HVAC Vacuum hose diagram?

Tiny
WHITAKER52
  • MEMBER
  • 1987 CHEVROLET SUBURBAN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 218,000 MILES
I took off all of the hvac ducting and went through all of the hvac choices to watch the action of the blending doors. They all are functional. However, the middle door, controlled by the heater/ac actuator with 2 ports, moves in exactly the opposite way that it should given the hvac selection. When choosing ac, the door closes off the passage to ac ducting, opening the passage to heat ducting. Hence, ac comes out the heater openings. When choosing heat or defrost the middle door opens to ac ducting, closing off heat passage. Hence, heat would come out ac vents and no defrost. The defrost actuator, with one port, operates correctly given hvac choices.
I have found only one decent vacuum diagram. It is for a 1982 K25 with ac. It shows the blue and tan vacuum hoses on the heater/ac actuator switched between the 2 ports from the way I installed them.
So, does it look like I need to switch those 2 hoses to get the middle blending door to work correctly with hvac choice?
Thanks for your help.

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Friday, August 1st, 2025 AT 3:37 PM

5 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
It does sounds like two of the lines are mixed up. Here are the vacuum line diagrams so you can compare the check the routing. Check out the images (below). Please let us know what happens.
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Saturday, August 2nd, 2025 AT 9:44 AM
Tiny
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Thanks for the diagrams. The leftmost one is the most helpful. However, if one were to take as gospel the port location for hose 3, mine is not oriented on a side (or "face") of the actuator. Instead, my port for the blue hose is on the left side "edge" (non-flat) portion of the actuator. The tan one is on the backside flat "face" of the same actuator, as shown in the diagram. I realize I may be expecting too much specificity from the diagram. That aside, I do remember that looking from the passenger seat towards the engine bay, the blue vacuum hose was to the left of the tan one. This seems to agree with the diagram. If that is the case, I would assume that I must have made a mistake upon reinstallation even though I was trying to be careful. However, is it possible that when I removed the metal axis rod, upon which the errant blend door rotates around, out of the distribution box to get enough clearance to remove the heater core, that the door rotated 180 degrees and then I reinstalled the metal axis rod without realizing the rotation of the blending door? Then, even though the vacuum hoses may be connected correctly the door would operate in exactly the opposite way it should be?
I will be trying to uninstall the heater distribution box this coming Monday so can check arrangement of vacuum hoses and let you know the results. I am very interested in your answer to the "rotating" door question though.
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Saturday, August 2nd, 2025 AT 7:23 PM
Tiny
KEN L
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O ya, if you get the arm on 180 out it will operated in the exact opposite of what it should. That should fix it.
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Sunday, August 3rd, 2025 AT 11:59 AM
Tiny
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Well, all of the problems were my fault. Even though I knew the correct placement of the 2 vacuum hoses on the ac/heater vacuum actuator, for some reason I switched their locations upon installation (obviously not paying attention). So upon reswitching to correct location, blue on the left-most port, tan one the right-most port, everything works as designed.
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Monday, August 4th, 2025 AT 8:18 PM
Tiny
KEN L
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It happens, don't be to hard on yourself. Glad you could get it fixed, thanks for letting us know. Please use 2CarPros anytime we are here to help.
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Tuesday, August 5th, 2025 AT 8:33 AM

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