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?

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1990 190E 3.0L