OK, I'm Completely Stumped Now!!!

[GregL]

I'm really at my wits end here guys.
I've been having some trouble with my HVAC system, getting the doors to actuate properly. Seems to be stuck on blowing out the floor vent only, no matter where the control is set. So I took it all apart, tested all the vacuum actuators and all are functioning fine.
So I reconnect the switch and move it thorough its paces, none of the doors move. I disconnect the vacuum connector and jumper each position from the hot vacuum line and every door opens as it should. Given this, it seems obvious to me that the switch is the culprit. Opened it, cleaned it re-greased it. Re-install it, no change. Still no action at any doors.
So I step up and buy a brand new (NOS) switch. It arrives today and I install it.

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Start the truck up and move the new switch through its paces.

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And I STILL have no action at any of the vacuum actuators!

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Remove the vacuum connector again and check each port with vacuum, all doors function perfectly.
This is absolutely crazy. WTF am I missing?
Again, this is the factory C60 HVAC system in my 1977 K20. The vacuum connector only uses 5 of the 9 ports. The hot vac port is at #4.

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As usual, I appreciate any and all input.
Greg

 

[Bextreme04]

When I had this issue, the connector had come off the bottom of the accumulator pot in the engine bay. Have you checked to make sure you are getting good vacuum into the switch?

 

[GregL]

I've got about 16 inches of vacuum at idle, at the vac reservoir line coming in.

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I've got the same at the vacuum connector inside.

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I'd like to see it a a couple inches higher, but this should do the job.

 

[GregL]

...

 

[GregL]

OK, this is bizarre but it looks like I'm back to the switch. I hooked constant vacuum to port #4 with all other ports capped off and it would not hold vacuum, which I thought was a little strange. Then I removed the caps and ran the switch through all it's paces, moving a vacuum gauge to each port without much success. At each switch position I tested each port with my vac gauge. All but one showed zero inches.

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With the exception of one port that read about 7 inches of vacuum.

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I can hear the vacuum leaking inside the switch as I move it to each position. Do you think this new switch could be suffering from dried up grease like any other old switch does? This appears to be my area of trouble again, even with a NOS switch.

 

[Blue Ox]

FWIW mine makes hissing noises as you select different settings. Maybe I missed it, but are you sure you have vacuum to the selector itself? I had a broken vacuum line in the engine compartment, and with no vacuum all the air goes to the floor.

 

[GregL]

Yes, as noted above, #4 port has direct vacuum on the factory connector. As I jumper this vacuum around to the proper hoses on that connector, all vacuum operated doors function properly.

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But with the switch in place, instead of the jumper hose, no functions work.

 

[dvdswan]

Hand held vacuum testers are your best friend in this case. You can do each individual item without the engine running.

Best of all most are under 20.

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[GregL]

dvdswan, I have a Snap-On vac tester similar to that one which I used on the bench to test the switch when all the ports were capped. This showed the switch was leaking pretty badly.
The test pics I showed were with constant vac that made my testing much quicker than having to hand pump each port.
Greg

 

[bucket]

You should only be able to hear a vacuum leak in the switch when it is changing positions.

 

[GregL]

I can hear it leaking, pretty badly, at all positions.