The air in my Tahoe only comes out the defrost vents. Some videos say that is due to a brittle vacuum line under the hood, while others say it is the HVAC actuator. Where should I start?

Are you able to change blower speeds or is that inhibited as well?
If it is inhibited try the blower motor relay behind the glove box.

I'm able to change the blower speeds.

My 04 Silverado did that. It was the blend door actuator motor.

I could be wrong about this as it has been a while since I've worked on a C/K, but here goes:

A vacuum leak in the function control knob will default to defrost on GM vehicles as a safety measure. The electronic controls will activate the AC compressor when the AC is switched on and also when defrost is selected (also a safety feature. The primary reason you are hard pressed to order a GM vehicle without AC anymore).

Rotate the knob to AC and turn the fan on high. You should have the compressor running intermittently and also the electric cooling fan if equipped (at all times).

Rotate the knob to vent and the compressor & fan should stop.

Turn it all the way to defrost and the compressor is supposed to start back up again.

The compressor is actuated by the electrical contacts in the switch. The actuator door is likely actuated by the vacuum.

Take note of how it functions in each position and go from there.

The vent positions are not vacuum-operated on this generation. Probable mode door actuator. The gears strip inside, sometimes but not always making a clicking noise. It is on the left side of the HVAC case, under the dash, and will look something like this:



Lining up the little gears and linkages is tough working on your back under the dash. You might find yourself having to remove the dashboard so you can see what you're doing.

quote:

Originally posted by egregore:
The vent positions are not vacuum-operated on this generation. Probable mode door actuator. The gears strip inside, sometimes but not always making a clicking noise. It is on the left side of the HVAC case, under the dash, and will look something like this:



Lining up the little gears and linkages is tough working on your back under the dash. You might find yourself having to remove the dashboard so you can see what you're doing.

Is the video applicable to a Tahoe?

https://www.youtube.com/watch?v=pgadjXyqjbA

I see this part, but not the part shown in front of it in the picture above:

https://www.amazon.com/Air-Doo...2001+Chevrolet+Tahoe

A '99, but close enough.

quote:

Is this video applicable to a Tahoe?

No, that one is a blend door, i.e., adjusts hot and cold air to regulate temperature. This one (from a Sierra 2500HD) is probably closer.


https://www.youtube.com/watch?v=h5_g0WHF0dc

Poor quality, but you notice he's taking a lot of stuff off.

quote:

Originally posted by egregore:
The vent positions are not vacuum-operated on this generation. Probable mode door actuator. The gears strip inside, sometimes but not always making a clicking noise. It is on the left side of the HVAC case, under the dash, and will look something like this:



Lining up the little gears and linkages is tough working on your back under the dash. You might find yourself having to remove the dashboard so you can see what you're doing.

How did you drill down to that part using carid.com? I'm trying to see how you did that, to see the AC Delco part number. Is the part number part of the name of the .jpg file in the link for the image?

I just put these pictures (found on Bing Images) up for a broad picture or general idea of what the OP might be facing for this repair. It is not necessarily the exact part.

quote:

Originally posted by deepocean:

quote:

Originally posted by egregore:
The vent positions are not vacuum-operated on this generation. Probable mode door actuator. The gears strip inside, sometimes but not always making a clicking noise. It is on the left side of the HVAC case, under the dash, and will look something like this:



Lining up the little gears and linkages is tough working on your back under the dash. You might find yourself having to remove the dashboard so you can see what you're doing.

How did you drill down to that part using carid.com? I'm trying to see how you did that, to see the AC Delco part number. Is the part number part of the name of the .jpg file in the link for the image?

I replaced this on my 2000 Silverado. PIA to get to. Just to the right of the accelerator pedal IIRC.

Start the truck put the switch into something besides Def. Take a screwdriver and tap the actuator with the handle. See if it changes.

I can't thank you all enough for the help so far! I'll try the screwdriver trick then order the part listed above.