my 93 does not seem to fully shut off the airflow to the defrost ducts when on A/C. it makes this anoying fog/condensation on the windshield. is this a comon problem? what door is not completely closing?

 

Is that on the inside or outside?

Mine is outside and depending on humidity it can get annoying
That's also how i found out the rear window defogger don't work.

 

on both lol..

my defogger was just one of the plastic clips on the back window had melted.

 

Do you smell coolant ??

 

That is what id be wondering too, the only reason i can think of for the A/C fogging anything inside is steam from a leaking heater core.

 

My 93 is fogging up on the outside, and it is not due to the heater core since I have disconnected that. My Ranger does the same thing on the back window (not an extended cab). The reason? It is so f'in humid outside the cold air hits the windows and causes the condensation. This tends to only happen to me early in the morning or late at night.

I have found if you use the recirc button instead of auto, the window will fog less. One more thing you might want to check is the check valve under the passenger side fuel rail cover. Mine was broke and the vents did not work right at all (lowers would blow high, window would blow, it varied day to day also). I fixed it with an electrical butt connector and now all is good.

 

no coolant smell it is actually condensation that forms on the outside of the windshield due to the cold air blowing on it from the defrost duct. this happens in high humidity wich we ave all the time down here. for some reason the defrost door does not completely close off in the A/C position.

 

If I'm not mistaken, and if I am I'm sure a correction will be forthcoming, when you use the AC, a certain amount or air is supposed to be directed to the windshield.........and when you use the deforster, the AC will cycle on and off.

 

Oh well that is to be expected mate, my 87 and 94 both do that. Its when its on the inside with the A/C on that you need to worry about it.
First bit seems to be in every Corvette ive ever driven, 2nd bit may be a climate control thing, although it never did on my 87 with C68, my 94 has C60 and it doesnt run any different on that setting. The compressor will cycle on and off to regulate the pressure anyway, that is how it works.

 

Here is the part I'm referring to. I dont know if any local stores carry this part (if yours is broke and you find a store that has it, please let me know)



(this may not even be broke, but its worth looking at as it only takes 5 seconds)

Found it... Advanced Auto, Part #47150 $3.48

 

.........there is a "proceedure" that the earlier-mid c-4's do that when the a/c is switched on, the air will blow out the bottom first and then after maybe 5 seconds or so, will switch to the "proper" vent/duct...this is to prevent the windshield from fogging up.....if you do a search, you'll find where SunCr has addressed this issue a week or so ago.......

 

GM directs a small amount of air to the windshield to keep it from fogging over (and some to the heater vents to cool the floor) when a/c is selected. On start up, there's a purge function (at least for Electronic units) wherein all of the air is directed to the floor vents. This allows some dehumidifying to take place. It isn't perfect as the engineering seems to have been to strike a happy mid point; ie, no a/c system is designed exclusively for the dry temps of the desert or the exceedingly high humidity of the South, it's all a compromise. A lot of fogging can be: 1. a leaking evaporator - refrigerant is below zero at normal atmospheres - that will show up on either side of the windshield, sometimes as frost - have it leaked checked. 2. a plugged up drain hose or hole allowing the dehumidified air to pick up too much moisture. Jack it up and make sure the hole or hose is clear. Use a toothpick or other soft object on any drain holes so you don't poke a hole in the Evaporator. 3. A leaking heater core which if there's coolant in the radiator, you're going to smell it. 4. Leaking duct work so the air doesn't go where it was designed to. The defroster vent is pretty much a force fit with a small piece of insulating foam to seal up the joint. That foam seems to have a life span of about 10 years and a leaking evaporator wears it out faster. Removing passenger and driver's side hush panels to see if there's air blowing behind the dash is a quick way to determine if things are still sealed up. 5. Vacuum controls though most of the OEM's (including GM) sucks the defroster door shut until defrost is requested at which time it sucks it open - though gravity can accomplish the same thing. When they're broken, all you get is defrost.

 

......what he said!.....

 

It's been a while, but it seems to me that on my '77 there was one wire in the connector to the fan that if disconnected will eliminate the always on fan. I have the same trouble with the '90 vert that I just bought but haven't had a chance to check it out. If my poor memory serves correctly it was either brown or brown/white. I don't have a complete wiring diagram, so I'll do the process of elimination. I hope this helps.