Here is a post I made years ago that covers the 1997 HVAC system diagram & layout:
Your details on what color goes where do not match up with the factory data in the quote above. The gray line would be the one that goes to the engine manifold source (including the vacuum reservoir/ball under the hood). If you do not have any engine manifold vacuum reaching the HVAC controls, it would default to the positions shown in the layout; that being defrost & floor heat.

Black & Brown are two sides of the lower mode door actuator. If you push smoke into one and get it out the other as in air flows rather freely between the two, then the actuator is shot. If rather, air does not flow freely between the two, but flows out for a time then stops you could just be pushing the air out of the open side of the actuator while pressurizing the side you are blowing into. This actuator is a dual action actuator. The HVAC control module vents one side and pulls a vacuum on the other to move the door one way, then will reverse that to move it the opposite way. When both sides are vented to atmosphere, the door should be in the middle (neutral) position.

 

My experience is only with older models but...my vacuum is drawn off of the manifold up top and if I try to draw a vacuum in the direction you have stated here, I can't get any because it draws air up out of the manifold. Can you block the other end of the hose with a nail or something then test the hose for vacuum?

 

Thanks,
Yeah...I was working off that diagram for a 99', and assumed black was the main vacuum line.
The grey holds vacuum fine.
So the blue runs the heat/defrost actuator, and the black/brown run the center duct actuator?

Looks like an adventure coming! Good thing I work cheap.

 

I don't know what works what, I never had to deal with that. Is there one main line feeding the HVAC controller? If so, can you apply vacuum to that from the outside and see what you get?

 

There should be a check valve in the line somewhere between the engine and the first tee. The check valve should not allow air to flow from the engine into the vacuum lines if it is present, installed correctly, and functioning properly. This is what keeps the vacuum powered accessories under vacuum while experiencing low manifold vacuum, namely during wide open throttle in which case the reservoir stores enough for those transient occurrences. The check valve should be installed in such a way that air only flows towards the engine intake manifold connection.

If the grey holds vacuum fine, then everything on the engine side is doing what it is supposed to do. You'll have to check whether you have leaks on any of the other 4 lines to see where your problem lies. You could also have a leak in the HVAC controller itself. It's mostly plastic (if not all plastic) and rubber seals and after 25-ish years of service they sometimes give up.

 

Yeah...I did just that...blocked the line from the engine compartment and tried to draw a vacuum from the cabin at the mode switch.
I was thinking the vacuum source was the black tube, because it starts out black under the hood and must change to grey somewhere, and the diagram for the 99' has it black all the way to the mode switch.
The grey tube tests good.
Seems I have 1 or 2 leaking actuators.
No wonder the heating seemed kinda vague. A lot like the steering.

Thanks!

 

I just started chasing my vac system last week and replaced the top hoses. If there is a checkball on mine, it would have to be in that first T, I'll have to look into that. Thanks

 

It looks like I have at least one bad actuator (the one on the black/brown lines) and a leaky blue hose or a bad actuator.
It doesn't help that AllData uses different names for different actuators that don't track over to the component locators.

What do I need to do to access these actuators?
Thanks!

 

I'm not finding anything that looks like a check valve. I'll snoop the webs and see what I can dig up. Any tips on what it looks like?

ETA: Nevermind, I see it. mine is built into a T and non-functional.

 

Yup, you found it. It is often part of the first tee in the system:

AC Delco 15-50568