ToreB

Great, I guess the test will reveal any problems.
The Climate Control Unit is in charge the HVAC servo control, and this unit has indeed the speed signal connected to it. But, as far as I know is this used for other purposes. Changing the speed will not affect the servo operation.
Cheers,
Tore

techman1

Ken, Tore's site is great for diagnosing and getting a understanding of how the systems are tied together.

In your description of the problem, I believe your MAX ax button is making intermittent contact when it is used.

When it is used, it sets everything to optimize cooling. Full cold, recirc, full speed interior fan, etc. It will override the settings of the lower ac button.

Here is a guess of what is happening: Lower ac button is pushed, set fan speed to 2 or 3, temp to cool. All good.
You then push the MAX ac button, and everything should change. I have found that the MAX button does not get used as much, and can develop a bad connection. So it will periodically "flicker" when it is used.

Test this by using only the MAX button. your settings should all change, look at the settings on Tore's site. It should stay this way until you disengage the button. If it is flaky, like mine was, 5 minutes spent cycling on and off repeatedly might clear it up. Worked for me.

Jlbarnett

Timely thread, I just used my Max AC button the other day for the first time and had the exact same symptoms. Everything works when in normal AC mode, all good. As soon as you hit the Max AC button things start acting erratic and the fan goes on and off intermittently.

KeninBlaine

Thanks Techman1 and ToreB. I'll still replace my resistors (ordered them last night), but will continue with testing the other mentioned AC components using ToreB's procedure. ToreB: I have to admit that I had a hard time believing that the resistors would impact my interior fan, and I appreciate your suggestions for me to look further for the cause of my current issue.

I didn't realize that the MAX setting was supposed to make the fan operate at higher speed regardless of my selected fan speed setting. It does seem possible that it is an intermittent contact issue in the button itself, so hopefully some testing and possibly some contact cleaner will locate the cause here. It was fine a few weeks ago when I first bought the car, and only started this week when we've had some warmer weather. Being a mech engineer, I'm a glutton for fixing things and solving problems like this, and it helps familiarize me with the car.

Outstanding advice and support here guys!

AllMine

I had issues with my interior (evaporator) fan functioning intermittantly. Turned out to be a poor fuse connection. I think fuse 11... Just a guess now... Anyway i turned on the A/C on high and when i wiggled the fuse the fan would shut off. Thats how i found mine
Curt

KeninBlaine

Tore: In re-reading your post, and noting your terminology in your DIY document, when you say check the operation of the rear fan, I assume you are referring to the fan in the engine compartment?

ToreB

Yes, the rear/AUX fan is placed in the engine compartment.
Cheers,
Tore

KeninBlaine

So, I just checked my rear fan at the relay, and the low speed is not working. So I just ordered a new ballast resistor for it. When I get all 3 resistors delivered, I'll replace them before I do any more trouble-shooting. Hopefully the new resistors will fix things.

I do find it curious that the 2 front fans and the rear fan have faulty resistors that prevent low speed fan operation. But for all I know, they may have each failed over a long time period, and I'm the first one to check them all.

KeninBlaine

Update: My rear aux fan ballast resistor arrived today, so I immediately installed it. Using the jumper test at the relay, I now have both low and hi speed aux fan. I started the engine and pressed the Max AC button, and found I still have the erratic interior fan speed when set at Max AC (regardless of **** setting for fan speed). I'm going away for a week, and my new front ballast resistors should arrive while I'm away. I'll replace them, and if the problem persists after replacing them, I guess I'll have to remove the CCU and start looking for the cause there.

P-daddy

Keni- can you post a pic of your rear aux fan ballast installed on the car? Thanks

KeninBlaine

Here you go. It is about 2" long, and is located down on the side of the duct below the auxillary fan. The coil resistor is inside the duct when in place, so you only see the 2 wires on the spade connectors. The swap takes about 2 minutes, with no tools needed.
BTW, this is the photo of the old one that was replaced.

P-daddy

^ Ok I recognized this gizmo now. Thanks for posting.

KeninBlaine

Another Update:
Yesterday I installed new ballast resistors on the AC fan and oil cooler fan. Turned out that my bumper is one piece, so since I had to essentially remove it for access, decided to do both resistors while the bumper was off. Accessing the resistor on the oil cooler is a PITA. They couldn't have put it in a more awkward location. Anyway, I now have all 3 fans working properly with both low and high speeds. I'm not sure yet if the original problem of erratic AC fan speed in the car is resolved, but I'm glad the other fans are working properly now. If the interior fan is still erratic, I'll pull the CCU and see what I find.

PS: I took the car for a drive today, and I think the AC is working OK now! When I press the max button, the fan comes on strong and stays that way and is no longer erratic. So it appears the problem was related to the faulty ballast resistor on the AC fan under the front left fender.

pooshee

I had the A/c fan resistor issue and had it replaced. True, it could not be in a worse place to get to... The trick is to leave old one in place, unplug and plug in the new one and mount it behind the head light. You will have to use an "L" bracket of some variety. Saves hours.

The fan speed seems to be erratic in mine as well, but I have ice cold ac and will leave it as is. In my eye it ain't broke and taking things apart back there always leads to things that potentially get broke for no good reason

You have a/c in an aircooled Porsche, count your blessings.

KeninBlaine

I sort of agree with you, but on the other hand, by diagnosing what might be causing my vent fan to act erratically, I did find all 3 ballast resistors were shot and replaced them. It isn't unusual for me to discover other faults when trouble-shooting something.
My only remaining challenge from this episode is getting my right headlight to work again. After taking it out to access a bolt for the oil cooling fan, the headlight no longer wants to work. Being addressed in a separate post.