Pontiac Repair: Bonneville climate control, temperature control unit, salvage yard

Question
I recently had another engine put in my 98 Bonneville SE (3800)and the climate control temperature readout started flashing.  The AC had only worked on floor and defrost before the engine replacement soI went back and had them replace the control unit for the climate control. Now when I turn on the car the temperature flashes for awhile and when it gets really hot outside, it switches to heat instead of AC.  If I'm parked outside and it's super hot the AC won't come on at all.  When I park in the garage and the engine isn't hot the AC works like crazy until I park in the hot sun and turn the engine off, then when I turn the car back on the temperature readout is flashing again and it is blowing hot air.  The mechanic tried replacing the temperature control unit in the dash but that didn't do anything.  Is it possible that he put in a bad control unit (it was from a salvage yard) that's occasionally not closing the door to allow heat from the engine to be blown into the car, or does it have to do with the computer not recognizing the codes in conjunction with the newer engine?  I've been having all the work done by a mechanic at a salvage yard and I'm wondering if he really understands how to reset the computer that controls it.  I read somewhere that you need to unhook the battery to reset the temperature control and I don't think he did that because the clock and radio presets were still there.  He basically threw up his hands and said he couldn't figure it out (after I paid him for replacing the control unit). I can't afford to take it to a GM dealer and I'm hoping the solution is something simple that perhaps I can fix.  I'm no dummy when it comes to mechanical things but I need instructions. [:-)   Thanks for your time and info!

Answer
Hi Debi:

well from the sounds of this and what you said you had a issue pior to the engine rpleaced but a different issue so there is an underlying problem here. the fact that the display is flashing menas there is a fault in the system some where. My guess is that there is either a temp sensor or a sun load sensor probelm with this the fact that it works fine if the car is parked in the sahde but not when it's in the sun would lead me to belive the sunlaod sensor may be the problem. That sensor is in the middle of the dash by the windshield.. You could also have a temp sensor inside the dash unit that is bad and when it gets hot it's not reading right and the control head doesn't know what to do. What this needed is someone to scan the HVAC system for codes and to see what if any codes are in the system and then go from ther instead of just throwing parts at this hopping it will eventually get fixed by trying the right part. If the control head was changed it's imperative that the same part # was on the used unit as the original. It could be a head out of a year older or year newer unit and just because they look the same doesn't mean they are inside. the program inside the control head is different from year to year. Good luck :}