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AC/Heater Vents Question



Hi all,

My vette has had a terrible smell in it ever since I got it, so I went searching for anything that might be trapped in the AC/Heater vents this weekend. (Bugs, mice crap, ...)

I also noticed, that while driving, when the A/C-Heater is turned off, there is still hot air blowing out of the vents - which really heats the cabin up quick... I did the trick to turn the blower off when the setting is on "off" on the control, but hot air (straight from the engine compartment it seems) still blows out of the vents.

While looking for any stinky stuff in the vents, I also looked to fix this problem.

Could some one explain to me what these doors do in the AC/System. I think "1" controls the air to the defrost vents, and "2" controls when the blower is on..... But, there is another door that I can see inside of "2", that sticks open all of the time. I think that this is what controls the MAX/Normal AC settings. If this is sticking open all the time, is this where My hot air from the Engine is comming from?

Also, how do I get to the actuater that controls that door? It seems buried in that box, behind door "2".

Sorry this post is sooooo long.

Thanks for any help,

Aug 29, 2001
Norcross, Georgia, United Stat
2017 Arctic White Grand Sport
I've been playing with my 77.

First thing I would look at is the hot water control valve in the heater line from the front of the block and the vacuum line that goes to it. I've read a lot of posts about this and it appears to be a problem. It is cheap and I would start out by replacing it.

Assuming the 78 is the same as the 77, vacuum is supplied to the valve when the heat control switch is all the way cold. Mine is about 6 psi, I don't know what it is supposed to be. If you are getting no vacuum, then you will be getting hot water to the heater core when on cold.

If you are not getting vacuum, there is supposed to be an adjustment that you can do that will control this. I'm not sure how this works but got the impression that the adjustment of the cable that runs from the temperature control slider to the hardware on top of that area you have labeled #2 impacts this, though I'm not 100% sure on this. There is a device on the cable that by turning it you adjust the door that controls the amount of hot air that comes from the heater core. The vacuum switch is up there as well. I don't know if adjusting the cable impacts the vacuum switch.

If you can't get this to produce vacuum at the hot water valve, you may have to replaces some lines.

As to the area marked #1, that appears to be the collector that diverts the air to the defroster. I'm not 100% sure but think the door marked #2 controls the amount of hot air coming off the heater core and if it is, I understand that that can be adjusted to get a better seal to keep hot air from coming in to the car from the heater core.

The 3rd door, appears to be the one that would control the flow of air from the floor, to the vents and then to the defroster. Does yours switch among the three outputs when you slide the switch. Ths is also a vacuum controlled function.

Hope this helps. Some it if I played with myself, the rest is from postings here. A search will come up with a lot of information on this problem.



Thanks bobchad

Thanks for the quick reply.

I have verified that the control that opens/closes the heater hoses works. That was my first thought, and luckily and easy fix. I'm still trying to figure out why the air is still "blowing" (Once I get up to speed) when the controls are all "Off", and the blower is disconnected...

Thanks :)


hi T, sorry to hear about your problems. But I have the exact same problems. :( I don't however have A/C so some of the controls may be different. I don't know what to say. I can't offer any advice either. Except that I am having my vette mechanic look at it this spring. Good luck.

Scott :w


Motor head!!!!
Feb 13, 2001
Valencia, CA,USA
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!

Stick you hand into the area where the evaporator is and remove the "Felt" pad of JUNK that has accumilated on that side of things.

You will be suprized what you pull out.



I can't help with the foul odor . . . don't have that problem. On my 75, I was getting warm air out of the vents when the controls were in the 'vent' position. All of my ducting doors were working properly, so rather than pulling my hair out, I decided to just 'live with it'.

By chance, while replacing all of my weather stripping, I also decided to replace the rear hood seal (it was missing when I bought the car). This seal REALLY helped to reduce the temperature of the incoming vent air. The air intakes on this particular model are in the outermost corner of the windshield wiper trough. Without the hood seal in place, it was allowing hot air off of the engine to whistle between the hood and the bulkhead that seperates the wiper trough . . . and consequently, into the air vents. This seems to be a HUGE design flaw. Without the hood seal, you are not only subjected to the hot air, you are also breathing any noxious vapors coming off of the engine - obviously a dangerous situation.



as far as the control valve in the heater hose goes...

Mine stopped working and prevented me from getting any heat into the car so I removed the valve and threw one of those t fittngs that comes in a flush kit in the line and plugged up the end of the vacuum line with a nail and a baloon (I can hear you guys laughing now) and I don't seem to feel any extra heat in the car when I don't want it as long as I slide my controls all the way to the left. I hope this helps.

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