Please let me introduce myself and appologize up front for the long post. I have a 99 Blazer 4x4, 4-door (Trail Blazer edition) with the 4.3 liter V-6 that I purchasedthis pastJune. It has the auto climate control heater & A/C system. My problem has to do with regulation of the temperature coming out of the vents. Last summer when I had the A/C turned on, the only way that I could get air that wasn't frigid (48F at the dash vents) was to set the temperature **** to nearly full hot (clockwise). Now that I need heat,if I have the temp control set anywhere other than at full hot it blows cool air. This is with the A/C set to off. If I leave it turned on the air is even colder. If I set the temp **** to full hot, I get plenty of hot hot air, too much to leave it at this setting. It seems to me that the system is working as I get the full extremes of very cold air from the A/C and very hot air from the heater, but just can't regulate them to a comfortable level. I've checked all the fuses, electrical connections and vacuum hoses that I could find, and everything looks ok. Initially the vents were not working properly but I found a disconnected vacuum linebehind the glove box.Reattaching that has restored proper airflow with all of the vents opening and closingproperly. TodayI pulled the climate control unit to checkits connection. Again everything looked ok. It also appears that a previous owner may have replaced the climate control unit as the one currently in the car has the word "Blazer" hand written on the plastic case with a pen. I'm wondering if a used one was installed and could be faulty.
Thanks in advance for the assistance. This is a great forum. I joined right after purchasing the vehicle. Following the instructions for cleaning the throttle body, the air sensor, and the EGR valve increased my to & from work gas mileage from 17.2 to 20!

 

Anyone???

 

Your HVAC system is feed via vacuum off the engine.
Some model years have a problem with the input seal on the t-case. This seal fails and the tranny fluid overfills the t-case. The vauum switch fails and the fluid travels up the vacuum line and into the HVAC system. This causes havoc with your HVAC system.

I dont know how to fix it, short of paying the dealer to do it. They have a TSB on it I believe, if not the techs are familiar with the problem, at least at my local GM delaership they are. I had a nice long converstation iwth one of them about it.

Anyway, to check, take out the top (fill) plug on the back side of your t-case. If oyu have a guisher, you found your problem. I can walk you through the t-case seal replacement, $5 part. However I don't know how to fix the HVAC system. You can also chek the vacuum line on the back side of the engine. Look behind the distributor cap, there should be a hose coming off the intake on the back side, this hose goes into a tee fitting, one leg feeds the t-case vacuum switch, and the other leg feeds your HVAC system. The vacuum switch leaks and you have fluid in your HVAC system. That vacuum switch should be replaced too.

Not saying this is your problem, however it is a place to start.


By the way, welcome to the site Jarhead.
Semper Fi.

 

i have the exact same problem, when the temp gauge is set on the blue dash, it get cold quick but if i set it one notch higher, the fridgidity (is that a real word? COLDNESS...) goes away. i can move the **** all around the different settings and the temp is the same. it seems to work a little better in the winter. if i want it hot, i have to set it to the red dash on the right. then, if it gets a little too toasty and i move the **** one setting up, it stops blowing hot air and seems to be just 'somewhat warm' i havent found a fix for it yet but i have a slight feeling that my heater core isnt in the best shape. i read somewhere that if the pass side floorboard is wet (and mine is), thats a sign of a faulty heater core

 

Hi everyone

For the last few weeks, my computer climate control, turn off completely now and then. Nothing at all. No power. When i'm on the road it's seems to stop and start randomly. I suppose that is some kind of bad contact. Do you have something to suggest to find the problem? Somewhere to start with?
Thanks
I’m waiting for you!
Martin Blazer LT 2000 4d

 

I would start by removing the HVAC control module from the dash and opening it up. It is quite possible that it has a broken connection in one or more of the solder joints on the circuit board. I haven't seen this in these HVAC modules, but I have seen it on a few other components in these trucks. That would be where I would start.

You could also try to find another auto control head at a junkyard and put it in. If you experience the same problem, then the issue is possibly in the wiring supplying power to the module.

Hope this helps!