Nissan Repair: idle problem, running down the road, stepper motor

Question
Engine will not stay idle when the ac is running.
Engine will idle when the ac is turn-off.

Answer
Danh,

If the car is equipped with a stepper motor for the fuel injection or carburetor it could be broke and that would cause the car to idle down to the point of killing the engine.  Also, if the compressor is starting to lock up this will cause the condition too.  Now, you did not say if when running down the road that the engine would keep going when the ari was turned on?  Does it lug the engine down?  If so, then the compressor needs to be replaced.