By head unit, I meant the HVAC panel with the *****/switches.

So a different HVAC panel did not do anything different....still loose defrost after warm-up, right?

Check the condition of the vacuum lines under the hood, especially the ones in the upper driver's side. Replace any hose that is cracked or leaves black residue on your hand. There are several "T's" and joints in this area and also the really small, almost impossible to see connection to the reservoir in the driver's front fender.

I purchased about 10' of hose and replaced all the vacuum hose at one time when I found one length of hose all cracked.

 

Correct, nothing changed with the new head unit. I will look into the vacuum hoses. How do you think the vacuum can make th defrost not work when it warms up? Thanks

 

I originally thought there was a vacuum leak to the actuator that controls air flow between defrost and dash vents; however I still think that with no vacuum, air flow should be to defrost....for safety reason.

But the problem occurs after warm-up. So that is why I went to the controller actually commanding dash vents after it got warm. But the replacement HVAC controller does the same, so only vacuum remains.

However, there is a door in the air plenum that is prone to breaking and perhaps this is the problem. But I have no "first hand" dealings with this door, have just read about it and the usual problem is no heat to floor.

So couple of questions:
1) Does the switching from defrost to dash vents happen after a certain time period everytime?
2) If you leave the HVAC in Defrost position, shut off engine for a minute, then back on, where does the air blow from..defrost or dash? Or does it take several minutes before the switch from defrost to dash vents?
3) If instead of defrost, floor vents are selected, does the air flow change to dash vents?


This sure seems to be an odd one.

 

1. The switching happens in the morning when I drive to work, usually when it's colder out it tends to stay in defrost mode longer. My drive to work is less than 10 minutes. Only once or twice has it stayed in defrost the entire way. Usually it will go to vent mode half way through my drive. Also, before it switches to vent mode I can change the position to any of them and it will work. After it 'breaks' and goes only to vent mode it doesn't matter what the selector is at it will only come out of vent. When I leave work and go home at night it will only work in vent mode. I don't think I have ever had it work on defrost twice on one day. And some days when I go to work in the morning it will only be in vent mode from the get go.

2. I think I answered that above. It does not change anything shutting the car off and back on again.

3. Yes, I have actually tried this. Couple mornings ago I put it on floor mode in the morning on my way to work and it worked great on the floor for a few minutes then the air hissing sound and it goes to vents only.

I also looked at the hoses this morning and I didn't see snything that looked bad, but I didn't see them all. I was told by someone once that in this vehicle of the 4x4 works and cruise works then the vacuum is fine, which they both work on mine. That's not saying that there is still something wrong witht th vacuum. I have to get this fixed, it's getting dangerous at times when I can't defrost my windows coming home from work. Thanks again for your help.

 

When you changed the HVAC control panel, were there vacuum lines on the backside of it or just an electrical connector?

 

Electrical

 

Okay! I had a misunderstanding between the manual and auto units....and as you stated you have the auto.
There is a separate module that the vacuum lines connect to, that is under the dash. This is the unit that is failing. There is a picture of it in another current thread here. Suggest removing the vacuum controller, resolder all the connections and try it again...may still have to replace it. I am not familiar with the unit used on Blazers, not many of them have the auto HVAC.

 

Ok thank you. Do you know where that picture is? I did a quick search and didn't find anything.

 

Look for "98 Blazer No Floor Heat"...post #74 has the picture of the vacuum controller. The unit is probably similar to the ones I have worked on, but knowing GM methods, I am sure there is something "special" about the ones used in the Blazers.

Anyways I still think this is an electrical problem and not a vacuum problem as you do have defrost for a certain period of time....time enough for the electronics to get warmed-up. I know in the other GM cars that I have worked on that have a similar vacuum controller, they are pretty much bullet proof...and never have had a problem with them.

So hopefully this is just another case of a cracked solder joint and simply reflow of solder will fix it.

Let me know what you find.


Later.

 

Well..

I took it to the GM dealership and gave them a few dollars to diagnose it. It is the HVAC Vacuum Control Solenoid Valve (16171742). They quoted me right at $1,100 to do the repair because they said the entire dash has to come off. I obviously said thanks but no thanks. So, here is my question. I have done no research yet to see what this part is or how to replace it. Does anyone have any history with this? Is there a way to replace it without taking apart the dash? Thanks.