My 98 blazer LT with the HVAC system has decided to cause me an issue. Everything works I can change positions of the air flow, I can change from heat to a/c. I have heat and I have a/c. You can hear the blower change speeds. I just have no air flow. It stays the same flow even when the blower speed changes. Ive checked for a blockage everything is clean. Im totally stumped. ANy help would be greatly appreciated

 

You have bad vacuum lines or door actuators under the dash or going into the firewall. They are very thing black vacuum lines

 

That was where I was going to look next. Could the blend door actuator cause this?

 

possibly, something is block it
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Most of the time it is not the HVAC internals but the vacuum.

The vacuum lines may appear soft and still intact but what breaks in old cars is the plastic junctions.
Especially the ones in the engine bay around the vacuum source.
Depending on the engine (TBI or MPFI) you have different vacuum port locations.

The hoses attach to the source by rubber T's or L's or straight pieces that tend to split on the lower side. (I had three of them on my 94 TBI split on the lower side towards the engine that could not be seen.) They had held up until I had to pull them to access a sensor. Afterwards no more heating control. Replaced and worked again.

Good luck with fixing it.

 

if the blend door isnt the issue, check to see if a mouse built a house in the blower box. maybe take out the blower motor and see if installation falls out.

another thing ive seen before is people installed the wrong blower motor or squirrel cage but doubt its your case if it worked fine at one time

 

The a/c works well, fan also works but not heat flowing. Las winter worked fine. This is a 2000 blazer

 

The blend-door actuator controls the temperature.

Next to the blower motor, at least on my "climate control" (auto temp control system) there is a temperature sensing component that can be removed that will give you access to the blower motor side of the AC condenser. The picture below is what I found when I was replacing my blower motor. I pulled about 6 large handfuls of tamarack needles out. Ugh.

However, it is likely the vacuum supply lines that have rotted allowing air into the vacuum system.