I have a 1992 K1500 Chevrolet. The air blows very cold @ 49 F. However, the air does not blow out of the vents veryu much at all even when the blower is on high. All the motors that control the flaps in the duct work are operating properly, and you can here the blower fan whirring away. What's going on?????

This could be what's called a blanket on the evaporator which is when enough junk like pollen, dirt, pine needles etc., get to it and reduce air flow thru it. It's a nasty job if you conclude that is so as you have to take it out and clean it which is a pretty involved job.

FYI - blowing cold is better than 49F Most cars/trucks can do low 40s on medium fan setting,

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/

How do I get to the evaporator? I have tried to lower the blower, heater housing, etc... from under the dash, but have had no luck.

You aren't going to like what I have to say about this one. If it's anything like my own 97 K2500 you would have to drop the steering wheel, completely remove the dash and lay it on the seat with all wiring and connections carefully noted and then you could get at the distribution box. This could be an all day or more job if you have all the little tools to get it all apart. When distribution box is opened you could then clean the evaporator but you probably would want to replace it with that much work and that involves opening up the A/C refrigerant system and all that goes with recovering refrigerant, adding some oil, suggested new accumulator/drier then vac and charge by weight of refrigerant. WOW!

You might be able to find the condensate drain and just carefully back fill it with a soapy water and let it drain without taking anything apart but the results are iffy but might be worth a try but that would probably get the interior wet for trying -- untested by me so if you even think of that you are on your own,

Hit back if you need more help as I know someone out there who does these routinely,

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/