Jeep Repair: OBD / Air Conditioning, grand cherokee laredo, climate control systems

Question
My air conditioning in my 03 Grand Cherokee Laredo seems to have a mind of it's own when it comes to blowing cold air.  Sometimes it's hot air when I get in the car and crank it up, but I could get in the car 1 hour later and it blows pretty cold.  Or it may start cold but then after driving for 5 or 10 minutes it gradually builds up to warm and hot air again.  I just recently got new frion, but this did nothing.  Does this have anything to do with my check engine light, which has also been on for the last month or so.

Answer
The check engine light may or may not have anything to do with the a/c working.  I know that the coolant fan is tied in to both the a/c system as well as the engine control system.  Also, when the a/c compressor kicks on you will notice the engine pick up rpms ever so slightly to accomodate the extra load on the engine.  Thus pointing out another situation where the engine controller and the a/c are tied in together.

After that things get a little more tricky.  There has been a revised HVAC housing for jeeps with dual climate control and even sometimes the single climate control systems have problems as well.  What frequently happens is that there are electronic motors that open and close the blend door.  This door is responsible for giving you the desired temperature from the vents.  When the motor fails you could have irratic operation.  When the doors break you will frequently see uneven temperatures from the left side of the dash compared to the right.  MOST of the time the temps do not change once the doors break.

I would first suggest that you find out why the check engine light is on.  It may be something simple under the hood that needs to be replaced.  If it's something inside the dash then you're looking at least 8 hours labor plus whatever parts may be needed because the doors are only serviced with the entire HVAC box removed from the vehicle.

Also, check a few of my other responses because there have been several HVAC related problems caused by vacuum lines being loose in the engine bay.
Doug