HVAC Vacuum switch testing - Please help

[robertmee]

Finally got all my vacuum leaks under control (running BL of 130 now). However, to do so, I had to completely disconnect the vaccum feed to my HVAC unit (supply line held zero vacuum). I'd like to have AC in the summer so I started diagnosing....Here's where I'm at:

Pulled HVAC dash unit and glove box. Disconnected vaccum header plug under dash and started testing:

Purple line back through firewall (supply line from check valve) holds vac
White line back through firewall (heater CV) holds vac
All other lines to flapper and vents hold vac once they move to position

So, all the lines from the one header to all devices check fine. That leaves me with the other side of the header to the HVAC vacuum switch.

Checked the purple and white pair that loops from the switch back to the HVAC...These hold vac

Checking the purple supply to the switch I hear an audible suction around the switch no matter what position I switch the heater control slider. No vac. Mind you, I'm plugging all the other ports at the header connector...I should pull vac on one of them, no?

Checking all other lines back to the switch, same thing.

So, I think it must be a bad switch. I order a new one from AA. Before putting it on, I try to suck vacuum through the various ports to see if any hold. None do and I hear the same audible sucking sound around this switch. WTF?

Can somebody please explain to me how this switch is supposed to work and how best to test it. I assumed it just routed vacuum to the various vents and if you put the switch into a particular position and closed off that port that you could pull vacuum from the supply port. Doesn't seem to be the case. Is it possible that I have TWO bad switches??

Help?!

 

[Nick Micale]

The info has me a little confused, but let me try to answer with what is usually a common problem.

I "assume" the switch you hear leaking is the rotary switch on the AC control panel? This has all the vac lines connected and does the routing for heat, AC, defrost, etc.

It is very common for this switch to come apart. If so, your local auto parts store should have a replacement, I just picked one up last week.

 

[robertmee]

The info has me a little confused, but let me try to answer with what is usually a common problem.

I "assume" the switch you hear leaking is the rotary switch on the AC control panel? This has all the vac lines connected and does the routing for heat, AC, defrost, etc.

It is very common for this switch to come apart. If so, your local auto parts store should have a replacement, I just picked one up last week.

Yes, you assumed correctly....I too picked one up from Advance Auto (AA - in my post), but before installing it, I was trying to figure out how to check it on the 'bench'. That's where I ran into trouble, because the bench top test with the new switch seems to behave just like the current switch on the HVAC head unit. It doesn't hold a vacuum when I connect my vacuum pump to the inlet and plug off all the other ports. It just gives me a sucking sound. I was trying to see if there was a more determinate method of checking the new switch and understanding its operation before I put it on.

 

[robertmee]

Any help?

 

[87NAT]

That switch is actually 2 pieces on a rotating assembly. The "veins" inside harden over time and do not let a seal take place as you rotate the switch. The new one should work as long as it is "new" and not dry rot.

 

[robertmee]

That switch is actually 2 pieces on a rotating assembly. The "veins" inside harden over time and do not let a seal take place as you rotate the switch. The new one should work as long as it is "new" and not dry rot.

That's what I thought, but the new one (it is a new part) doesn't seem to behave this way either. If I put my vacuum pump on port 1 and block off all the other ports, no matter which way I turn the new switch, it doesn't hold vacuum. It might jump to 3 or 5, but then quickly dissipates. I guess try a different switch again?

 

[Razor]

HVAC= High Voltage Air Conditioning

Your AC switch routes vacuum. Thats it pretty much. I dont know if diagrams exist on what ports behave on what position.

 

[cruzn57]

close....

HVAC= High Voltage Air Conditioning

Your AC switch routes vacuum. Thats it pretty much. I dont know if diagrams exist on what ports behave on what position.

HVAC= Heat- Ventilation- Air Conditioning

 

[robertmee]

Your AC switch routes vacuum. Thats it pretty much. I dont know if diagrams exist on what ports behave on what position.

Then maybe I'll take this opportunity to create one....I've already traced out all the colored vacuum lines. They even change colors through the disconnect connector (you'd think GM would have kept the same color).

 

[Razor]

Then maybe I'll take this opportunity to create one....I've already traced out all the colored vacuum lines. They even change colors through the disconnect connector (you'd think GM would have kept the same color).

Great idea.

 

[Razor]

HVAC= Heat- Ventilation- Air Conditioning

I stand corrected

 

[salvageV6]

There's a very clear diagram in the service manual on how it all works or is supposed to work anyway, page 8A-65-0.

Assuming the new one has 7 ports and all sealed but 1 it should hold vacuum.

 

[robertmee]

There's a very clear diagram in the service manual on how it all works or is supposed to work anyway, page 8A-65-0.

Assuming the new one has 7 ports and all sealed but 1 it should hold vacuum.

Thanks, I just happen to have the manual. I'll take a look.