HVAC stuck on defrost and floor vents

Tiny
LIPS81
  • MEMBER
  • 1995 BUICK LESABRE
  • 3.8L
  • V6
  • 2WD
  • AUTOMATIC
  • 125,000 MILES
The heater/air conditioning is stuck on defrost and floor and will not change to any of the other vents.
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Wednesday, June 17th, 2020 AT 6:41 PM

3 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

The air flow direction is controlled by what is called a mode door actuator. On this vehicle, all actuators are controlled via engine vacuum which is supplied from the engine intake manifold via a black vacuum hose. That hose connects to the rear of the controller in the vehicle where you select air flow directions. Then, based on your selection, engine vacuum is routed to the different mode actuators which open and close different doors allowing the air to flow in different directions.

_________________________________________

Since you are getting air at both the defroster and the floor, I suspect the vacuum supply from the engine has failed. In most cases, the plastic hoses crack and break off. So what I need you to do is simply craw under the dash and locate the vacuum hoses to the rear of the controller. Locate the black supply hose and remove it from the controller. Start the engine and place your finger over the end of that black hose to see if there is a vacuum (suction) available from the engine. If there isn't, we will need to locate where the break has happened.

__________________________________________

I attached a pic below that shows the different actuators and vacuum lines. Note that I highlighted the supply. That is the black one you need to check.

Let me know what you find or if you have other questions.

Take care,
Joe
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Wednesday, June 17th, 2020 AT 7:00 PM
Tiny
LIPS81
  • MEMBER
Would it have anything to do with the plastic cover over the blower motor?
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Thursday, June 18th, 2020 AT 3:36 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
I'm not sure what you are referring to. Are there vacuum hoses going to something at that point? If it is just a cover, I don't see how that would cause this.

Joe
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Thursday, June 18th, 2020 AT 8:24 PM

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