HVAC Help Needed

[WOOS1]

I apologize in advance if this has been covered on this forum before. My searches came up empty. I have lost all circulation in my HVAC system of my 1990 K5 Blazer. I pulled the fuses and none "appeared" blown. The fan motor is working. I am not getting anything out of the vents. Going down the road at 60 MPH, there is some heat forced into the cabin. Moving the selector to cool does reduce the temperature. But nothing is coming thru unless being forced thru at highway speeds. Is this an issue with one of the flaps? Thx
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[68Timber]

Does your 90 model still have the vacuum operated "head unit"? I don't know squat about those, but would start with at least verifying the fuses. Keep us posted how it goes.

[WOOS1]

Originally Posted by BRUISER

Does your 90 model still have the vacuum operated "head unit"? I don't know squat about those, but would start with at least verifying the fuses. Keep us posted how it goes.

I do see a vacuum diaphragm (?) down by the floor above the hump. Is that what you are referring to?
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[68Timber]

I was calling the HVAC switch in the cluster the head unit. The newest K5 I've had was an 83, and it still had vacuum switches. That's what I was asking about, I don't know how long GM used them. Up until 1991 for all I know. They do go bad, I had to replace one on my 83 while I owned it. But it was so long ago I don't recall the symptoms when it died.

If you've got a vacuum diaphram down by the hump it's likely for one of the damper doors and yours is all vacuum operated... Which answers my question, but I still don't know how to trouble shoot. Have you had a chance to test the fuses?

[RJGeister]

Under the hood there could be a vacuum canister. If the line has a leak, canister is damaged or where it gets it's vacuum from is clogged it will not provide enough vacuum to operate, through the Heat-A/C selector, the diaphram actuators.

[OutlawDrifter]

i would pull the cover under the dash to expose the flapper...i bet the little white clip that connects the vacuum arm to the flapper is broke.

do you get forced air out the bottom no matter what selection you have up on the dash?

check and make sure you are getting correct vacuum at the bulb in the engine bay first if it is so equipped.

[Ski-me]

When you move your lever from heater to vent to defrost, do you hear any vacuum air moving around? If not, start checking for leaks at the controller.

If you do hear some air, I'd then check the door clip mentioned above as it is VERY prone to breaking. If you expose the lower door (where the clip is located, you should see the actuated arm move as you change your selector on the controller.

Another area is on top of the box. There is another rod that moves another door (or part of the same one....can't remember). But check to be sure that is moving too. It's a threaded rod that can actually be adjusted too.

Here is the clip that breaks a lot! I went to the junkyard and every truck I looked at had the clip broken. I eventually had to just buy a new $0.43 part!! And also probably $4.50 shipping for that part.....

Blending Door Tab

[onedayjake]

I would pull the blower motor out and see if the wheel is full of leaves or a mouse nest.