T_MaX

1989 928 S4 Climate Control System

The symptoms are:

The Climate Control System either blows nice cold air (when the temp lever is in the 65-degree setting) or hot air when moved just below the 65-degree setting.

Let me also say, I have checked the Rennlist archives (under AC controller / A/C controller unit / HAVAC Control Unit / Climate Control System), searched though as many 928 web sites as I could find, read though the PSM and the Porsche Climate Control M563 Supplement.

Here is my Checklist thus far.

I've checked the complete Vacuum System for leaks. (Holds as designed)

I’ve checked both the inside and outside sensors. (Both good all the way back to the Climate Control Unit)

I’ve checked all the Flap positions. (All operating as designed)

I’ve checked the center vent micro switch (Operating as designed)

I’ve checked the heater control valve (for vacuum operation). (Operating as designed)

The last item is the Climate Control Unit!

The only thing I have found to be out of tolerance is the potentiometer that controls the servomotor for the heater core flap (temperature selector lever operated).
The range for this potentiometer is from 760 +/- 100 Ohms (@65f degree setting) to 1760 +/- 100 Ohms (@85f degree setting).
I can get the 760 Ohms @ the 65f degree setting, but at 85f degrees I can only reach 1100 Ohms maximum.

Question
1. Should I test the Climate Control Unit hooked up or on the bench (I did it on the bench)?
2. Have I over looked something?
3. Is this difference in the potentiometer reading great enough to cause this problem?
4. If so, has anyone come up with a suitable replacement for just the potentiometer?

All responses welcome.

Nicole

This is clearly a Wally question...

Best of luck, Max!

Niels Jørgensen

FWIW, mine does something similar, and it always has. I figured that was just how A/C's worked in 1987

Given the climate in Denmark, A/C is not a major priority; as long the heat works I'll be fine .

Nicole

Niels: The problem is that if you don't use the AC, the seals dry out and you'll have an even higher repair bill later. Also, the AC is pretty helpufl on rainy days (keeps the windows from fogging), and even on cold winter days (keeps the windshield from icing from the inside). I drive with climate control on all year round - both Saab and Porsche.

Niels Jørgensen

I know, and I almost always have it on (summer and winter) for that exact reason. Never the less, about a month ago it started dripping on my right foot, so now (and until I figure out why) I only turn it on occasionally.

I just meant to say that I'm not very fuzzy about how "well" it works, and usually sets the temperature quite high (maybe that's why it's leaking now?).

My problem is that I can't stand the cold A/C air - I always turn it off in hotel rooms, I hate flying because the A/C makes me ill and, in spite of the lovely climate in California, I usually leave Calif. with a cold because you people seem to think a working enviroment should be cold enough to keep a moutain dew frosty outside of the fridge .

MikeN

Definitely a Wally item for a perfect answer, but I think you pretty much narrowed it down.

I believe the A/C controller has a set of override parameters whenever the temp slider is set to either full cold or hot. So the 760 Ohms that you are getting at the 65 degree setting (full cold) is correct....but as you have stated you seem to have incorrect readings in between full hot and cold....which will cause problems.

What can you do?? Most of the time you might be able to find someone on this list that has a spare/used one sitting around that you can swap out to see if anything changes. Sometimes new/used/rebuilt ones show up on Ebay. Or get a rebuilt one from the big three......don't know if the potentiometer can be changed out or not....lucky for me I never had to delve into the innards of one before.

T_MaX

I will resolder the connections to the potentiometer than check the Climate Control Unit hooked up to the systems to see if there is a difference in readings and operation.

Thanks to all that have responded.

AO

One thing you did not mention was if you checked to see if your system is holding vacum. I assume you did check, just did not tell us. Anyway...

Not sure on the S4's but on my 82 I had a similar incident that the PO had given up on, and I was about to too. I had full heat or fresh air only - no AC. I thought it was the head unit or perhpas one of the temp sensors (one in the dash, and the other in the driver's front wheel well under the head light). Everything checked out. Then someone suggested I look at the connector above the accelerator pedal which is where the outside temp sensor is routed IIRC. Long story short, I pulled the tray, got upside down to look up under the dash, and wouldn't you know it... The connector (about 12-15 wires) there was on the plug cockeyed! Son-of-a... I pushed it back on, and presto! Everthing works just like it was designed to. Hope this helps.

T_MaX

Andrew, your right I did check the vacuum but did not list it, thanks for pointing it out (I did revised the above for historical purposes).

"I pulled the tray, got upside down to look up under the dash, and wouldn't you know it... The connector (about 12-15 wires) there was on the plug cockeyed! Son-of-a... I pushed it back on, and presto! Everthing works just like it was designed to."

That's a good Idea! I guess while I'm in there and have the trays removed I might as well clean and apply conductive electrical grease to the MP grounds and as many of the connections I can find.

T_MaX

Well, I resolder the control board, checked all connectors and related (and nonrelated) ground points. Reinstalled everything and I still don't have climate control, just cooled and hot.

Due to a little storm out in the Gulf of Mexico I decided leave everything intact and move on to a few smaller problems (Hatch Motor and a 5.06 amp batt. drain).