Vents stuck on defrost

MARK MULDER

MEMBER
1997 CHEVROLET LUMINA

The air conditioner/heating system is stuck on bi-level. I can change the temperature and the fan speed, but not how the air is coming out. It is always on defrost and feet (bi-level). I was told the problem is probably the mode door actuator, so I bought the part and attempted to remove the instrument panel carrier. I was able to remove everything until it came to the last step (#24) in the repair manual, which was to remove the instrument panel from the vehicle carrier. I just could not figure out how it was held in place. There may have been some clips up near the windshield, but was not sure. I am on the right track to replace the mode door actuator to repair the problem? If yes, how do I remove the instrument panel form the vehicle carrier?

5 Replies

BLACKOP555

MECHANIC
10,371 POSTS

Usually defrost only is a sign of a loss of vacuum to the controller. I would start under the hood and verify the source hose is attached and doesn't have any breaks in it. They are usually plastic and can break from age

Here is a guide to read, it will help you figure out what is going on

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

See the diagram below for location of source hose

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Sunday, June 13th, 2010 AT 10:20 PM

SAM SARANTOS

MEMBER
1 POST
1999 CHEVROLET LUMINA
3.1L
6 CYL
2WD
AUTOMATIC
180,000 MILES

Where is the exact location on the passenger side?

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Tuesday, August 13th, 2019 AT 6:38 PM (Merged)

STEVE W.

MECHANIC
12,961 POSTS

Basically straight behind the lower edge of the glove compartment. To get to it you remove the sound insulator under the dash on that side, then remove the glove compartment and the actuator is right on the HVAC case in front of you. That is the one that controls the passenger side temperature.

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Tuesday, August 13th, 2019 AT 6:38 PM (Merged)

ONE1WAY

MEMBER
1 POST
1997 CHEVROLET LUMINA
6 CYL
FWD
AUTOMATIC
171,000 MILES

About a year ago our 1997 Lumina's heating and air conditioning air flow from the vents stopped flowing like it is supposed to. When accelerating or going uphill the air flow stops coming out of the front vents and starts coming out of the defrost vents and the floor at the same time. I am suspecting that either a vacuum or electric actuator is bad. Or that maybe a plastic damper door in the duct work that directs airflow has slipped, come loose, or just plain broke. But before I took the car apart, I wanted to ask the 2carpros. Any help in pointing me in a direction would be greatley appreciated!

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Tuesday, August 13th, 2019 AT 6:38 PM (Merged)

MERLIN2021

MECHANIC
17,250 POSTS

Find the vacuum line that comes out of the firewall, this is the supply line, and it goes to a small reserve tank in the engine compartment or under the das, then anothe line goes from the tank to a check valve, then to manifold. If the reserve tank cracks, or the supply line breaks, air will divert to the defrost vents when accelerating. The reserve tank supplies needed vacuum to A/C system during these times!