No defrost

[hondarider188]

 I have a 1983 Chevy k10 with a 350 engine. My defrost doesn't seem to work right it just seems to blow everywhere and not very much at that either. The vent and heater settings work great but defrost doesn't. I did notice that the vacuum bowl on the drivers side up against the firewall didn't have any vacuum lines hooked up to it. Does anyone no what the problem could be?

[bd]

...the vacuum bowl on the drivers side up against the firewall didn't have any vacuum lines hooked up to it....

Vacuum is what operates all those fancy doors under the dash.

[hondarider188]

Hate to say it but I don't know what you mean when you say fancy doors under my dash. But any ideas on why the defrost doesn't work to well at all.

[zieg85]

On factory AC cabs the vent/floor/defrost (top slide on the heater/AC control) there are door actuated by vacuum.  There is a vacuum line from the round canister to inside the cab that runs everything...

[bd]

Thank you zieg! 

Hondarider - movable 'doors' control the airflow through the Heater/AC/Defrost ducting under the dash.  You can think of them as being similar to railroad switches that direct train traffic from one set of tracks to another, thereby controlling the destination of a train.  The doors control the destination of airflow out of the Dash vents, Defrost vents, Heater vents, etc.  Assuming you have factory A/C, there is a small vacuum operated rectangular door (called the Mode Door) located above the center floor hump directly in between the Heater/AC housing and the Defroster ducts.  As zieg stated, there are several other vacuum operated doors, as well.  There is also one cable operated door, the temperature door (also called the Blend Door).  With a GM factory AC/Heater system, vacuum is necessary to make the Defroster (Mode) Door function.

Regarding airflow volume, you need to reconnect the vacuum supply and repair/adjust the various doors for proper function.  I highly recommend you clean out the cowl below the windshield, as well - follow this link... http://forum.73-87chevytrucks.com/smforum/index.php/topic,12521.0.html

[hondarider188]

How do. Which vacuum line it is and which canister it comes from, the reason I ask is that I have to canisters one with a line coming from it and the other has nothing to or from it.

[bd]

Do you think you could post images of the two canisters?

The one for the AC/Heater system is black plastic and round like a softball.  A 7/32" rubber hose connects the 'ball' - through a small in-line plastic check valve - to manifold vacuum.  A plastic tee fitting between the 'check valve' and the 'ball' connects to a second vacuum hose that runs inside the cab through the firewall.  If you look around, you might find the original vacuum line into the cab.

[hondarider188]

Yeah i will try to get some pictures tomorrow. But saying that the a/c has a canister too make me think that the one that isnt hooked up goes to the a/c because i am missing the a/c compresser and the on is near this to, but at the same time the a/c still blows air but it just isnt cold.

[hondarider188]

sorry didnt get a chance to take any pictures. does anyone know where the vacuum line for the a/c-heater would come through the firewall?

[Captkaos]

It comes out right about the AC box.  You can see it here:

[hondarider188]

thanks for the pic to. i found the line yesterday and pulled it off the canister and started the truck and it is sucking air so that leads me to the dash. I belive that either the doors need adjusted or i Have a leak on the vacuum fo the doors, but either way i need to pull out the dash. Does anyone know which way to go in like from the top or bottom of dash? thanks.

[bd]

You can access some of the vacuum hoses and the Blend Door adjustment through the glove box opening after you remove the glove box from the dash.  You can reach some of the vacuum hoses and the Mode Door vacuum motor from under the dash after you remove the plastic duct that runs under the steering column from behind the ashtray to the leftmost dash outlet.  And, you can access the vacuum hose connections to the back of the Heater/AC control head assembly after you remove the large plastic instrument panel that covers the gauge cluster and radio. 

You probably ought to begin by simply listening for door movement and vacuum leaks under the dash as you operate the Heater/AC controls while the engine is idling outdoors.

Vacuum runs from the engine compartment reservoir to a multi-port vacuum switch on the back of the control head, then on to the vacuum motors that operate the various doors.

[hondarider188]

Thanks for the help. I pulled the glove box out yesterday and found the door for the vent control and the hot or cold setting and they both are working fine. But does anyone know exactly where the door for the defrost is, because thats the one i need? Do you know if i would be able to just pull off the entire top dash cover and be able to find the doors? from what i can see it looks to be a little easier to get off than going behind the dash and pulling out everything from behind.

[bd]

...does anyone know exactly where the door for the defrost is...?  Do you know if i would be able to just pull off the entire top dash cover and be able to find the doors?...

The defrost door is in the very top of the heater housing at its leftmost end, directly over the center hump and center floor outlets, below the defrost ducting.  The vacuum pot that operates it is sandwiched between the heater housing and the firewall.

There is no access from the top of the dash.

[hondarider188]

well i finally got the defrost fixed. It me a good 8 hours to get it done finished and put back together. In reality it shouldnt of taken that long, but i also wired in the front dash speaker since they had been cut out and i messing around the the stereo system figuring out how it all worked and ran together. On top of it all i ended up taking the entire dash apart and out of the truck down the metal base support only to find and realize that the defrost and the normal heat worked perfectly, but the vent door( wish is the simplist and easiest to get to) was the only thing broken.  The door pully had broke so it couldnt close the door more than half way, just letting all the air kind of out everywhere with the other door open to. So thanks for the help I couldnt of ever found any of it without this sit.