2001 Cadillac Concours

Blowing heat when set to 90 is normal, it should still be warm at 83 considering what the inside temperature is. I would check for a possible thermostat that is stuck open not allowing full operating temperature to be reached. Try also pressing the AC Off button, and see if the temperature gets a little warmer. A moderately warm heater air can be cooled more with refrigerant flowing through the evaporator core, and when less than max heat is selected the blending with cool air will have a noticeable effect. This would suggest a possible thermostat stuck open or not regulating temperature properly.