loss of heat controls

i have a 1993 olds 88 royale with the automatic/electronic heat and ac controls. two weeks ago i lost my heat. i need help fiqureing this out.well here goes, this car has the controls on the steering wheel too. the heater blows cold air all the time except when i shut down and restart then it blows hot for about 90 sec. then switches to cold the hot cold air works with an elec. motor not vacumm. i tried changing the main control panel in the dash which has digital readout and the problem seems to occure after the readout changes from the outside temp to the interior. the change didnt help i also changed the control mod under the dash beside the computer, no change, as i said the control lever in this location works electronically and you can watch it flip over after 90 sec the control mod is a black box the size of the computer and i was told it is the heater a/c computer, changing this didnt help. i was wondering about the sensor that tells this unit that the interior is up to temp then slows the fan motor automatically but dont know where to locate it. im in ned of any imput that would help please!!!!! the car temp has always been normal and the heater hoses ar both hot its just the small door that keeps closeing thanks

It is the airmix door actuator most likely if it was a sudden loss. It is a pain to get to. you should be a be able to test it with a scan too.

Heater cores dont just go out so it is most likely the airmix door actuator

found this photo on the net:

http://img.photobucket.com/albums/v155/randman1/1993SSEi/DSC01066_edit.jpg