Auto Parts: AC, most newer cars, vacuum problem

Question
My heater and AC are both operating at the same time. I have turned the temperature knop to cold but the coolant from engine keep flowing. What do I need to do fix the problem?

Answer
    On most newer cars the coolant flows through the heater core all the time at a constant rate.  Then, the heater controls regulate the temperature of the air going into the cabin by moving a door back and forth to control the proportion of heated to cooled air.  If you can sense heat in the air when the control is turned all the way to cool (and you've given it a minute or two to actually cool down), then the problem is most likely the door.  They are, in most cars, controlled by a vacuum motor.  If the controls are not correctly regulating the vacuum to the motor, then the system won't work.  If the vacuum is not reaching the controls or the motor because of a vacuum leak, then the system won't work.  Lastly, if the door has become stuck for some reason (dirt or debris in the system the most likely cause), then the system won't work.  On most cars there is room to reach your hand up above the heater box and operate the lever which moves the door manually.  If you can't move the door by hand, then it is stuck and you will have to remove the heater box and repair the fault, whatever it is.  If it moves easily, then try cycling the controls from cool to hot while you feel what it is doing with your hand.  If there is no movement, then you have a vacuum problem.  Although the controls could be at fault, I would look for a vacuum leak first.  You'll have to do this with the engine running to provide a source of vacuum.  Trace the hoses from the motor and then the controls with your fingers, looking for cracks or any feeling of suction.  The most common source of vacuum trouble is a cracked or degraded hose, but the vacuum reservoir, a small spherical or cylindrical can under the hood, can also develop a leak.  Repair is just a matter of replacing whichever part is leaking.  The vacuum motor could be leaking, as well, but that hardly ever happens.
    Of course, if you have an older car with a heater control valve (some Japanese cars still use them, such as Mitsubishi), then you need to find out why it isn't closing.  Most of these are also operated by vacuum, so diagnosis will be more or less the same as above.