1972 Blazer (No defrost)

[72 Blaze]

Ok I have read through all the old post. I'm a little SLOW so I'm still not understanding. Does my 72 need vacuum to run the defrost? I noticed from an earlier post that the cable on the lower unit. Right above the trans hump switches from flooor to defrost. It seems to be working. I had all the air/heat ducts off and I'm replacing them now.. Just put in a factory radio and discovered that some of the ducts were cracked and broke. I hate to put it back together and not fix this problem. The two vac hoses that are attached to the dash controlles seem to be ok. It blows great but will not blow on the floor or the defrost. I dont think I need to crank the truck to build vacuum? Do I? What else can I check. I read about a white plastic clip. Does a 1972 have the clip? Also my fan blows on 1,2,3 but when I go to 4 it stops or slows down.. I'm not that worred about that. It blows hard enought on 3! And ideas or help would be great. And thanks in advanced. I't a cts so it had air.. But now its just a 350 crate with no air hooked up.

 

[British Slave]

You need it running to have vacuum, unless you have a vacuum pump for testing. Was the vehicle originally equiped with factory air conditioner?

 

[72 Blaze]

Yes Sir.

 

[Confedneck]

yes, the truck needs to be running to create vacuum

 

[72 Blaze]

I know I made my self look stupid. I know it has to be running to create vacuum. But does it need the vacuum to blow through the defrost? I can't just have the key on and the fan while switching the dash control to and from defrost? Thanks guys!

 

[Confedneck]

well it needs the vacuum to actuate the directional doors

 

[72 Blaze]

10-4! I will crank it up tomorrow and see what I get. Thanks for your help! Would crank it tonight but the neighbors might not like it! Lol!

 

[72 Blaze]

I wish thinking it was all mechanical on the first gen blazers. Thanks again.

 

[72 Blaze]

OK guys I'm starting figure this out I think.. BUt I still need some help. I cranked the truck up.. No luck. So i started tracing the vacuum hoses. Looking at the motor behind the trans tube on the fire wall I found two hoses un hooked. I took the bigger one and put in on the carb. Just for now to pull vacuum. It seemed to help I now have vacuum on the door that opens and closes on the kick pannel. I could hear a noise and I found a Tee under the dash.. Now that thats fixed the noise is coming from the top of the control unit (fan heat defrost controls) where the two hose attached... could that be bad? It looks to un screw with two screws but I didnt see where I could buy a new one. But Im sure the noise is coming from there. I have no ac any more or a black ball.. Should I attach the other hose to something or plug it?? Thanks guys I have been chasing this for awhile! Also whith it blower on and lines hooked it I can pull the vacuum hose off the disc on the kick pannel and it shuts with it on it stays open.. Is this correct? no matter where the switch is it stays open. I think im getting closer but I'm not quite understanding it yet!

Thanks

 

[British Slave]

Defrost is actuator #53. It can be changed in vehicle.

 

[British Slave]

You only have two hoses to the control switch? If it was factory AC you would have 5 vacuum lines.

 

[British Slave]

Your control says Off-vent-heat-defrost?

 

[72 Blaze]

Ok I ran out to the garage! Thanks for that diagram! Here's what I have. The noise I have is coming from the 1st pic.

 

[bigblock72]

The vacuum switch is only used to run max a/c; it only opens the vent door on the kick panel, closes the door in the cowl and shuts off the coolant to the heater core. You'll only need to mess with that if the a/c actually functions.

ALL other functions are handled by mechanical cables. I'll bet the door that switches between vent and floor/defrost is broken and is stuck in the vent position.

 

[British Slave]

That's a little different from mine, but you can get that switch at LMC, maybe at a local auto parts. Chevrolet dealer for sure.

 

[British Slave]

page 148

http://www.lmctruck.com/icatalog/bzc/t.aspx?Page=148

 

[72 Blaze]

Thanks for the replies! I will pick up the switch before I put it all back together. But the flaps only work by mechanical cables? So really I don't have to worry about the vacuum with no AC. I'm just trying to get my defroster to work. Thanks again for the help!!

 

[Fordum]

Don't have one of these beasts, so I don't know when they changed from mechanical to vacuum, but you have to have all vacuum lines plugged with something.
For two reasons.
One, an open line will kill or seriously degrade the vacuum to the whole system, and two, an open line = vacuum leak, which if you read things here, is considered a bad thing.

Note: Plugged does not necessarily mean plugged. It means hooked to something that blocks the vacuum.

 

[bp71k5]

Ok I ran out to the garage! Thanks for that diagram! Here's what I have. The noise I have is coming from the 1st pic.

Those little vacuum switches in the first pic fail a lot so that might be where your leak is. It could also be the hoses has dryrotted, or even a splice/tee that's bad. Since your switch only has two ports(some have three), you should have a tee somewhere under there with a third hose going to one of the flap doors.

-Brian

 

[72 Blaze]

Ok I think I found the one problem! Thanks to y'all. The flap in side the heater box. Controlled by the Ac / Heater cable... Any one know where to get one?