what is the standard alternator current?
 

car is a 1990 190e 2.6 with a standard 80 amp bosch alternator. when running the car I test at the battery at 13.6, but when I turn on the a/c, high speed aux fans, and rear window defroster the voltage drops down to 11.5. is this normal or should the current be maintained at 13+ or does the voltage regulator need to be replaced?

Normal "voltage" readings...to measure "current" you need an ammeter.

Even with a new voltage regulator, I think with that kind of full load you can expect to see a voltage drop of some level. Generally as long as it doesn't drop below 12.6 when the engine is above idle I think you would be OK unless you do a lot of stop/go/sitting in traffic where the engine would be at idle, sitting at <12V. That will eventually kill the battery.

I upgraded my 190E's alternator to a 143 amp unit and have not had a power supply problem since. My 300D has a 150A unit as well.

Also you'll need a clamp-on ammeter to measure this level of current. Your meter can only do up to 10A.
http://www.amazon.com/Triplett-Clamp-On-Resistance-Temperature-Frequency/dp/B006GD0LC2