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arveetek 09-06-2019 08:03 AM

No air out of vents on A/C
I'm having an issue with my 1995 Tahoe. I have very little air coming out of the vents on the dash. I've never thought there was proper air flow out of the vents, but this past week it suddenly got worse. With the blower on high, there is very little air coming out of the dash vents.

Here's what I know:

1. The blower is functioning properly. New blower and wheel in the past year. Works on all speeds.
2. The mode door is functioning properly. I can watch the actuator move from floor, to vents, to defrost. I seem to have more (normal?) air flow on the floor and the defrost, just not the vents.
3. The blend door is functioning properly. I can change the temp from cold to hot, and it appears to work as normal.
4. The recirculation door is not functioning properly. It broke a few years ago, however I have it propped up in the recirc position.
5. A/C is working. I have cold air, just not air flow out of the dash vents.

So what else could cause a blockage or restriction in the dash vents ductwork?



DmaxMaverick 09-06-2019 09:30 AM

Possibly a misaligned duct connection behind the dash. They are foam-wrapped inserts. The foam can deteriorate or unglue and go missing (usually causing a restriction), allowing the ducts to get out of place. If the fan is blowing, the air is going somewhere. Also, moving bellcranks doesn't necessarily mean moving doors, or fully moving doors.

More Power 09-06-2019 12:10 PM

To what Greg mentioned... See if you can reach up into the area where the dash vents are, and feel whether there is a lot of air blowing around in that area. If so, the vent ducting may have become disconnected/dislodged. It could be that some prior work (even before you got it) was the beginning of this problem.

You can remove the instrument panel and glove box/liner to see more into that area.

arveetek 09-06-2019 12:15 PM

Ah, there are more linkages and flaps that I was not aware of. Skip to 11:05 for a good shot of the linkage and mode doors in this video:

I can visibly see the actuator and at least one arm moving. I'll have to get back under there to check the other arm that I didn't know was there. That's got to be my issue. Hopefully I can address what I need to without dismantling the entire dash.


arveetek 09-07-2019 04:49 PM

Problem Solved!
Turns out there are actually 3 mode doors / flaps in the HVAC system, all of them controlled by one electric actuator. It's pretty neat how one electric motor can move all three flaps at different times. However, after careful visual inspection, it appeared that all three flaps were moving correctly. I could not see inside the HVAC box to actually see the doors, but everything seemed normal from the outside. I would have to disassemble the entire HVAC system to get to the mode doors.

I decided to remove the fan motor to check the cleanliness of the evaporator coil (I cleaned it a few years ago). Lo and behold, I discovered the evaporator was about 1/2 clogged with debris. The past 2 weeks at work, the feed mill directly behind our dealership has been drying corn, resulting in a ton of corn husk chaff to be blown all over our parking lot and vehicles. It's almost like brown snow. Apparently my rig has been sucking that chaff into the HVAC system where it got blown up against the evaporator coil. I washed it all out with a garden hose, and now the air flow is back to normal. What I thought was just reduced air flow from the vents was actually reduced air flow across the entire system.


DmaxMaverick 09-07-2019 06:37 PM

Yep, that'll do it. I have 2 vehicles waiting for cabin air filter replacements, now (the evap's and ducts get a good blowing while I'm in there, too). I usually wait until the end of the dry season, but the reduced airflow we are now noticing will require it sooner. It's been a year of very heavy pollen, and too hot for me to find the gumption to do it. It's cooling off, so under the dashes I go this week.

arveetek 09-09-2019 06:31 AM

I wish the older vehicles had cabin air filters...


DmaxMaverick 09-09-2019 08:27 AM

Yeah. It's a mixed bag of benefits and (in)convenience. The Sierra/Silverado GMT-800's began the cabin filter option in 1999. It was later discontinued about 2004-ish, with the reason given that owners weren't servicing them, and complaining about poor HVAC performance due to that. Initially, they (GM) just left out the filter panels, but the option of installing them was up to owners after delivery. At least one model year omitted the access cover opening, so they couldn't be used even if you wanted to (although they could be easily mod'd to accept them). I think it was more of a bean-counter decision, but facts remain. If filters are plugging up, whatever it is that's plugging them is being passed through the system w/o filters, and onto passengers, or allowed to block airflow through the evap core. Poor HVAC performance will almost always result in a dealer visit, at least on in-warranty vehicles. Maybe GM figured out that despite the health and comfort benefits of cabin air filters (I doubt they give a hoot about that), the additional maintenance item would be overall beneficial to dealer service departments bottom line, and almost all GM vehicles have them, since. Generally, people don't read their owner's manual. There's profit in ignorance, willful or not. Later models are MUCH easier to service. On my 2001, the lower HVAC cover has to be removed (3 blind screws and a thick, semi-rigid cover that doesn't remove w/o distortion), and one screw retains the filter slot cover. Once the cover is removed and filters slid out, any debris caught by them immediately dumps out on the floor (easily avoided, but not mentioned in the manual, BTW). The passenger cars have them easily accessed behind the glove box, which is about a minute to remove. Our 2017 Volt has a carbon/charcoal filter, which provides noticeably fresher air (why aren't all of them like this?). If an owner doesn't like the filters, they can just leave them out. The system will function as before w/o them. A benefit of having the option, whether or not filters are used, the access cover opens immediately before the evap core, and allows for MUCH easier removal of debris that may block airflow, as well as cleaning of junk that leads to mold and stinky air. I like them, and will continue to service them.

In your case, a filter option (however effective) may have prevented this thread, entirely. Low airflow is a simple issue to diagnose, and having a likely suspect almost always makes it a simple process. Not unlike MAF codes, the usual suspect is almost always a fouled air filter, and we almost never hear about them because they aren't worthy of discussion. We'll hear about the occasional duh moment, but they are the exception, not the rule.

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