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.

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
Hi all,
1985 c10 Silverado with AC. The compressor has been taken out so that’s not setup right now. Previous owner put a new rebuilt 355 in. All is running good, however it seems in the process, everything related to hvac under the hood is not there. Other than the blower motor and the accumulator. Inside the cab all the vacuum lines from the controller seems to be intact. So what do I need under the hood that then goes through the firewall?

I know I need to get a tee of sorts that screws in to the intake manifold behind the carb. Where do I run that line? I found the vacuum reservoir ball at the junkyard. I’m thinking I run from the intake manifold to a tee that ties in the reservoir and then where?

Any help greatly appreciated!

Shane
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
There is a vacuum line coming in to engine bay near heater hoses that needs to be connected
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,889
Reaction score
4,024
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
You must be registered for see images attach
You must be registered for see images attach

Part #3 - Dorman #47150
You must be registered for see images attach

Part #10 - Dorman #47077 or ACDelco 1572234
You must be registered for see images attach


This is for a fuel injected motor, but the bottom part of the diagram is correct for an '85.
You must be registered for see images attach


Here's the cruise control tie in.
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
This is so very helpful! Thank you so much. Looks like I need that check valve, some vacuum tubing and maybe that cruise control Servo!!
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
I have a cruise control servo for sale, but the rod on it would need to be shortened as it came from tbi truck
 

smoothandlow84

I'd rather be draggin' frame
Joined
Jan 4, 2015
Posts
2,121
Reaction score
784
Location
Arizona
First Name
Steve
Truck Year
1984 Frame...87 motor and 700r trans
Truck Model
1500 r10 pavement scraper
Engine Size
350 TBI
If installing the cruise control, you may want to also replace the switch located at the back of the brake pedal with the vacuum line attached as this deactivates cruise when the brake pedal is pressed. The compressor mounting brackets can be tough to source, but if you have a salvage yard close by, thats probably your best option.
 

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
I had not planned to worry about the cruise control yet. I've been more concerned with getting the vacuum lines sorted out for the hvac but they are certainly all inter-related. I had no idea there was a vacuum line attached to the back of the brake pedal. Is there any more parts associated with the cruise, other than the servo, the controller on the steering wheel and then the switch on the brake pedal. The servo must attach to the carb or throttle with a rod. Anything else?

yevgenievich, how much for your cruise control servo?
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
I had not planned to worry about the cruise control yet. I've been more concerned with getting the vacuum lines sorted out for the hvac but they are certainly all inter-related. I had no idea there was a vacuum line attached to the back of the brake pedal. Is there any more parts associated with the cruise, other than the servo, the controller on the steering wheel and then the switch on the brake pedal. The servo must attach to the carb or throttle with a rod. Anything else?

yevgenievich, how much for your cruise control servo?
There is a speed signal sensor on the back of the guage cluster that is used for getting speed signal. But if your truck had cruise before, it shoulf already have it. $10 + shiping for the cruise servo if you want to get it. The rod on it can be shortened to work with a carb.
 

chengny

Full Access Member
Joined
Feb 22, 2012
Posts
4,086
Reaction score
1,008
Location
NH
First Name
Jerry
Truck Year
1986
Truck Model
K3500
Engine Size
350/5.7
I had not planned to worry about the cruise control yet. I've been more concerned with getting the vacuum lines sorted out for the hvac but they are certainly all inter-related. I had no idea there was a vacuum line attached to the back of the brake pedal. Is there any more parts associated with the cruise, other than the servo, the controller on the steering wheel and then the switch on the brake pedal. The servo must attach to the carb or throttle with a rod. Anything else?

yevgenievich, how much for your cruise control servo?

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
 

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
Chengny, this is such excellent info! Thank you.

The only question I have (right now) is where does the vacuum line enter through the firewall. Is it over by the vacuum ball reservoir or is it over where the red arrow is in your picture close to the blower motor?

Thanks again for such great info!

Shane
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,889
Reaction score
4,024
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
It's by the red arrow next to the A/C box.
 

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
Does this look like the right one?

You must be registered for see images attach
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
Does this look like the right one?

You must be registered for see images attach
that is the line. Your heater controls might leak vacuum when hooked up if they have not been used for a very long time. But they the selector valve can be taken apart, cleaned and reassembled a bit tighter with a bit of silicone grease.
 

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 I cut the end of the line off (about an inch) to take it down to the auto shop to match things up. They said that it definitely was not a vacuum line. The line is tiny, maybe like a 1/16 of an inch. So I’m back trying to figure this out again.
 

Latest posts

Forum statistics

Threads
41,857
Posts
903,627
Members
33,370
Latest member
mitchell1128
Top