Chevrolet Repair: 03 Suburban front A/C not cold enough, rear coil, quarter panel

Question
QUESTION: I have a 2003 1500 Suburban with the automatic HVAC system.  The front A/C is not as cold as it should be.  The rear A/C vents get as cold as 47-55 degrees. The front air vents don't get below 70 degrees or so at best.  I traced the lines from the compressor to the passenger side of the firewall.  The narrower upper line is very cold to the touch, about 60 degrees, but it's hard to get a real accurate reading with all the hot air blowing around the engine bay.  I assume that's the supply line from the compressor to the evaporator coil.  The lower thicker (return?) line is warm.  It seems as if the blend doors are not closing off the heated air completely.  I can turn the temperature all the way up on both sides and the air blows hot, but when I return the temperature dials back to the lowest setting I just don't get air much below 70 degrees.  Suggestions?  Where is the rear evaporator coil located?  Are the front and rear coils in series or parallel?  I.e., does the freon for the rear coil pass through the front coil first, or are the separate?

ANSWER: HI ROB, THE REAR EVAP IS IN THE REAR INSIDE SECTION OF THE QUARTER PANEL,SOME ON THE LEFT AND SOME ON THE RIGHT SIDE. LOOK UNDER YOUR TRUCK AND FOLLOW THE LINES INTO THE REAR SECTION.YOU WILL SEE IT. YOU DO NOT HAVE A PROBLEM IN THIS AREA ANYWAY..THIS ALL SOUNDS GOOD. THE COMPRESSOR PUSHES THE FREON THROUGH BOTH THE FRONT EVAP AND REAR EVAP AT THE SAME RATE AND  TIME [SEPARATE OF EACH] THEY EACH HAVE THERE OWN LINES TO ANSWER YOUR QUESTION.. I AM ALMOST SURE THAT YOUR TRUCK HAS A PROBLEM WITH THE DASH CONTROL HEAD. VERY COMMON, MY DID THE SAME THING AS MANY OTHERS DID..THE TEMP SENSOR IS PROBABLY WORKING CORRECTLY IT IS JUST THAT THE BLEND DOOR IS NOT WORKING CORRECT BECAUSE OF THE CONTROL HEAD OR THE GEAR IN WHICH IT TURNS ON IS BAD.IT IS PLASTIC, I HAVE SEEN SOME OF THIS AS WELL. NOT VERY COMMON AS THE HEAD, BUT I HAVE SEEN IT.... UNDER DASH WORK !!! THE ONLY WAY TO REALLY CK IS TO GET UNDER THE DASH REMOVE AS MANY PANELS AS NEEDED TO SEE WHAT IS GOING ON WHEN YOU MOVE THE CONTROL HEAD BUTTONS..I HOPE THIS HELPS YOU,,TOM..LET ME KNOW IF YOU NEED MORE INFO. GLAD TO HELP..

---------- FOLLOW-UP ----------

QUESTION: Thanks for your quick response.  So are you saying you think it is the control unit with the two (dual zone) temperature control knobs and other control buttons?  The temp control knobs do work at least some because I can turn the heat all the way up, but when I turn it down the temp never goes below 70 degrees. To me that means the blend doors are working at least partially.  I would expect the blend doors to work or not work if the motor or gear failed.  I also assume there are two blend doors, one for each side that work independently.  Since the problem is the same on both sides, it makes more sense that it's the control head.  I'll look at it.

Answer
HI ROB,THE TEMP SENSOR IN THE HEAD IS USUALLY THE PROBLEM.[DUAL ZONE CONTROL] YOU MAY ALSO NOTICE IT WORKS EVERY NOW AND THEN AS WELL. AS FAR AS THE PLASTIC GEAR FAILING IT BREAKS IN ONE AREA AND GETS COCKED ON THE CONTROL STEM. THIS IS WHAT I HAVE SEEN...I WOULD JUST REMOVE THE LOWER AREA OF THE DASH UNTILL YOU START SEEING THINGS ACTIVATE OR NOT..I HAVE ALSO LOCKED THESE DOORS IN THE POSITION TO ONLY GET COLD AIR FOR THE SUMMER TIME FOR PEOPLE THAT DID NOT WANT TO SPEND  BIG $$$$$ ON THIS..LET ME KNOW.TOM..