Defrost blows only on cold air setting
 

This just don't make sense to me. Defrost blows really hard on cold air hardly at all on hot. Everything else works pretty good. New heater core, new thermostat, plenty of heat in the floor. Plenty of vacuum at the flap door. It does flap but thats the other control not the heat...thats why I cant get any sense here. The adjustment of the flapper is weird as well. I can get either defrost all the time or floor heat all the time and no happy medium.

Any body got any ideas........besides an exorcism?

It will be appreciated!

RE: Defrost blows only on cold air setting
 

What are ya workin' on? Year, model, options (auto or manual HVAC)?

RE: Defrost blows only on cold air setting
 

Sorry, 1991 S-10 Blazer 4.3 L with AC 4WD

On way to work this morning confirmed that blows really hard in defrost on cold air setting, closer I move to hot air thew weaker it gets and blows out into the floor. AC, setting is working with exception that I need to retrofit to 134a so AC actually is not working. On heat setting I cannot remember exactly how it acted but I do remember it was weird also.

RE: Defrost blows only on cold air setting
 

Sounds like there may be a problem in the HVAC control module. I believe that everything is vacuum controlled on a '91. I don't have my references with me at work, but if I have some time tonight between work at fire training, I will try to look up the information for you.

RE: Defrost blows only on cold air setting
 

Thanks

I have been reading a lost of posts and you seem to help a lot of people. I just want you to know that we appreciate you a great deal! I bet your vehicles are top notch!

RE: Defrost blows only on cold air setting
 

There is also the chancethat somethingfell down the dash vents and it's found it's way into where thehot and cold sides meetis and is blocking the baffle from shifting over to the hot side. Hard to diagnose this without pulling the vents apart.

RE: Defrost blows only on cold air setting
 

OK Swartlk this post is for you. I believe your probably the most knowledgable person on here.

How's that fer suckin up? hahah

Anyways after replacing the HVAC control (which was a pain) I have discovered that the problem is the vacuum door at the bottom near the floor vents. I can manually make the system work either way defrost or floor. I have vacuum on both lines but the door does not move.

How hard is it to replace and where can I find this vacuum canister/door assembly?

RE: Defrost blows only on cold air setting
 

I just looked in my references and I cannot find good information on how to remove the defrost door actuator. From the diagram that I have, it looks like your biggest problem is going to be one of room to work. You should just have to remove the mounting screws, the vacuum lines, and disconnect it from the door. But again, the difficulty of this repair is dependant on how well you can get to the fasteners...

Do you have vacuum on both lines at the same time??

RE: Defrost blows only on cold air setting
 

No.....one line sucks for defrost and the other sucks for floor heat...come to think of it they all been suckin lately!

Its funny.....I traded a woreout 95 Toyota Avalon for this vehicle and its like I really don't have any money tied up in it except for what I have spent fixin up. With all the hard work and knuckle bustin I have really fallen for this truck!

So Far:

[ul][*]Replaced leaky heater core and Thermostat...........(Wish I had really checked out the ductwork while the dash was out!)[*]Replaced wire to rear window defrost[*]Replaced Rotors/calipers/lines/pads cleaned ABS Sensors, right front sensor is bad and needs replaced[*]tightened up upper control arm nut.that was a scarry moment before I found what was wrong.....I thought I had really been taken to the cleaners on the trade. Also did a "CountryBoy alignment".[*]Replaced HVAC Control[*]Replaced Light Switch[*]Cleaned/replaced Underhood lamp[*]Adjusted Transmission Shift Linkage[/ul]
Still Need to:
[ul][*]Fix that Vacuum defrost door ( I chose to now now have all defrost, no floor heat, everything else works Vent, Bilevel, AC.[*]Line up the front end & replace shocks[*]Retrofit the AC to 134a[/ul]
This truck runs like a Singer Sewing Machine (as we say here in Kentucky) and only has 113,000 miles. I can see myself getting 250,000 easy. Everything works except the HVAC Problems. I kinda get the feeling from reading on this website that might be a common problem with these earlier versions of Blazer.

I will post a pic of her soon whenever I get time from maintenence work. Seems like I have worked on this truck every day since I have owned her. Can't wait to get her the way I want her to be!

Thanks for all the help you have been. It is appreciated!