The normal airflow configuration is to allow outside air into the car at all times except when the RECIRC function is set and, even then, the system will only allow minimum RECIRC until the system balances out. There are 6 or more ambient, cabin, duct and heat load sensors in the system. One glitch anywhere and things go wrong. Outside air is also heat controlled even with the system OFF. (A/C does not run, of course, but the system still tries to maintain the last temp settings.) Air enters the system, moves through the cabin, then into the trunk and out the two flapper vents in the lower inner rear fenderwell quarter panels.
I think there's a cabin air temp sensor in the pre-2000's, but the intake is up in the center of the dash area - at the center dash vents - instead of a little grille just left of the steering column. If that sensor (thermistor) is covered with dust then any temp balance compensation is slow.
Read your owner's manual for settings and basic operation, then go through your GM Service Manual for HVAC system diagnostic trees.
The doors and diverters are inside the HVAC stack itself - for replacement the entire dash, pad, and console front must be removed. The blend door motor controls are on the side of the HVAC stack in front of and beside the glove box - it needs to be removed. The rear console diverter door is down lower, near the passenger's left calf.
I think there's a cabin air temp sensor in the pre-2000's, but the intake is up in the center of the dash area - at the center dash vents - instead of a little grille just left of the steering column. If that sensor (thermistor) is covered with dust then any temp balance compensation is slow.
Read your owner's manual for settings and basic operation, then go through your GM Service Manual for HVAC system diagnostic trees.
The doors and diverters are inside the HVAC stack itself - for replacement the entire dash, pad, and console front must be removed. The blend door motor controls are on the side of the HVAC stack in front of and beside the glove box - it needs to be removed. The rear console diverter door is down lower, near the passenger's left calf.
