HVAC Vacuum setup

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,819
Reaction score
3,400
Location
Place
First Name
Name
Truck Year
Year
Truck Model
Model
Engine Size
Size
It is a tiny line. I believe it supposed to have a rubber adapter on the end to match other lines.
 

turtle1173

Junior Member
Joined
Jul 12, 2018
Posts
19
Reaction score
3
Location
Western KY
First Name
Shane
Truck Year
1985
Truck Model
C10
Engine Size
355
Well if you are sure that’s the one I need to tap into, then I will keep pursuing it. They don’t have any vacuum lines that small. How can I piece this together?
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,819
Reaction score
3,400
Location
Place
First Name
Name
Truck Year
Year
Truck Model
Model
Engine Size
Size
The hose slips over it.
You must be registered for see images attach
You must be registered for see images attach
 

turtle1173

Junior Member
Joined
Jul 12, 2018
Posts
19
Reaction score
3
Location
Western KY
First Name
Shane
Truck Year
1985
Truck Model
C10
Engine Size
355
Yeah, it looks like that has to be it. Here’s a couple better pics of mine.
You must be registered for see images attach


You must be registered for see images attach
 

erv1994

Junior Member
Joined
Mar 10, 2024
Posts
9
Reaction score
0
Location
Los Angeles
First Name
Estevan
Truck Year
1986
Truck Model
c10
Engine Size
305
Other than sharing a common source of vacuum to operate their respective controls, the HVAC and CC systems are entirely independent of each other. You might want to consider tackling the re-activation process one system at a time. Say you just want to re-establish the vacuum supply to the HVAC control system for now - and then at some point, want to get the CC system up and running - all you need to do is to tap into the common vacuum source. That only requires the installation of a tee and a short length of vacuum hose to the CC servo.

Connecting the HVAC controls to the vacuum source at the engine is simple enough.

The HVAC control system has only one section of tubing (gray) that passes through the firewall and enters the engine compartment. The engine side is connected to the primary vacuum source. The source supply can be taken directly from the intake manifold or any unported tap on the carburetor.

Whichever source you decide to use, that vacuum is applied to port 1 on the HVAC controller valve.

Although there is only that one section of tubing that leads from the controller and passes into the engine compartment, once on the engine side it has a branch line. The branch line is connected to the main line with a tee (or Y fitting) and is lead over to the vacuum reservoir tank.

Another critical component is the check valve. It is inserted into the main line. Be aware that it must be located in the section of tubing that runs between the vacuum source and the point where the line to the vacuum tank is branched off.

A couple of images showing what I mean. Blue is vacuum source applied to both the controller and tank, yellow is flow in and out of the tank (explained below) and red is vacuum (from either the tank or manifold) applied to the controller:

You must be registered for see images attach


You must be registered for see images attach

Description of vacuum tank and check valve operation - in response to a question of whether the check valve is necessary:

If that check valve is damaged, does it absolutely have to be replaced, or can you just do a straight shot of vacuum line?

You could do that, but mode control of the HVAC would suffer during periods of low manifold vacuum (e.g. heavy loads/WOT).


The function of the check valve (and the reservoir tank) is to allow for proper operation of the mode control even when no vacuum source is available from the engine. They work together to generate and store a supply of vacuum.

When a deep vacuum is available from the manifold, it is used to operate the diaphragm controlled dampers. The tube that supplies manifold vacuum to the controls is tee'd off in the engine compartment. One side of that tee is lead over to a vacuum tank mounted on the firewall. When manifold pressure is low, a negative pressure is also developed in the vacuum tank. There's also a check valve in the section of tubing that runs from the manifold to the tee. Under ambient conditions, it is held open by a light spring. The spring allows any negative pressure that exists in the manifold to become common with the HVAC control system. Under cruising conditions, that common negative pressure rises and falls slightly - along with any minor fluctuations in engine load/manifold pressure.

But when a heavy load is suddenly applied and the manifold pressure shoots up - the check valve snaps shut. The seal created by the check valve isolates the two sides of the system. On one side is the vacuum stored in the tank (along with any vacuum currently in the control system). On the other side is the higher pressure air in the hose that is connected to the manifold. Without that check valve installed, the higher pressure air on the manifold side of the system would be able to rush in to the HVAC side - operating vacuum would be lost.

The volume of the tank is such that, with the check valve closed, it is able to supply the HVAC controls with sufficient vacuum for full operation. How long it can do that depends on the degree of leakage in the control tubing. But it is designed to provide operation for a reasonable amount of time.

When you let off the accelerator, the throttle plates close down and the pressure in the manifold starts to drop. Now, with the throttle plates trimmed down, the pressure in the HVAC control system eventually rises above manifold pressure (or the manifold pressure drops below the HVAC pressure - whichever way you want to look at it). Either way, at some point the check valve opens. When that happens, the higher pressure air in the control system will begin to flow towards the engine. Eventually the pressure differential between the two systems will reach equilibrium - at manifold pressure. After that, the HVAC side will kind of float along with the manifold side - until the next low engine vacuum event.

Sometimes - because the HVAC controls are operated by vacuum rather than pressure - it's difficult to visualize the direction of flow. Also, in theory there should be no net flow, just the back and forth motion of the air in the tubing. It's kind of like AC power. The only actual flow of air - through the vacuum supply hose to the manifold - is what leaks into the system on the HVAC side. The arrow on the check valve should point in the direction of flow; towards the intake manifold.

Vacuum schematic for HVAC controls applicable to your model year:

You must be registered for see images attach
Thanks for the information. If I’m not mistaken, I don’t need a separate vacuum reservoir for The HVAC controller? I have an 86 c10 305 that has one vacuum reservoir on the drivers side and I’ve been told that it’s for Cruise control. Can that vacuum reservoir be used for both the HVAC controller and Cruise control using a T tap? Or should I install another vacuum reservoir strictly for the HVAC controller
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,236
Reaction score
2,160
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
The small diameter vacuum line is sold in two sizes, and it’s often referred to as GM hard vacuum line. Parts stores used to stock it, but I’m not sure they still do. Dorman 47417 1/8” line is an example. https://www.dormanproducts.com/p-27841-47417.aspx

It’s easy to use small sections of regular rubber vacuum line to splice the hard line. Just push it on over the end of the hard line. FYI - There are multiple hard lines that run across behind the dash to reach the controls. Replacing complete lines with rubber vacuum lines is usually not feasible because you just can’t easily snake it all through.
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,913
Reaction score
4,114
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
Thanks for the information. If I’m not mistaken, I don’t need a separate vacuum reservoir for The HVAC controller? I have an 86 c10 305 that has one vacuum reservoir on the drivers side and I’ve been told that it’s for Cruise control. Can that vacuum reservoir be used for both the HVAC controller and Cruise control using a T tap? Or should I install another vacuum reservoir strictly for the HVAC controller
HVAC and cruise control use the same vacuum canister that originates on the driver’s side. Here’s a diagram. Disregard the area on the left side in the box, it doesn’t apply to your truck.

You must be registered for see images attach
 

erv1994

Junior Member
Joined
Mar 10, 2024
Posts
9
Reaction score
0
Location
Los Angeles
First Name
Estevan
Truck Year
1986
Truck Model
c10
Engine Size
305
I promise this is my last question on this issue lol. I’ve been trying to understand the diagrams but the CC servo diagram throws me off. I am trying to figure out where to attach my AC vacuum line. On my current system

Blue line goes to Vac reservoir
Yellow line goes to Cruise control Servo
Red line goes to Intake

1. Can I Just disconnect the yellow line from the Cc servo and connect it to the ac vacuum line since I won’t be using CC?

2. Or can use a Tee connector within the blue line and connect the AC vacuum there?

Thank you for your help. god bless
 

Attachments

  • IMG_4474.jpeg
    IMG_4474.jpeg
    132.2 KB · Views: 13

Latest posts

Forum statistics

Threads
43,975
Posts
946,899
Members
36,029
Latest member
ChorizoBang
Top