HVAC demons...

[gardiam1]

Howdy - My '96 four-door Tahoe (258K) is starting to torture me with small but hard-to-diagnose problems. Case in point - the heater not longer heats, and the defroster setting only blows air if the temperature control dial is centered or on cold.

More details: the heat just went away between one drive and the next. All three of the control dials (fan, temp and mode) appear to work; at least, the fan goes through all speeds, the vacuum-powered heater water valve in the engine compartment actuates, the air goes from coming out the floor vents to the dash vents to the defroster vent (more on this in a moment). *But*, there's no hot air anywhere! So maybe the heater core suddenly got blocked (unlikely but possible)...but also, the air will come out of the defrost vents only if the temp dial is *not* turned to hot. If I turn the temp control back to the middle position or colder, the air starts coming out of the defrost vents again.


There's a gif file attached from the AllData site that shows the HVAC ventalation system. I'm too tired to go on any more right now - I've got carpal tunnel surgery in the early morning. Any and all theories are welcome. Thanks so much for your collective help!

 

[rmodel65]

get under the dash on the drivers side look up laying on your back to the cienter of the dash there is a little black box with some levers attached. that is the blend door actuator. the default setting for the actuator is to blow from the dash. these things can stick move the arms with your hands. my 97 silverado needed a new actuator but my 97 tahoe just need a slight nudge they were stuck from sitting without being used and now they work fine..

 

[Donovan]

I have a similar issue with mine. It looks like the same thing would work. Example.... I have not had to use my heater very often but have noticed it heats and cools when ever it chooses. First the blower motor would not blow on the last 2 settings just the first 2. I changed the resistor and now it works, except on the last setting. If I smack the glovebox it will kick on full blast. It's funny but it sucks. During the summer it blows cold air just fine, but from the defrost vents and it fogs up my windows. Then it will blow through the dash vents. This morning I let it warm up for about 15 minutes and all it did was blow cold air with the heater on. When I left work 11 hours later the heater was blowing warm. From what I have read there is a few door actuators on the whole system right? So what do I start with, the blend door? I saw them on LMC almost 300 bucks for all 3, is that a good price? do I need to replace all three?


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[Bamba]

What ended up working for you? I have a similar issue

 

[Jimmyy]

This is a 7 year old tread. You may want to start a new one for your problem. Lots of help on this forum.

 

[Bamba]

Yes 7 years is a stretch but I thought I would give it a try. Thanks.. Will do