Q? Vacuum lines for A/C-Heater...

[Stepside81]

New member, first post...so grateful to find this site!

1981 Chev Silverado. Leaking heater core. Had to pull under dash plastic distribution box out. Original, very dirty so I'm trying to clean it up, replace heater core and "refresh" with new seals, etc.
Several vacuum lines broken/missing?? Checked Factory Manual. Manual illustrations show more vacuum lines than I have. I have 5 vacuum lines coming out of control on dash. Factory manual shows I should have 7 connections... (6 actual vacuum lines, black, blue, tan, red, orange, pink....... as the black tees off and connects twice to back of switch)... at back of switch, I only have 5. I'm missing the red and pink lines.
Checked out diagrams for 1980 and 1982. All the same.
Anybody out there who has experience with these systems? And no, nothing ever seemed to work exactly right, but I would get Heat and A/C, but the Defrost was always "weak", at best. Any help/info would be very appreciated.
..........Thanks in advance, Dominick.

 

[Charlie]

 

[animal]

 

[75gmck25]

Those vacuum line pictures are even more confusing than a wiring diagram. My book also has a table that is supposed to show how the vacuum controls work, and its really hard to figure out.

The lines you need to have are:
Vacuum in from the reservoir under the hood
Vacuum lines to four vacuum valves (as GM calls them)
1- Air inlet valve (or kick pad recirculate valve for Max A/C)
2 - Plenum valve (connects to the plenum for outside air intake)
3 - A/C or heater mode valve (has two connectors)
4 - Defroster valve to the recirculate door in the left side kick panel
Valves 1 and 2 work together to go from standard mode to recirculate.
- If there is no vacuum on the orange line (fresh air), the kick pad valve has no vacuum and it snaps shut. The plenum valve also has no vacuum but it snaps open.
- Vacuum on orange line (recirculate), kick pad has vacuum and is pulled open, and the plenum valve has vacuum and is pulled shut.

These are the vacuum lines at the controls in my '75. There is no Red line.
Gray line - to ports 9 and 9A, providing vacuum from reservoir
Black line - loops between ports 4 and 4A
Orange line - To air inlet and plenum valves (connects to both #1 and #2 valves)
Tan line - to A/C valve (#3 valve)
Pink line - to heater mode valve (#3 valve)
Dark blue - to defroster valve (#4 valve)

Your vacuum lines are probably a little different than mine because of how the Max A/C works on my controls. I don't have a separate max or recirculate position on the mode control slider. When I slide the temperature control to the far left it will go pass a small detent mark, and that will kick in the max/recirculate mode and turn the fan on high speed if the mode control is also set to A/C. On the newer trucks they added a Max A/C position as part of the mode control.


Bruce

 

[Stepside81]

Hey Bruce,

THANKS! for your reply. After a bit more scouring the manual I realized the back of the controller I have is different than the one in the manual shown for "C" and "K" models. Apparently the controller I have is designated for a "G" model vehicle (whatever a "G" model is???) and the schematic shows 5 vacuum lines (as I have) BUT the heater box I have IS the one shown for "C" and "K" models and vacuum pots in the "G" schematic does match with what I have under the dash AND I don't know enough about any of it to "figure it out".............

One Q: How many vacuum lines come off the "black ball" vacuum device mounted next to the fan on the firewall?

How many vacuum pots do you have? The factory schematic for "C" and "K" models show 4 pots. The "G" (Fig. 1B-9 Vacuum Schematic-G Models) model diagram shows 3 pots, which is what I have.
This rig I have is designated a 1981 model in the VIN# BUT when I did the brakes, 1981 parts did not fit, I had to use 1980 parts. Other oddities exist. I think this was built very early in the 1981 model year and GM used up various "leftover" previous year(s) parts.

As in the G diagram, my controller has a total of 9 "ports" at the back but in the "C" and "K" diagrams the controllers shown have 11 "ports".

And as you wrote, trying to read these diagrams is difficult because the don't have enough detail.

Someone who really understand these systems might know what is going on. I considered ordering a new controller but then I'm stuck with the "G" model configuration for the vacuum lines at the point of connect at the back of the selector.......

Any ideas-suggestions--comments-----anyone?

 

[75gmck25]

The G model is the step van, so it seems to be a cross between a van and a truck chassis. Based on my repair manual, the HVAC system seems to have quite a few differences.

Bruce

 

[Stepside81]

Howdy Bruce, I'll keep looking around and try to get it worked out. Thanks for that clarification.

 

[75gmck25]

I will continue to work on figuring out a simple way to explain how the controls for the HVAC doors work, since I really need to understand it for my own truck.

I was able to get the system recharged and working well last summer, but I'm not sure all my controls really switch like they are supposed to. For example, I've had a problem with the clip on the recirculate door linkage since it seems to pop off about half the time I put it into recirculate mode. Its hard to tell what might be coming apart on the other vacuum motors that are up under the dash where I can't easily see them.

FYI - you can still buy sections of GM hard plastic vacuum line from either Autozone or Advanced Auto parts, and then use short sections of rubber line to splice it into the system. The hard line is a lot easier to fit and route up under the dash.

Bruce

 

[Stepside81]

Hey Bruce,
Thanks again for your reply. Good to know the vacuum line is still available. I'm going to study the diagrams again in the shop manual and try to get it connected as per factory specs before I start playing around with modifications.
This whole "project" started several months ago when I had a new Jasper rebuilt engine put in the truck. Apparently the higher pressure blew out the old heater core. I removed the heater box in the cab and found vacuum lines just hanging, unattached. I'm usually pretty careful to label things as I take it apart, etc.
I did find on this site another post where a man posted lots of pictures of the heater box at various points of disassembly. I'm trying to study those pictures and see what I can figure out from that.
If you have access to a Factory manual, there are "flow charts" for the various positions that detail what should be working at each setting of the dash controller, again, the chart is different for the "C" & "K" model and the "G" model has it's own flow chart.
Wish me luck and best luck to you too in figuring out the mess, HA! My best, Dominick.

 

[Frankenchevy]

Correct me if I’m wrong, but the G model is a full size van. The P model is a step van. Not sure if that will help the OP or not.

 

[Stepside81]

Thanks! for that clarification. I think your "Frankenchevy" is spot-on!
This '81 p/u I have is absolutly a FrankenTruck!!.......seems to have been built with a mess of leftover parts. The brakes require 1980 parts, the A/C Heat controller is from a Model "G" and on and on.......

 

[Stepside81]

Hey Bruce,
I keep going back to the Factory Manual and studying the schematics.

In the "G" design no plenum vacuum pot is shown and my truck does NOT have a plenum vacuum pot.....at least I can't find it.
Where is it in your truck? Do you know? Have you ever torn into your system??

In the diagrams/schematic for the "C"/"K" C60 System/series there are 4 vacuum pots. In the "G" series I can only find 3 vacuum pots........exactly like what I'm seeing in this mess I own.

All I can see from the diagrams is the plenum pot is located somewhere in the cabin. Looks like somewhere on the upper right side.

And I'm not counting the "Master" vacuum control located on the engine firewall in any of this.

Nick.

 

[Stepside81]

Hey Bruce or anybody else who might look at this thread: I need to run this truck and I'm not going to try and rush this heater box fix.

What would be the best way to "TIE OFF" or otherwise connect the two heater lines under the hood? A 1/2" piece of copper tubing is too big to easily fit the 5/8" line, I could probably force it and too small for the 3/4" line, I don't think a clamp would seal it reliably. A piece of red PEX is too small overall.

Do you think if I folded the ends of the hoses over themselves and then really tighten down the hose clamps it would hold pressure/antifreeze?

Any other ideas to connect the hoses together or other easy/quick fix?

Thanks in advance for any ideas/suggestions.....Nick.

 

[Frankenchevy]

Can you run from water pump directly to the manifold? You should be able to find a threaded fitting with a barb in the right size...

 

[Dougnsalem]

Hey Bruce or anybody else who might look at this thread: I need to run this truck and I'm not going to try and rush this heater box fix.

What would be the best way to "TIE OFF" or otherwise connect the two heater lines under the hood? A 1/2" piece of copper tubing is too big to easily fit the 5/8" line, I could probably force it and too small for the 3/4" line, I don't think a clamp would seal it reliably. A piece of red PEX is too small overall.

Do you think if I folded the ends of the hoses over themselves and then really tighten down the hose clamps it would hold pressure/antifreeze?

Any other ideas to connect the hoses together or other easy/quick fix?

Thanks in advance for any ideas/suggestions.....Nick.

A Google search took like a minute....

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