Need to get air coming out of Defroster vents

[clay1937]

I just picked up a 1971 Blazer that I'm in need of some heater & A/C control info for (yes, it's equipped with factory A/C...although it doesn't presently work).

When taking it in for the required state safety inspection, they failed it due to fact that no air is coming out of the defroster vents onto the windshield.

No matter how the sliders on the heater and A/C controls are adjusted, air will not come out of the defroster vents.

I think this is at least partly related to the heater and A/C contols themselves. The top slider control (the Inside / Air / Outside / Defroster slider), which is likely the culprit, doesn't appear to be connected to anything at all...a quick peek at it from below the dash shows that when moving this lever, it moves independently, and just flops around, back and forth. I'm not sure what this needs to connect to.

The two bottom slider controls (Outlets and Temperature sliders) appear to be working, such that they are connected to cables that they move.

I noticed another cable though, that is easily seen without crawling under the dash...its attached to a lever near the driver's side defroster hose and floor heater vent. None of the sliders are moving this cable, so I'm suspect that the top lever that isn't attached to anything, should be moving it, however, manually moving it with a pair of pliers didn't cause air to come out of the defroster vents either.

Also, when the engine is running, there is a constant "hissing" noise that sounds like air leaking out of a tire. I think is related to the problem. When the engine is turned off, the hissing noise stops. It doesn't happen when the key is in the On position...only when the engine is running. This hissing gets irritating to hear while driving.

So in a nuthshell, what I'm looking for is:

1. Info on the heater and A/C controls to get the defroster working (including where and how to re-attach the top slider )

2. What is causing the "hissing" noise when the engine is running...I think it's related

The blower motor is very noisy, but at this point, I don't really care...I just need to get the defroster working so that I can get the safety inspection passed...I can fix that later.

I haven't torn into this much yet, but am hoping there's a good reference someone can point me to (I've ordered the factory service manual, but would like to fix it sooner than this will likely arrive). Or, if someone is really familiar with this system, some tips and info would be greatly appreciated.

Thanks!

 

[70jimmy]

Check the hoses going up to the defroster vents from the heater box in front of tranny hump and make sure they are hooked up and intact so that air is actually getting to the defroster vents.

I don't have air con but on the front of the heater box a little toward the passenger side a cable should be hooked up to a sliding lever, this is what moves the (butterfly) or whatever you call it that directs the heat from the floor to the defroster.

 

[bigblock72]

You're probably missing the cable that runs the heat/defrost door or the heater control lever could be broke or the hoses to the defrost vents are missing.

The hissing noise could be caused by a broken vacuum hose under the dash or a broken vacuum switch on the back of the heater controls. Vacuum only runs the recirculate diaphragms and really isn't needed if the A/C doesn't work.

Looks like you're in my neck of the woods so swing by and I'll help you figure it out.

 

[clay1937]

I should have mentioned that I did check to make sure the defroster hoses were hooked up, but I'll check the other suggestions and let you know.

Thanks!

 

[Greg72]

If it were me I'd check to make sure that the system can blow air out from somewhere (not necessarily the defroster vents). That will tell you that the fan motor and switch are OK.

After that, I'd take off the defroster hoses where they connect to the plenum box and see if there was a way to move the flapper than re-directs the air....somehow force those flaps to stay open to direct air upwards.

It's a crummy job to deal with. The heater plenum is tucked into a tight spot and there are hoses everywhere (especially with an A/C system). Those factory cables from the dash controls always seem to freeze up and never slide worth a damn....at least none that I've even owned.

 

[bp71k5]

For reference, the lever that controls the defrost (upper vents) is about 4 inches above the floor outlet for the heater (right on top of the tranny hump)

 

[NV_K5]

Wow!

 

[clay1937]

Thanks for the info. I got it working well enough that they shoulod pass it off for inspection.

I think next year I'll pull the whole thing out and put a Vintage Air system in.

Thanks!