Heat and AC has a mind of its own...
 

Not sure how to explain this or even try to search for it but here it goes....

Whenever I use the heat or AC the air output seems to cycle on and off.... If I set the output to the dash vents the air (hot or cold) exits there for a bit then acts like it turns itself off. Then after a minute or so then it will come back out the dash vents even though I can still hear the blower motor running. If the AC is on it does the same thing - exits a bit from one area then shuts off then back on again.

Hope that makes sense but anyone have an idea what it may be or point me towards a way to search for this fix?

Thanks in advance,
Ed

does it stop blowing completely, or does the air just start coming out somewhere else (I think windshield is default)

Is the blower fan cycling or blowing steady?

Does this happen only while driving, or parked?

Any speed setting?

Blower motor still runs - just acts like the output air is coming out somewhere else. Happens anytime it’s on - driving or stopped in traffic. Does it as far as I can tell from any fan speed setting.

it’s got me confused for sure.

Ed

what are your vehicle details? year? miles?

Sorry - thought I had it in my signature.

2009 3500 with the 6.0 and 160,xxx or so miles.

replace all the engine vacuum lines, the check-valve, and test the reservoir for leaks too.

the door actuators are vacuum controlled and if you have a vacuum leak the doors will change position when the engine vacuum is low...off idle.
do you hear a hissing sound when the engine is idling?=vac leak

exactly what tech2 said. Your vents are vacuum controlled.
And in my experience, after 160k/14 years, those vacuum lines are going to be falling apart in your hands. You get microscopic leaks that are hard to find. The valves degrade, and tanks crack too sometimes. The heat in the engine bay destroys everything and you're better off just refreshing it all. Luckily it's all very inexpensive parts.
It sounds like the effect is delayed on yours which indicates a pinhole leak so you may not even hear any hissing.

So finally got chance to dive in.

replaced the T connector and the vacuum line from the T to the canister under the battery tray and the line from the T to the motor. Checked the connection of vacuum lines under the dash on passenger side and still no luck. Guess it’s time to locate a smoke tester and dig deeper.

S
https://cimg4.ibsrv.net/gimg/chevrol...a1ed6018f.jpeg

so checked this plug here and when I open up the brown/black I have vacuum there. The part that has me is all the other lines are color for color matched in the plug but this one once connected is brown to black plus in other online photos I’ve see they have a black as well…

I went and ordered a smoke vacuum tester so will get that and see what’s next.

I can select air output to defrost or floor or defrost and floor but nothing to dash vents. Doesn’t matter if idling in driveway or driving and under any kind of load.

More troubleshooting coming soon 🤦‍♂️