JPS

So intermittently I get an odd AC issue. If I turn the AC on, and the temp gauge even a little up from full cold, the cold sometimes disappears altogether and goes warm (but not hot), even if I turn the temp back down. This is true of all the vents, not just one.

Usually if I fiddle back and forth on the temp again, it will go back to blowing cold.

I've read through

http://forums.rennlist.com/rennforum...diagnosis.html
and
www.porschehvac.bergvill.com

So it seems to me this is likely a mixer/flapper/servo problem? Here's the rub. If it was a servo issue, wouldn't there be warm coming from just one place rather than all the vents? Or am I missing how the system works?

Thanks,
JPS

Leander

When I had the servo flapper issue, I got super-hot air out of all the vents.

JPS

Was it one of the 3 in the boot or one of the 2 in the footwell?

nile13

JPS, please try to isolate the issue this way - does it only exist when small snowflake button is pressed or is it also the case when the big snowflake is On?

993/907

It's quite easy to access the servo's in both footwells to see if they are responding to signal from CCU....so easy in fact, I was able to do it.
If one or both do not close, that ducting will bring incredibly hot air into the dash distribution system.

JPS

Big and small

LOL

nile13

I'm going from hazy memory here, but I believe it's a relay near the rear blower that's intermittant. On the other hand, big snowflake forces CCU into "full cold" mode in theory, that's why I've asked about the differences.

See if you can contact Tore.B, he might know. Randall.G was my guide on this years ago but he's no longer here. Read through this thoroughly, though: https://rennlist.com/forums/993-foru...l-anymore.html

ToreB

JPS, you are describing a typical failing mixer servo(s) scenario.
The mixer servos can act like this if they are beginning to fail. The variable resistor inside is wearing over time, and sometimes it will give the wrong position feedback to the CCU, and thus will stop moving in the wrong position. Therefore the full heat. Wiggling the temp control will make it move again, and thus maybe move into an area of the resistor that is OK again.
I'll suggest to check both mixer servos, I have seen several cases of both of them being shot. After all, they both move siimultaneously, and thus get the same abrasion.
You could also do a OBD check for error codes in the CCU, that would determine any previous servo faults if they suddenly decide to work OK during the testing.

Having A/C on or off in this has no influence on the test. But, pressing the big snowflake button in addition will set temperature to minimum, and you would thus not be able to adjust the temp button to exercize the mixer valves. See my HVAC page for details on the CCU **** operation.

Cheers,
Tore

nile13

Tore, I'd suspect the same thing. What's different, however, is that JPS isn't getting hot air as I'd expect. He's getting warm/ambient air if I understand correctly.

ToreB

I agree nile13. I wouldn't be surprised if more servos are failing in JPS's car, especially the fresh air intake. This, and the two mixer servos, are the ones that move the most in our cars. Inert fresh air intake and/or mixer servos would make it difficult or impossible to regulate the air temperature.
A OBD diagnosis would determine most faults in the 993 climate system, but all servos could also be tested in minutes using the procedures described on my web pages.
Cheers,
Tore

pp000830

low RPM = engine warmer and ac compressor less functional.
higher RPM= better engine cooling and compressor more functional.
May simply be at near idle the AC is not able to push out cool air.
Just a thought,
Hope this helps,
Andy

JPS

I've reviewed the links but will do so again. I wasn't able to dig in this weekend, but interestingly was driving the car and needed the heater for I think the first time since I've purchased the car.

At first it wasn't getting very hot, more ambient, sort of like the air wasn't getting cool, more ambient. Seems to also point to the mixer/flapper/whatever servos in the cabin. I hope to check them out this week.

Thanks.

nile13

I would seriously consider blaming CCU, actually. What i'd start with is cleaning the internal fan. Methinks it's confused about internal temperature and what's asked of it. That's actually easy to check with heat. Turn the temp all the way clockwise and see if it gives you full heat all the time or if it becomes ambient at some point.