Defrost help

[Dr.Destructo]

Hi;

I need to get the defrost working on my '83 Blazer so that it will pass safety certification up here in Canada.

The heater and heater motor works fine, and the lower vents blow hot air. When the selector is set to "Defrost" (the vents on the upper dash near the windshield), no air blows through them.

I thought I would ask here to see if anyone had a similar problem.

I am planning to take the upper dash out of the truck to see if I can investigate the problem. Could one of the ducts have been kicked off? Or maybe it is plugged up? Is this fixable by a diy-er?

A mechanic friend said it may have something to do with the A/C on the truck having dust accumulation?

Any ideas would be greatly appreciated.

I have the service manual for the truck but it doesn't go into the venting/heating system.

Thanks.

[kevinr1970]

I had a similar problem on my '82.
I got it to partially work - it has heat coming out of the defrost vents, but not as strong as I think it should be.
I checked the vaccum line to from the engine to the resevour ball (next to AC box(?) on firewall, in engine bay), from ball through firewall to back of heater/AC control panel.
The line going into the control panel was broken or became disconnected. I assume it had something to do with the radio as it was right behind there and the dash was cut to put in a modern style radio (square, with out knobs).
I notice when I move my heat control to the middle to low heat position, it is still hot on the floor, but cold in the vents/defrost.
It has been suggested to me that there is vaccum switches on each side of the cab and that I should try to switch out both of those to see if it fixes it.
Good luck and let us know what you do to get it fixed.

[Dr.Destructo]

Hey guy.

Last night I took my truck apart and figured it out.

If you take the dash pad and glove box out, you will see the screws for the defrost vents and the dash vents. you have to remove all these screws and take the secondary venting out. Approximately 5 screws for defrost venting and 3 screws, 1 clip for the dash venting. you pretty much have to get it out of the way to see what's going on.

On the bottom of the venting system, you'll see where it blows onto your feet. It's held on by only ONE screw accessible just below the ash tray. I also noticed that the defrost venting was not securely fastened to the heater system...it looks like whoever replaced the heater core didn't really do a very good job in putting the venting system back so it was losing a lot of air. When I put it back

After I took off the bottom venting for where the air blows to your feet, i snaked using a coat hanging and found that just above that, there was a little vent/fin that switched from defrost to floor.

I assume the motor for it had siezed and it was stuck on the floor vent no matter which switch i had the heater control on.

I unbolted the spring that holds the vent on, and opened the vent so the air was directed to the defrost vent instead. then i reassembled, and presto, full air through the defrost now.

It took me about 4 hours as an amateur to find that little vent. Later on tonite when I get home, I will try to take a digital camera picture so you can see where it is located.

I assume this is your problem too.

Also, if you have a service manual for the truck, look through the section on removing the heater core. Don't go that far as to remove the heater core, just get to just before where you remove the cowl screws and no further.

I will try to post pictures later tonite and give you more descriptive

[Dr.Destructo]

Use coat hanger and/or mirror to see up into this vent and manually open door with your hand...awkward work...may also have to make sure defrost vent is secure where it connects to the interchange above this...would have to remove dashboard venting system (middle selector) to do this.