mike cap

I thought I should post the solution to an A/C problem that I was having with my '97 C2 for some time. Not so much for the fact that anyone else is experiencing this problem now, but for the 'archives' and future reference. This was a tough problem to chase down and something that is really an inherent defect in the 993's A/C system.

First of all my car has about 28,000 miles on it and I've had an A/C system problem since I've owned it the last three years. In the cooling season the A/C would operate normally per the Shop Manual's performance chart. Center vent temps would be 48 F to 52 F depending on ambient temperature, engine speed and settings. Normal (and anemic), but good enough to handle the Upstate NY summer temps. As most of you do, I keep the car in the shade with a window shade in place if it is blistering hot outside, to keep the interior from heat soaking. The initial cool-down is where the A/C system shows how under-sized it is.

My car's A/C problem became evident only under certain conditions. After a 30 to 45 minute drive in extremely hot 85 F + and high humidity days, the A/C center vent temperature would start to climb from the 48 F to 50 F range up to 65 F to 70 F. The center vent temps would level off in this range and not go back down.

After getting the car back home and parking in the garage you could not see any dripping from the evaporator drain - the evaporator had completely iced up solid. About 15 minutes later this ice would begin to melt and the water would run in a stream from the drain area under the car.

Cantech Automotive in Syracuse attacked the usual suspects and did a thorough job of charging and pressure checking the system, checking compressor operation, replacing the evaporator expansion valve, evaporator temperature sensor and dessicant tank. The problem persisted and really made me avoid driving the car in hot and humid weather all together.

Finally, Cantech brought the car back into their shop and ran more tests on the system. Fortunately the weather cooperated with extremely high temps and humidity while the car was at their shop. They checked that the evaporator temp sensor, CCU and compressor were all cycling properly as the evaporator iced up (as it is supposed to do), but that by the time the temp sensor mounted on the evaporator reached the correct temperature and set a signal to the CCU to shut-off the compressor, the evaporator was iced up solid. The problem was that the compressor was continuing to run long after it should have, causing the evaporator to ice up solid. No air flow and hence the high center vent temps.

Cantech felt that the only solution would be to relocate the evaporator temp sensor lower on the evaporator core so that it will see the lower temps (ice-up) sooner, and send a signal to the CCU to shut off the compressor. The evaporator then has an opportunity to melt off the ice, the compressor turns back on and the cooling cycle begins again. Discussing this with Cantech I remembered some posts from several years ago here on Rennlist from guys that had this ice-up problem and used the temp sensor relocation as a fix. I found three or four discussions of this topic using the 'search', and while the discussions were specific as to the fix, they were buried in threads from several years ago.

Cantech relocated the evaporator temperature sensor lower on the evaporator core by about 2" or so and after driving the car I noted that the center vent temps would now fluctuate by 2 or 3 degrees while driving at a steady highway speed - meaning that the A/C compressor was now cycling on and off when commanded to do so by the evaporator temp sensor via the CCU. Since this change was made I have driven the car several times in temps and humidity that would have, in the past, caused an ice-up event and the A/C has worked fine.

Sorry for the length of this post, but I thought a thorough explanation was in order as this is more of an inherent manufacturing issue that an owner may have to deal with, rather than a typical maintenance issue. Hopefully this post may save a future 993 owner a lot of hair pulling and cussing. Thanks to Rocco at Cantech in Syracuse NY and Randall G. (Rennlister) in California for their help.

InTheAir

Mike, thanks for the wonderful account. It is helpful to have a thread dedicated to the subject, including the fix that worked. This one should be easy to find with the search feature now.

kdurg

Great Post Mike......

Do you know where the Evaporator Temp Sensor is located ? Is this a DIY ? How/What did they use to affix the sensor in its' new location ?

mike cap

Jeff,

Thanks. Meant to tell you previously that I enjoyed looking at your aircraft photos. Your Cardinal RG is a nice looking airplane. I have a '87 Bonanza A36 and have flown into Troy MI many times. I really enjoy staring at that manufacturing building and power lines on take-off. No place for an engine failure.

Adrienne

Mike - You deserve a Rennlist lifetime membership for that one, especially considering the timeliness of your post. I guess this just proves you are, ahem, one very cool guy....(somebody had to say it)

(by the way...note my avatar's painted grille, thanks for the assistance)

mike cap

Keith,

Hi. You should be able to DIY. Access is at the center of the cowl - you can see the expansion valve (silver block). I'm not sure what parts and pieces Cantech had to remove to expose the evaporator though. Possibly the ww solvent tank and metal "bulkhead" that the plastic cowl cover snaps on to. No gas tank removal was needed. I don't know how Rocco re-attached the temp sensor. You can email him at: rocco@cantechautomotive.com. He is on holiday this week and may not answer an email till next week.

Texas993

I want to know the same.

This is exactly what I am experiencing. Cooling well, then just stopping after 20-30 minutes. I suspected the thermostat located in the evap. based on previous posts. Your explanation is perfect, and it shows all the items that are NOT the problem. My car goes into the shop Monday for this fix. You can bet I will be armed with a printout of this and the past posts.

Thanks so much for taking the time to detail your experience/outcome.

grouchy

Pictures and little red arrows during dissection would be very nice.

This sounds like something my car needs as well.

p_carfan

Great post.......I've had the same problem for years . I'll be looking forward for more info. from you and others to hopefully put together a DIY.



One of the many reasons I love rennlist !!!!

mike cap

Everyone,

I guess I'm not surprised that others have experienced this evaporator ice-up and loss of cooling problem. The reason for my post was to save others the hassle of chasing down the wrong path and replacing a lot of expensive parts when the fix is fairly easy and cheap.

Keith, Patrick, Brent and Steve:
I was putting the trunk liner and pieces back together last night and looked closer at how the evaporator and temp sensor were accessed. Remove the trunk liner, and plastic cover that is at the base of the windshield - the one that covers the HVAC servos. There is a metal bulkhead just forward of the cowl - the one that plastic cover's rubber strip slides on to. This bulkhead has to come out. Also, there is a plastic tank above the gas tank that may have to be removed. Not sure what this tank is? Maybe an expansion tank for fuel? With this stuff removed the evaporator and temp sensor should be visible. The temp sensor was moved on my car 2" lower on the evaporator. Attachment method is unknown, but it may be as simple as re-using an existing clip. Be aware that the temp sensor may need to be grounded to operate properly. Hope this helps.

Adrienne:
Thanks for the compliment. I always wanted to be a "cool cat". Your rear (grill on your Targa of course) looks great!

Texas993

Thanks Mike, you are a true Rennlister. I plan to take mine in to the shop Monday. You very likely saved me a lot of money and time. Thanks.

CalvinC4S (former Porsche Tech) recommends that I replace the evaporator expansion valve too. If that involves taking out the coolant, I may skip it.

BTW, I drove my car to work this morning with the ac blowing cold for 40 minutes. It was low 80s. My troubles will most likely surface this afternoon on the way home. I am hoping it will, then I will try to take a look a the evap, or at least at the water pattern.

mike cap

Patrick,

If the A/C is blowing cold at 48 F to 52 F (you can use one of those inexpensive HVAC probe thermometers) as it did this AM on your way to work AND it blows warm when the "ice-up" occurs (and no dripping from the drain for 15-20 minutes after you get home), then skip replacing the expansion valve. It would be a total waste of labor and material $. Just have your shop relocate the evaporator temperature sensor. Advise how you make out.

avillena

Great post!!!!!

Wachuko

Great post!!

I had this problem from day one on my first 993 C2S (a Guards Red with no spoilers)... same situation you describe... this was in Puerto Rico (hot temps and high humidity is common there...). Imagine taking the car out (my first Porsche and a brand new car at that) from the dealer and heading out for a trip around the island… just drove the car from early morning until I covered the whole island… wife (at home) thought I was crazy… anyway… ½ hour- 1 hour into the trip and the AC is not blowing cold… I was mad as heck… I just lowered the windows and kept driving… took the car in the next day. Not sure what they did but it was fixed and did not give me any problems after that.

This and the alternator belt breaking in pieces were the only two problems I ever had with that car… do know that I drove that car hard… ended up trading for the C2S Vesuvius. Never had a problem with the AC on that car… but the belt breaking was always an issue… even after the upgraded pulley…