BS911

So my A/C has been getting progressively ... intermittent ... ever since I bought my car 4 years ago now. I've fuddled my way through replacing several items that I have been able to (mostly) determine as failed, such as my 3 level pressure switch, compressor relay, both condensor and oil cooler ballast resistors, vacuum leaks, low coolant pressure (under 5 psi), and a dead fresh air flap servo. But every time I fix something, it works for maybe a few months and starts acting up again... intermittent, of course. So lately I've been stumped on why my compressor fails to run.

Problem:
Push the A/C button and no compressor. The condensor fan was starting so I knew that the CCU was doing something. The plug in the engine bay at the compressor was connected, but no voltage. The fuse for the compressor was good, the relay was good. Still, no luck. The darn thing just wouldn't start.

So I gave up and started digging in to schematics. Rather than buying the $$$workshop manuals$$$ or a pirated, bootleg pdf of said manuals, I bought a subscription to alldata.com. I would HIGHLY recommend that service for DIY tinkerers that are tired of guessing and want the REAL information on their car. $25 a year, can't beat it.

So using the right information from the schematics now, I found that my evaporator temperature sensor was reading 0.28 kOhms instead of the 3.3 to 7 kOhms that it was supposed to be reading. (Read between pins G18 and G22 on your CCU connector plug.) Oddly enough, the sensor had been "relocated" lower on the evaporator by the PO's shop and was apparently bent in the process. I wonder why it had a limited life span?

Since I can't completely throw away my nature of having to "fuddle" through and Jimmy-Rig things, I started taking apart junk electronic components that I have been hanging on to for reasons I am only just starting to become aware of. Couldn't find a resistor reading in the right range, but did find two 1.9 kOhm resistors. Connected those little suckers in series for a 3.8 kOhm resistor. Right in the operating range. Connected it in place of my Evaporator Temperature sensor, and proceeded to FREEEEZE on my way to work this morning. and any other smiley that might represent my overwhelming joy of cold air on this 85 degree day with 70% humidity! Of course, I got to work and looked up the cost of a new evap temp sensor... $87!!! Oh well, I'm just glad I could solve it before I had to resort to a stealer looking at it as I'm sure the lying ba$tard$ would tell me my CCU was kaput.

On a related note, I've been up and down, inside and out, and back and forth through the 993 A/C and heating system now and have learned to diagnose almost every piece of it. If anyone is having any weird problem that they just can't trace down with a compressor not running or an otherwise inoperative or just plain weird climate control system, I'd be glad to help diagnose.

rkass

man your knowledge isn't limited to hot air! just kidding. you were instrumental in helping me with cold air flap, hopefully won't have to tap into your a/c database, but glad it's there. congrats on your fix.

TheOtherEric

Very cool post (no pun intended). What is the CCU connector plug? If this is a way to diagnose problems, we all need to know more!

And where exactly did you look to find that the evap temp sensor was supposed to be 3.3 to 7 kOhms?

tj90

Good job Brian! The relocation of the sensor was probably due to the freezed up evap problem that everyone talks about. I too have an Alldatadiy.com subscription for all my cars and it is extremely useful! I think you get a discount if you order more than one car (makes printed Chiltons manuals obsolete)!

BS911

Wait! My knowledge IS hot air. At least that is what my wife says. Glad I could help though!

For the CCU connector plug...

Pull CCU out and uplug it straight from the back. They say you need the "Porsche Radio Removal" tool to get it out, but I found that 4 6 penny common nails fit the holes perfectly in the holes around the edges. Reach up under the dash to behind the CCU and push it out. Just make sure you don't dislodge that vacuum line.

There are 2 plugs... G and K and it is imprinted on the back of the CCU which is which. G is the longer one. On the plastic plug itself, the pin #'s are imprinted 1 - 18 on one row, 19-35 on the other. Pin 18 is the sensor ground connection. Pin 22 is the lead from the evap sensor. Using that ground pin 18 and then the other appropriate pins, you can test all the different sensors like your oil cooler sensor, temp mixing valve sensors, engine temp air duct sensor, evap sensor, and outside sensor to name some. They all have slightly different Ohmage (Not sure if that is a word, but sounds good anyway) requirements, which are shown on the schematic. So you can compare your readings to what should be in range. Everything is usually in the 3-9 k Ohm range as normal. Obviously, any open loop or closed circuit will be a completely bad reading on a temp sensor. Looking at the schematic from alldata (who licenses it from Porsche) at the evaporator temp sensor, it showed 3.3 k Ohm to 7 k Ohm as the normal range. So reading 0.28 showed a major problem. I'll see if I can write up all the basic diagnostics I went through and the specifics of ohmage per each pin.

I need to find out more about the "proper" location for a relocated sensor though. Mine was shoved in between the cooling fins at the bottom of the evap. I understand it needs to be low, but there has to be a good mounting method. Anyone have pics possibly?

long_beach_968

so the evap temp sensor, is it next to the evap, or is it the same sensor used to sample cabin temp? the reason I ask is my cc will pump out lukewarm to hot air, even with the a/c on, unless the temp selector is on the very last click to the blue, in which case I FREEZE my ***** off. it is just a slight annoyance I would like to tackle. so im thinking I want to replace the temp sensor.

BS911

The evap sensor is different and only tells the compressor to stop when the evap ices up.

Your interior cabin temp sensor is inside the ccu itself. Because of that, it doesn't have a direct pinout on the cable. BUT... pull your ccu out, using a flat blade screwdriver, depress the 6 tabs holding the faceplate of the ccu onto the case and gently remove the face plate. You can then detach it from the ribbon cable to make working easier. You will see the temp sensor (couple wires with a blob of something at the end) that is soldered onto the circuit board and leading up to the vent on the left of the faceplate. Mine was also having the symptoms at one point of "cold on blue dot, warm to hot on anything else." I simply cleaned the collection of dust off the sensor probe and then it worked better. Check the resistance across the ends of the sensor at the solder connections and I think you should get about 4 kOhms at around 70 degrees. That is just a guess on that one because it isn't in the schematic. Because it is hardwired into the CCU, I'm not sure if you can replace just the sensor. Also, I unfortunately didn't measure resistance on mine BEFORE I cleaned it, so don't know if it really was mucking up the works.

Looking at the schematic, there isn't even a range provided for the outside temperature sensor. The outside temp sensor leads to pin G-9 on your plug. Test the resistance from G-18 to G-9 and it should be similar to the resistance on the inside temp sensor. If one is WAY off of the ~4 kOhm ballpark, that could be your problem.

TheOtherEric

Is "CCU" = climate control unit??

long_beach_968

outside temp sensor? I didnt think any of our cars had one unless we had the on board computer option

BS911

Cars with the OBC have 2 right next to each other. You should have one in your left front fender just ahead of the condensor - and an empty plug for the OBC one tie wrapped out of the way. Not sure if visible from the grill through the bumper though. You will definitely see it if you take off the wheel and wheel well liner.

jnor10

Great post. Can I take you up on your diagnostic offer?

Two issues that began this year from an otherwise trouble free HVAC system.

1. No heat on the drivers side of the car. Having read several heating posts, it seems one of my servos is not functioning. I need to narrow down which servo controls the heat to drivers side of cabin?

2. My A/C functions, but there is a definate wheezing taking place which varies as a function of the fan blower speed. My guess is an air flap is not staying in the proper position and the A/C air is working against it. My hope is the two problems are interrelated to the above mentioned servo.

Appreciate your comments.

Jeff N

rkass

jeff probably two different issues but brian is the man for this stuff. on your driver side, there is a panel covering the servo that's probably causing your heat issue. It's the plastic black piece connected to the large tubing. just undo the clamps and see if the servo flap moves or if it is stuck (most issues re: no heat or just heat can be fixed by freeing up the flap). if you do search you will find tons of useful pics/steps etc.

brian posted something about wheezing a little while ago. search for that as it sounds like you have some kind of vacumm issue, maybe. anyway good luck.

BS911

Hi Jeff.
The driver's side servo is actually under the drivers side foot well kick plate. Uh... sit in driver's seat and kick your left foot left... that. Take that off and you'll see the cardboard tube with a little motor inline. Undo the vent tube so you can watch the servo flap. With key on, go from blue dot to red dot settings and you should see that servo moving. If not, most likely failed. It could also be the mixing chamber temp sensor but I think on red dot it will still open the heat servo.

The wheezing can best be summed up here... http://p-car.com/diy/acdiag.html

- No air flow/Wheeze on recirc (or a/c) - you have a vacuum leak somewhere... most likely under dash behind the climate control unit. Find a little rubber elbow with a loose hard plastic vacuum line nearby.

- No air flow/Wheeze on fresh - your fresh air electric servo has died in the closed position.

jnor10

Thanks guys.

Looks like their likely unrelated problems.

I'll revisit the cabin located servo. I have been in there a couple of times to check its function. The last time I checked was because I was getting full heat all the time. That is the position the servo is designed to fail in.
ie, no power to servo, servo valve= full open. At that time I removed the servo, gave it a couple of light raps with a mallet and it was good to go.
so its possible that the servo for whatever reason failed in the closed position, might be time to replace the part.

For the wheezing issue, I'm pretty sure I have good vacuum, as I had already checked all the usual places and found no loose hoses. I also recall a couple of years ago I had a Varioram and wheezing issue that were both solved after I found a loose vacuum hose in the engine compartment.

Thanks for the help!

subdriver

Brian,

Thanks for your diagnostic efforts...I began having similar symptoms. Evap temp sensor reads 1.7 kohms, so mine is going as well. While I could get the resistance measurement, I'm having a tougher time actually finding the sensor itself. Can you point me in the right direction? Thanks!