Vacuum lines for hvac system

[K5Rob]

Hey guys, 87 K5 Blazer, just did an engine swap, I have a functional hvac blower but cant switch settings (defrost etc). Then i recalled I had never hooked up any vacuum lines, and Im not sure if they were hooked up before. Could someone describe where the line/s exit the firewall and where they should go from there? I also have the black plastic reservoir sphere on the DS by the wiper motor with what seems to be only 1 line coming off the top. Could someone help me square this up?

 

[Matt69olds]

Does the truck still have the emission label under the hood?

There should be a pink and purple hose. One is the vacuum supply line, it goes to the vacuum check valve and vacuum storage tank. The other goes to the heater control valve, when the heater temp is set to max cold or max A/C, the heater control valve is supposed to to close and shut off coolant to the heater core.

Now, the problem: I don’t recall which color hose does what. Which is why I asked about the emission label under

 

[K5Rob]

Does the truck still have the emission label under the hood?

There should be a pink and purple hose. One is the vacuum supply line, it goes to the vacuum check valve and vacuum storage tank. The other goes to the heater control valve, when the heater temp is set to max cold or max A/C, the heater control valve is supposed to to close and shut off coolant to the heater core.

Now, the problem: I don’t recall which color hose does what. Which is why I asked about the emission label under

I need to go upstream of this first. Where is the line roughly that comes from engine vacuum?

 

[Matt69olds]

I need to go upstream of this first. Where is the line roughly that comes from engine vacuum?

Look next to the heater case. There is a grommet

I need to go upstream of this first. Where is the line roughly that comes from engine vacuum?

I’m pretty sure the A/C harness and vacuum lines come out of this grommet.

 

[K5Rob]

Look next to the heater case. There is a grommet

I’m pretty sure the A/C harness and vacuum lines come out of this grommet.

Okay I am familiar with that grommet, i had an unknown plug coming out (maybe it was ac) i definitely had a big plug that went to the pcm under the dash (was a tbi truck) which was no longer needed. I do not recall a vacuum line coming from that location. Guess I will have to dig around. Does this line go to the control box? Maybe i can chase it backwards

 

[75gmck25]

This is a picture of the hvac on my ‘75 GMC.
- The flat round canister up next to the a/c plenum is the vacuum reservoir for the hvac controls, and it has a rubber vacuum line running from it over (down low) to manifold vacuum.
- There is also a small diameter hard plastic vacuum line coming from the canister and it runs over the top of the plenum and through the grommet in the firewall.
- inside the cab, the hard plastic line runs across the back of the dash and over to the a/c controls.

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[edgephoto]

On an 87 the vacuum reservoir is on the left side next to the wiper motor. If you have cruise control there is a vacuum check valve Tee. I imagine without cruise it would just be a check valve one only one nipple on each side. You need the check valve so under acceleration your HVAC does not change positions when you lose manifold vacuum.

On my 1987 7.4L on the emissions related components show on the vacuum diagram under the hood (VECI label). I have not found a source for the non-emission control vacuum line diagrams. I have the factory service manual, wiring diagrams and the unit service manual. If someone knows of a source for these diagrams please help us find them.

 

[K5Rob]

On an 87 the vacuum reservoir is on the left side next to the wiper motor. If you have cruise control there is a vacuum check valve Tee. I imagine without cruise it would just be a check valve one only one nipple on each side. You need the check valve so under acceleration your HVAC does not change positions when you lose manifold vacuum.

Thanks, yes this is what I have, appears to have only one nipple and one line however. Guess i will have to look closer

 

[Broken85]

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[K5Rob]

You must be registered for see images attach

Okay so I need some line and some Tees,
Does #4 go thru the firewall and to the dash control panel?

 

[Broken85]

Yes, I believe so. I think the grommet that Matt circled above is for the wiring harness to the spark control computer from distributor, but just to the left is where the HVAC goes through the firewall.

 

[75gmck25]

FYI - Dorman sells the GM hard plastic vacuum line in small rolls. You can join/splice hard line using short pieces of rubber vacuum line. The original hard vacuum line should have a colored stripe that matches the colors in the repair manual. This makes it easier for figure out which line connects to which port under the dash.

 

[Matt69olds]

Yes, I believe so. I think the grommet that Matt circled above is for the wiring harness to the spark control computer from distributor, but just to the left is where the HVAC goes through the firewall.

Crap, I think you are right!! I was going by memory, and they say as you get older, memory is the first thing to go.

You know what they say about memory, it’s the first thing to go.

Anybody remember what the saying about memory and getting older??

What were we discussing??

 

[K5Rob]

Ok guys thanks for all the info.

So here’s my issue, I have an LS and manifold vacuum sources are limited. I have one that goes to driver side valve cover, drawn in Green. Can I just Wye here, and run my OE lines as shown in Pink? (Drawing A) This option replicates the diagram posted by Broken85

Or is the Vac Tank even nescessary? Can I wye off the manifold and run directly to the dash control box? (Drawing B)

 

[edgephoto]

The vacuum tank is needed. It is a reservoir so you have vacuum when the engine is not making vacuum. The check valve keeps the vacuum in the system and does not let it bleed off into the manifold.

That picture above is what I had found but it is not 100% correct for my 1987 V3500. The fitting in the intake connects a hard metal line to the vacuum modulator on the transmission. The metal line fits inside the fitting on the manifold.