hinckley

I was wondering whether anyone has experienced this problem . . .

After driving for about 30 minutes with my top down and the HVAC controls set to "A/C Max", all the vents close. The fan continues to run at max, but no air flows into the car. It's almost as if something has frozen inside the system as I can't get any air out of any of the vents even if I press one of the manual buttons for air distribution. But if I stop the car for just a short period - enough time to fill the gas tank for example - and restart the engine, everything returns to normal with air flowing through the center vents. Has anyone experienced this?

LexVan

Well, driving with the AC on and the top open is murder to your HVAC system and MPGs. But I get why some people do it. Just FYI.

On the very rare occassion that I'll run the AC Max with the top open (for example, when I come off the highway on a very hot afternoon [+90] and just want a little cool air for the last 2 miles before I can pull into my garage, but still want the top open) I've never seen this issue, but my drive is only a few minutes at that point.

What happens if you turn off MAX? Remember, in the MAX setting, the car is only recycling the cabin air, and not bringing in the outside (warm & humid) air. So, maybe there is a servo not working on the mixing door for the HVAC. I assume your frunk cabin filters are good, and the inside cabin filter under the glove box are good?

Needsdecaf

I wouldn't be surprised if your evaporator is freezing up from such hard use. I wonder if this is some defrost mode it's going into.

LexVan

Excellent post.

hinckley

Yes and yes.

I had thought that it could be something like that, but if that's the case, why wouldn't warm air just be blowing? If it's some sort of systemic shutdown (understandable with a heavy strain on the system), why would all the vents close but the fan continue to run at max?

Macster

Needsdecaf I think has the answer.

The system didn't shut down from the what you wrote. The evaporator froze up from the extreme cooling demanded and the high humidity. (This evaporator freezing up is a concern and the reason why the A/C controller will not engage the compressor in cold (~32F) temperatures or colder to prevent the cold outside air freezing any condensation that gathers on the evaporator from freezing the evaporator and blocking air from flowing through it.)

In your car's case the evaporator froze and air stopped flowing through it. The air control flaps were set to route all air through the evaporator for maximum cooling and this is why the air stopped flowing but the fan continued to run at max.

When you stopped the car and turned off the engine and the compressor stops running the latent heat around the evaporator thaws, at least partially, the evaporator. Then when you restart the engine the A/C works normally again, at least for a while.

I had a similar problem with an apartment wall mounted A/C heater system. The air intake was down low and the cool air exhaust was just 2 feet or a bit less right above the air intake. The A/C would run and the cold air would of course be expelled but would encounter a bed that blocked the flow of air. (Small apartment and no other place to position the bed.) Some of this cold air would be directed down and the A/C would pull this cold air in. On really warm days the A/C would run to the point the avaporator would freeze up and no air would flow. The fan would keep running though. I would shut off the system and after a while it would thaw out and then when I switched it on again after a while the A/C would run normally and cool. Until the next time it froze up.

I took the unit apart and was able to view the air flow arrangment with the sheet metal removed and spotted a panel in the outlet vent. I closed this and this blocked a considerable amount of air from flowing out the vent above the air intake. Instead the cold air was forced to flow through a duct that led to an outlet vent in the living room.

Afterwards the A/C would come on and run and cold air would be routed to the living room. The A/C as a result would pull in warmer air from in the bedroom and cool this.

The A/C worked better, kept the apartment cooler and the cooling was much more even/balanced and it never froze up again.

My recommendation is you avoid running the A/C at least set to max with the top down. If the evaporator freezes my concern is the liquid that flows through it is not transformed completely into a gas but continues on, at least some of it, as a liquid. I'm not sure what effect this might have on the compressor which is really intended to compress the refrigerant as a gas not as a liquid. (The liquid can't compress and this could damage the compressor.)

Additionally the vent fan contines to run but does not have any air to move and thus has not cooling air flow and the fan motor can run hotter than it would otherwise.

Over time the system can suffer I would hazard a guess expensive to fix harm.

gearFX

So do Porsche cabriolets include some kind of "logic" in the "Auto" mode that automatically changes to vent fresh air only or dials back the A/C when the top is down (unless overridden by "Max A/C" button)? Or is it up to the owner to continually make those adjustments once moving (and adjust it again every time one comes to a stop with no airflow coming from outside the cabin)? What do most people do (or recommend)? I didn't realize this wasn't a set-it-and-forget-it thing that might strain the HVAC system.

JCtx

'Max A/C' makes no sense with the top down, as you can't recirculate the air. Plus don't think it's meant to be run continuously (most cars don't have it); only a few minutes to cool down a hot cabin. Anyway, don't use it with the top down. Just crank the fan speed to max but in regular A/C mode, and see if your issue goes away. Good luck.

Noah Fect

It's surprising how effective the recirc button is with the top down in my 981. Good for eliminating diesel exhaust, etc. from neighboring vehicles.

Haven't seen any evaporator freezing, but I don't typically use MAX A/C mode with or without the top down.

evilfij

I had this on a BMW. The evaporator would freeze. Turned out it was a little low on R134.

hinckley

So do you use A/C at all with the top down?

That's probably it. Thanks.

Funny enough, but there was a post on Bimmerfest yesterday describing the exact same issue.

evilfij

I see nothing has changed since 2004 with BMW.

py0413

I have a .2 coupe and only once in early June, the AC stopped working properly for about 1/2 hour. It was a hot day so I could tell quite easily that the AC wasn't doing what it was supposed to do. Played around with the switches, turning fan up/down, max a/c, nothing seemed to work and all I heard was the fan kept blowing. So I decided to make a quick stop and restart the car after a 5 mins break, everything was back to normal and it never happened again.