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98 STS Blend Doors?

6716 Views 16 Replies 6 Participants Last post by  MoistCabbage
I have had the STS about 9 months now and having gone through summer/winter season, I think I may have blend door issues.

In the winter, the heat is working, but the car is still cold.
In the summer, the air is working, but the car is still hot.

I believe from the blend doors not closing completely and letting outside air into the cabin. Gets a little better on Recirculate, but still not blocking the outside air.

Where are the blend doors on a 98 STS? No codes.

Robert D
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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.
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To stop outside air from entering the car you use RECIRC. The little icon with a car and 180 degree turn arrow.

If the cabin air temp sensor ("aspirator") grille is to the left of the steering column, remove the grille with a paper clip hook and start the engine. Now blow out the tunnel with computer cleaner air. There's a thermistor at the tunnel entrance and further down, a board with computer muffin fan on it which fills with dust. This fan is also the occasional source of winter noise from the dash - buzzes and squeals. Several posts over the years in here and Tech Tips on how to replace the aspirator board or lube the fan.

Not sure just why you want to stop all outside air from entering - if the system is operating properly, the outside air has NO effect on cabin temperatures: The system can heat AND cool the air to uncomfortable levels with no intervention from you.............
Yeah - his description of uneven air distribution points to mode door actuators - something's either hung up OR an actuator arm is disconnected............ now I don't think the RECIRC has anything to do with this posted, regardless of how much outside air enters the car, the system will totally control airflow, temperature, and humidity if the system is operating properly.

May I suggest a subscription to ?? This is the entire GM service manual plus tons more info - and it's instant gratification: you can have the manual in front of you 20 minutes from now. All diagrams and diagnostic trees, all DTC's, all recalls.......
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