1999 Questions for all other cars RPM down while switch on t

With out the ac on the engine idles about 1000 rpms and when you turn the ac on the engine goes down to about 500. This is a normal thing. When the ac is off there is no drag on the engine so it runs a little faster. When you turn the ac on the compressor turns on using the engines power to turn the compressor. This is more common in 4 cylinder engines. You can see this while you drive the car and turn the ac on the car looses a little power. Especially noticable when passing someone and the compressor turns on.