk55ofoywyjg1anhef55&makeid=800016@Mercedes&modelid=1193699@300D&year=1979&cid= 18@Electrical%20%26%20Vacuum&gid=2412@Alternator