Stuck on floor vent, no mid, but defrost works?

Tiny
DOOTSIE5TIMES
  • MEMBER
  • 2000 BUICK REGAL
  • 3.8L
  • 6 CYL
  • AUTOMATIC
  • 200,000 MILES
Digital climate control module. When pushing "front" on control module, output switches to defrost vent. When cycling to "mid" on module, air outputs on floor vent, with a very small amount coming out of dash vents. Replaced control module with 2 others. Problem persists. Sometimes switching to "front"/defrost and then back to mid would correct the issue, but that is no longer the case. Checked vacuum line on firewall side coming in from the engine manifold. That line holds vacuum. The output nipple on the booster where the purple vacuum line plugs in also holds vacuum. However, when unplugging the purple line which then goes into a distribution "block" and testing it, it does not hold vacuum. I visually inspected the purple line and didn't see any cracks/holes. At this point I'm stuck and have no idea how to proceed. Does this car have a mode door actuator and if so, where is it located?
Friday, November 1st, 2024 AT 3:53 PM

3 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,512 POSTS
This system is vacuum operated, so it looks like you lost vacuum going to the HVAC controller. To confirm you can remove the HVAC control head and test for a vacuum with the engine running. Here is a guide to help and the vacuum line diagrams so you can see which line to test for the source vacuum from the engine.

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

Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.
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Friday, November 1st, 2024 AT 6:02 PM
Tiny
DOOTSIE5TIMES
  • MEMBER
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There is no vacuum line attached to the HVAC controller head.
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Friday, November 1st, 2024 AT 7:08 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,512 POSTS
It shows vacuum lines going to the HVAC control module and to the mode actuator? Check out the images (below). Let us know how it goes.
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Saturday, November 2nd, 2024 AT 9:45 AM

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