A/C plenum damper question

[Tom L.]

For YEARS I've been chasing a cause to the brutal flow of hot air from the floor ducts onto my feet in the summer time. It's a big block and many members have provided suggestions that might reduce the heat. I've addressed all of them but the heat continued unless I stuck rags into the floor vents in the summer. Something that I hated to do but was a Bubba type short term fix.

Recently decided to drop the dash in the car and perform some repairs to things that are under there which is unpleasant to perform. Since I didn't have a clear understanding of how the dampers work I used my vacuum pump (I'm in the refrigeration business so I have one) to operate the vacuum system with the engine off and with everything apart. Fortunately all of the actuators work properly when 20" of vacuum or more are applied. I did notice that the damper that diverts air to either the floor, dash for defrost or both was sluggish when the vacuum dropped to about 14". It didn't close completely. I'd love to provide a vacuum reading at idle but right now I can't. The last time I took a reading I believe that it was approximately 18". It's a bone stock build.

So here are my questions. Anyone have this issue? And being as it it 45 years old, what would be the best lubricant or method to lubricate the friction points? I'm thinking that during normal operation the damper isn't seating and that's my heat issue. Any help??

Thanks!!

[Mark E.]

Re: A/C plenum damper question

Tom,

I certainly empathize with the hot air problem, and the prospect of tearing into the dash.

The door which diverts air to the floor, dash and defroster is the diverter door. Although you may have found an issue with it, I doubt it's your source of hot air. This door merely routes the air; it doesn't control air temperature. With that said, post 7 in this link may help with getting it working properly: http://www.corvetteforum.com/forums/...ac-blower.html

Likely sources of hot air coming out of the floor vent:

- The temp control door (controlled with a bowden cable from the temperature control wheel) isn't working properly. This door opens or closes air flow through the heater core.

- The valve that shuts off coolant flow to the heater core isn't working properly. This can cause hot coolant flowing into the heater core when the heater is turned off. I live in Texas where a car heater is redundant (especially with a big block inches away from my feet), so I completely disconnected the heater hoses from the heater core. This resulted in much less heat in the dash. As a Yankee, you may choose to keep the heater operative.

- Hot air entering the plenum. The primary sources for this include: A poor seal along the seam between the AC evaporator box and "firewall" (the caulking tends to fail with age); missing or poor seal around the heater core water outlets; missing or failing drain nipple on the AC evap box; poor seal between the rear of the hood and top of the "firewall" (to check this, remove the cowl vent panel and check for a tight seal); poor seal in the kick panel plenum (where the two fresh air/recirculate air doors are located). The idea is to prevent hot air entering the plenum from the engine compartment .

You might find the FSM and this link useful with understanding how this system operates.

http://repairs.willcoxcorvette.com/category/heater-and-ac/

I also acquired a nice oversized, laminated schematic of the system for the garage wall. I think at least one of the Corvette parts catalog companies sells them.

Keep us posted.

[Edward J.]

Re: A/C plenum damper question

Tom, does the temperature blend door have a foam seal on the door? this was happening on my 63 letting hot air by-pass the door and while driving the air flow through wiper grill would let hot air flow around the door and into pass compartment at floor ducts. this seal was deteriorated so bad.

[Tom L.]

Re: A/C plenum damper question

Thanks for the replies, first here are some additional notes. As for dreading the tear down of the dash, I'm already there so I'm sure that the "diverter door" does move as I indicated and the seal is in fine shape. (A hard rubber seal on my '72). As far as the heater valve goes, there isn't one on '72 cars. I did put one in a while ago but it didn't help, only the rag did. I checked the temp control door, couldn't see it but when moved to the cool position it seemed closed and as far to the passenger (closed) side as it would go but left a small gap when in the cool position. Not something I could visually check but I did get my hand in there to feel what I could. I also checked the seal around the hood, it's not old and seems great.

The One thing that I'm not sure of in your reply is the passenger side kick panel seal. While in it's resting position (closed without vacuum) it seems fine. With the engine running and the a/c - heat system off it gets vacuum and is opened. Either way, it does open and closes completely. There doesn't seem to be a seal that can be replaced either way.

One thing I did not include was the cowl damper. When in it's closed position there appears to be about a 1/4" gap ( the grill is off so I looked closely).

Going back to diverter door. When vacuum is properly applied while in the OFF position it closes completely. Would't that prevent airflow to the floor vents?

Since it's all apart, and I'm looking at it work, that damper seems to be the only thing that would allow air to the floor vents.
Again, Thanks!!