I have just started having a problem with my temperature control in my 2002 XTreme. I don't drive this Blazer much at all. It can be anywhere from several days to a week or more between drivings. I prefer to put all the, to and from work, driving on my old Dakota that has 300,000 miles. First off, I have checked to make sure all the vacuum lines under the hood are in good shape and are hooked up. I have also checked under the dash for the same. All are in good shape and are hooked up correctly. My Blazer has always had good heat and AC but a couple of weeks ago I was driving and had to put the AC on. It was blowing hot air. When I was able to pull over, I popped the hood and the compressor was running but hot air continued to come out of the vents. I turned it off and left the windows down and dealt with the heat. A few days later I drove it again and just for the heck of it, I tried the AC again. Nice cold air. Switched to the heater and had good heat. This has happened a couple of times in the past. Fast forward to yesterday midday. My wife and I were out driving and needed a bit of AC and it worked fine. Stopped and got gas on our trip. Got back in the car and the AC was putting out hot air again. Last night we were on our way home and needed a bit of heat, which worked good. Just on a whim, I tried the AC. Low and behold, we had cold air again. This morning I am back to hot air again. I am about to pull what's left of my hair out trying to figure this out. Could it possibly be the "comfort controls" going bad? It has no problem shifting from floor to dash to defrost although it does seem a bit slow in comparison to that of my Dakota and my Ram trucks. Anyone ever experienced this problem?

 

 

Good morning George. I have made sure that the compressor is working and everything seems to be doing what it should. I am curious as to whether this is some sort of vacuum problem. Sometimes everything works as it should and sometimes not. Seems that switching from floor to vent to defrost takes a little longer that it should. Have drove the car the past few days and everything seems to be working right for the time being.

 

 

Hi there,

I guess I have become a bit (in)famous as someone who has made a study of the controls for the HVAC system in our 2nd gen Blazers.

The temperature control is primarily via an electric motor controlled actuator most often referred to as the "Blend Door Actuator". This will turn a door that directs the incoming air deferentially to go through the heater core or AC evaporator. Now, it is a bit more complicated in that there is also a vacuum actuated door call the "slave door" that controls where the air flows after it leaves the heater core or evaporator.

The failure mechanism for the blend door actuator is that the large gear that is directly connected to the axle of the blend door will crack allowing the gear to slip as it rotates around this axle resulting in the door not moving. (Look up "Blend door actuator replacement" or "blend door actuator repair" and you'll get lots of hits of videos on this topic.)

The failure mechanism for the slave door, and for the rest of the HVAC system control, is via leaks in the hoses that supply vacuum to the HVAC (and the front axle engagement on 4wd models). Depending on the engine load, the vacuum supply at the intake manifold can be sufficient to overcome the loss of vacuum due to the leaks. These leaks will occur because of slow deterioration of the hoses causing cracking or them becoming less able to seal at the nipples of the connectors for these hoses. In my .sig file at the end of this message I have link to what I did to replace these hoses when my HVAC system which would change to where the air was directed depending on the engine load, e.g. accelerating up a hill, at idle, or cruising on the highway.

To test if the blend door actuator is working, you can release the glove compartment stop and let the door flop fully down and then with the system powered, adjust the temperature setting on the HVAC system. (This is where the videos that show you where the actuator is located come in handy.) If I remember correctly you should be able to see directly or at least with an inspection mirror if the actuator is moving the axle of the door or not. Recall, the failure is not with the little electric motor, but rather the gear cracking so be sure to see if you can see if the door axle is moving or not. This might be easier to do by feel rather than visually. Again, look at those videos.

To test the vacuum system, the easiest is a visual inspection of the hoses in the engine compartment. They should be soft and be difficult to remove from any of their connections. However, because the replacement of these is so easy, especially if you have a 4-button transfer case system, I'd recommend looking into replacing them all anyway since they will eventually fail. The ultimate test is to use a hand vacuum pump (a Mityvac) to test if the system can hold a vacuum. If it leaks down, it will not work properly.

A couple notes here: There is a vacuum reservoir that can become disconnected that is very hard to see. It is worth checking to see if that is your problem. Also those hoses that are under the dash for the vacuum system are not made of the same the neoprene-like material as those in the engine compartment. These seem to be much more durable although there is the chance one of them became frayed or worn through somehow although this is unlikely. A vacuum hold test done where the under-dash hose comes into the engine compartment is easily done to confirm that there is no bad leak there.

I hope this helps!