HVAC vacuum diagrams?

Tiny
DALE OWEN
  • MEMBER
  • 2001 FORD F-150
  • V8
  • 2WD
  • AUTOMATIC
  • 135,000 MILES
While AC is running, when I accelerate the blower will switch from the vents to the defrost vents, then back again when I let off the accelerator. What is causing this problem and is it expensive to repair?
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Friday, August 15th, 2008 AT 1:36 PM

5 Replies

Tiny
MERLIN2021
  • EXPERT
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! Mot expense, usually the check valve or tank...here's a vacuum schematic:


https://www.2carpros.com/forum/automotive_pictures/62217_F150a_2.jpg



https://www.2carpros.com/forum/automotive_pictures/62217_F150b_2.jpg

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Thursday, July 30th, 2020 AT 11:49 AM
Tiny
TOM SZCZEPANIAK
  • MEMBER
  • 2000 FORD F-150
  • 4.2L
  • 6 CYL
  • 2WD
  • MANUAL
  • 137,000 MILES
The blend door actuator(s) has apparently stopped working. I get only intermittent air flow through the dash vents. How many of these actuators does my truck have?
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Tuesday, April 20th, 2021 AT 9:35 AM (Merged)
Tiny
STEVE W.
  • EXPERT
Blend door controls the air temperature. Mode doors control where the air goes.
If you cannot select hot/cold it is the first one, If you cannot select floor/dash/defrost it is the second.

In the case of an F150 you have a total of four vacuum actuators. One controls the fresh air/recirculate, one controls temperature and two are mode control.

All can be tested with a hand held vacuum pump at the back of the control panel.
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Tuesday, April 20th, 2021 AT 9:35 AM (Merged)
Tiny
TOM SZCZEPANIAK
  • MEMBER
I do notice air flow through the dash board defroster vents and air flow through the vents down near the gas pedal when the switch is set on defrost or floor/defrost.
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Tuesday, April 20th, 2021 AT 9:35 AM (Merged)
Tiny
STEVE W.
  • EXPERT
That sounds like the panel/defrost door is not operating properly. It is on the top left of the image. Connector 6 red on the control head. It could be a bad vacuum line or bad control itself. As you say it is intermittent, I would bet it is a bad line to the control. Sometimes it gets vacuum and other times it does not.
The control should be visible if you look up at the heater box from under the drivers side of the dash. The line should be easy to spot as well.
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Tuesday, April 20th, 2021 AT 9:35 AM (Merged)

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