A/C - RPM speed
 

I have a 1984 190d. up until about a month ago when i turned on the A/C the engine speed would increase. I suppose this was suppose to be normal. Now when i turn on the a/c, the engine speed stays the same. which one is suppose to be correct? anyone have a clue? with the engine speed running slower the a/c does not cool as well when sitting at a red light or stoped and the engine is idleing.

thanks for the help.