I have a 2002 ZR2. Manual Climate control (not the auto temp computerized one) Before I get crucified by forum admins for not searching the forums... I've been googling and looking on the forums for hours and I can't come to a definite conclusion what could be wrong with my blazer as I have multiple problems.

Basically heat and a/c blow out of all the vents. The windshield defroster, the main vents and the floor vents... all the time.

Also.... it gets stuck on heat. I can move the temperature **** all the way to heat and heat will come out...but when I move it back to full cold...heat still blows out. But if I wait or a try later it can switch back to a/c. ???I heard this could be a bad blend door actuator which I'm about to try to take out and look at.

I took apart the panels by the gas pedal to look at those actuators and they don't move at all no matter where I have the air selector switch positioned. I also took the connector off the vacuum actuator switch on the transfer case underneath the vehicle and a little Transmission fluid did come out which I know isn't good but I doubt is causing all the other problems.

Anyone got any ideas? thanks

 

The hot and cold blend door is controlled by an electric motor that is known to have issues. the other doors in the HVAC box are controlled by vacuum.


if you drop your glove box down and look on top of the HVAC box you can see the blend door motor to the left above the glove box and then you will see a little white plastic arm connected to a vacuum actuator to the left of that. That white plastic arm is what controls the defrost vs dash vents vs floor vents. with the engine running you should be able to change the vents and see that actuator and arm move. If it does not move You will have to start tracing vacuum lines to make sure none are disconnected or cracked. If your 2002 is like mine, check on the left fender near the ABS module. you should have a vacuum hose going into the fender because there is a vacuum reservoir inside the fender behind the ABS module. The vacuum hoses are know to get brittle and crack.


I only know this because mine broke that little plastic arm and its a PIA to replace but it can be done without removing anything except the glove box.

 

I have a 2002 Zr2 Blazer myself and can tell you what I have gone through with this as it does sound like you might have multiple problems, which I am sorry to say is why this is rather long, but figured I would give you all the info I could.

Also just to mention before I get into things that I am not some complete authority on all this, and for you something could be completely different, but figure I will just tell you all I went through and know (or think I know) as perhaps something will be of some help.

First off I am sorry to say but if you have fluid in that hose to your transfer case switch you actually have a major problem.

Basically just to mention on that side of things, yes the seal is bad on the switch in the case, but also when it is, it is really also a strong indicator that the rear tranny seal is leaking fluid into the transfer case and causing it to be much higher then normal. Not sure if this is what makes those seals go, but if they are and this happens what happens next is the level in the case gets to high and then through the bad seal in the switch the vacuum sucks that fluid up that hose and it can then get into just about every vacuum line and actuator and other part possible.

Is possible I suppose all of that is not happening for you, but from all I was told and read and as well found for me, that was the case.

To check this in a few ways you could pull the fill plug to the transfer case and if a lot of fluid is coming out you can strongly suspect the tranny seal is bad as is really the only way it could be overfilled. Also if you are having to top off your transmission fluid often where you have no other leaks you can see, that would be a indicator as well.

As far as the what to do about this aside from the transmission seal issue as am sure you know that part...all depends on what has become contaminated, but as you are discussing all the same issues I had, I would suspect it has gotten into your HVAC vacuum control system.

First of to start that beginning, naturally you want to take down that fluid level and replace that switch which just to mention, but careful on that as for me I found that all the replacements are just a hair smaller in thread diameter and despite how tight it was to get the old one out, as the transfer case is aluminum and the new part was smaller I stripped the hold like nothing.

Anyway the fix for that is another story so will just end by saying, yes replace that part and be careful and also depending on how bad your tranny is leaking into the case (if that is the case) you may want to also keep a eye on that fluid level as it could ruin the new switch too (although I hear there was a mode made to them by GM a while back once they realized this was happening).

So while we are talking about this part of things, if you didn't know this switch and the vacuum lines with it are all a part of a control to engage and disengage your front axle for your 4 wheel drive. So probably best to be sure those hoses have been cleared of fluid and holding good vacuum and that the large 4x engage actuator they lead too for that which is under the battery is working properly and not leaking vacuum air.

This would probably be a good point to say you should just go and pick up a hand pump vacuum gauge as they are pretty cheap but really the best way to really be sure an actuator is holding its vacuum 100%

So to finally get to the HVAC, the first place to look at is in the back upper drivers side corner of your engine compartment and there you should see a couple of vacuum lines where one comes from the engine and is your main source, this then goes into a small black and white check valve tee where one line eventually runs down to that transfer case valve, the other runs to a bit of a hidden vacuum reservoir inside your fender, an lastly after a few adapters one continues in a small black harder plastic capillary tube, down into the driver side which is what eventually feeds your controller in the dash.

So take a look at all these parts and hoses and be sure (again after replacing the T-case switch) they are clear of fluid and working properly (especially that check valve) and are tight and not leaking air.

So the next fun part is where that black hose goes to which is the far right control switch/distributor in your dash which is an interesting part in that internally it is made up of a rubber disk with all these channels in it that rotates from that **** over a bunch of fixed holes that all your vacuum lines to each control part is attached to which is basically how you control everything vacuum actuator in the system.

For me the first time I got that part out which is done by getting the whole control assembly out, I took it apart and found it well wet with fluid inside with as well even the vacuum manifold of hoses that plugs into had many of the hoses wet with fluid too.

Just to mention at this point, yeah its easy at this point to just replace the whole assembly and thus this the vacuum controller (kinda have to do the assembly as they don't sell just that one part), but even after replacing that T-case switch I did this twice and in both cases the control of everything only lasted a day or two before it was destroyed again.

Basically again, the control is by this round rubber channel part that rotates over the plastic manifold, so once any remaining fluid hits that, it just softens the rubber and it flattens out and is no good again...at least that is what I found.

For me I even did try adding washers behind the spring that applies pressure to hold this disk snug so it would have more pressure there, but then I just got a few more days and it was leaking again.

To move on, for me that is kinda where I left things years ago as the next step of tearing out the whole dash and going through every part was just not something I was going to do. All in all after the third switch and washers behind it I had about 50% control restored so that is where I left things.


Just recently though as I now have to replace my heater core which means the whole dash just about has to come out, I decided to get back into resolving this so at this point I guess between what clean up I did years ago, basic time and the several years of the vacuum running through everything with as well as the new T-case switch holding, there does not seem to be any fluid left in anything as far as my lines go.

So, so far as have not gotten the dash out I have been able to check out the 3 of the 4 vacuum actuators that I can get too.

For two you can actually get to relatively easy, they are behind the glove box which you actually don't even have to remove. Just open it up and to the upper left hand corner of the back part to it is this spring like tab that you can push up and then the box will completely fall open (obviously make sure there is nothing in it).

Once open you will have access to the two actuators I call the Recirculation Actuator, and the Slave Actuator.

With this access you could either possibly get these out and be sure they are clear of fluid, or at a bare mim hook up a hand vacuum pump and be sure they are not leaking.

For me I found my Recirc was holding 100%, but the Slave does leak very slowly.

Sadly as you cannot get the Slave anymore I just am going have to deal with. Note as that Slave one has an internal spring this one if its leaking bad enough will probably not move no matter what, but even if a little it will return all in its own eventually.

Next is the one you already got to which is the Defrost Actuator and if it is not moving means either there is no or so little vacuum going to it, it won't...or is filled with fluid or already destroyed by fluid.

Again can just check at the hose points to see if one you are getting any vacuum to it at all at the hoses, or two check the actuator with the hand pump.

For me as I just replaced this last week as for as hard as it is to get too it is possible to swap out once you have the access you spoke of...I can tell you that one for me was completely shot.

Basically it is a push and pull actuator so it has two ports on it (one above and one below the diaphragm) and for me I could completely blow air easily from one port to the other.

So now once putting the new one in I have a lot more control over everything although everything is not 100%, but it is much better.

For the last actuator this is the "Mode" one and is the one above the Defrost and if you can get to that one without removing the dash god bless you as was just too up there for me to see anyway to swap it or even check it at its ports so will just have to leave seeing if that one is bad till I do the heater core.

All in all with all I found elsewhere and that fact that I know it had fluid in it, it's probably not 100%.

Btw, just to get to something that was probably relevant to your question right off, both the Defrost Actuator as well as the Mode are push pull ones that very well could just be sitting mid point if they have no vacuum or are bad...which basically is what leads to air coming out of every duct possible depending on the problem.

As far as why your heat is always on, yes as ChuckNTruck mentioned this is controlled by a electric actuator via the center control dash **** which opens a door to allow the air to go through it (ie the core is always hot is just if the door is open and by how much or not is why you get heat) so does not have anything to do with the vacuum system, but those electric control actuators do have a great history of going too as they are a part that has many plastic gears in them that break or split over time, so could just be a coincidence.

If you want to know more on them can look up those things on youTube and see what I mean.

I suppose next I could talk about any of the internal to the main plenum box control doors being bad as those can either leak if the foam is bad, and or break too, but have made this long enough and those issues are probably easy enough to figure out without me telling you things.

Would just add one more thing in that as I just went through the collection of all these parts and doors as I was going to do the heater core that I can also tell you that for quite a lot of them...they just are not available any more.

All in all won't make this longer by listing what I was able to get an not get, but will just tell you many are not made anymore and for what NOS ones there were still around, I very well may have actually gotten the last ones.

At any rate if you want to know more about that feel free to ask.

So again, sorry this was all so very long but hopefully among something of it will be something helpful to you or perhaps others.

Best of luck and feel free to ask anything more.