Odd A/C problem

[rtrigg]

OK, here goes:
My car was in a front-end collision which required replacement of the A/C condenser...and naturally recharging of the system. All performed by an Audi Dealership. I was called to pick-up th car after after the completion of the repairs: body work, new radiator, condenser, inter-cooler, etc. The A/C appeared to be working fine upon immediate return of my car. However, the next morning my A/C is no longer blowing cold. No faults, no ECON light, nothing. So, I press ECON off and on a few times to see if anything happens...NOTHING. So, I leave it in ECON mode for a bit...but about 15 minutes later the heat is getting to me so I turn off ECON mode...and now the A/C is working. I conitue my drive to work, get back in the car at lunch...A/C is still working. I get in my car 3-4 hours later to drive home...the A/C is still working. So, I'm thinking OK, that is weird. I park the car overnight, get in the next morning...A/C is NOT working!!! I fiddle with the ECON button...again NOTHING. So, I turn ECON ON and drive to about the same spot I was the day prior...at freeway speed (60mph)...turn ECON off...the A/C begins to work again!!! Like the day before, it worked at lunch (about 4 hours later) and it worked on my drive home (about another 4 hours later). So, over the next few days (actually weeks) I noticed a 100% perdicable pattern as described below:

1) During all of this time, there have been no codes which can be read by VAG-COM.
2) If the car has been sitting for more than 6-8 hours, the A/C does not work until the following two conditions are met:
a) The engine must be a full operating temperature.
b) The engine RPM must be at a sustained RPM above 2,500-3,000 (60-70mph)...for initial engagement only.
3) If I am doing low RPM city driving, short burst of RPM, the A/C will not engage...even if the engine is at temperature. Also, If I drive to the nearest freeway where I can acheive a sustained RPM, if the engine has not been to temperature for a while, the A/C will not work for another 10-15 minutes...even at sustained speed.

So, it is VERY temperature/RPM related. However, once you meet these conditons it will work fine. Even if you stop at a stop sign, do city driving, whatever. Once it begins to work cruising above 2,500 RPM...it works normal until the car sits for 6-8 hours. Then the process must be repeated. This has been going on daily for about 2-3 months...with car being driven daily. Again it is 100% repeatable and predictable. But what the heck is causing this.

Any ideas? Again, I have ZERO codes in VAG-COM from any module!

Any help would be appreciated.

 

[Staz]

Hi and welcome.

Can I ask why you haven't taken it back to Audi?

Also when you say engagement do you mean once you maintain a constant 2,500-3000rpm you can hear that the system has come on line?

 

[rtrigg]

Hi and welcome.

Can I ask why you haven't taken it back to Audi?

Also when you say engagement do you mean once you maintain a constant 2,500-3000rpm you can hear that the system has come on line?

It is long story, but I will just be brief. The repair was quoted at 12 days by my insurance. The Audi dealership had my car for 44 days...and still the bodywork is not complete. My insurance quit paying for a rental car at 30 days. Anyway, I had to leave town on business trip and I needed my car back regardless if it was finished or not...as far as the paint/body work. So, after two weeks when I returned, I called the dealership. I had a long list of items done correctly...even to this point. So, since so much "re-work" had to be done, I insisted they supply a loaner/rental car. I complained enough that my request was honored...however, it was at THEIR convenience. Also, I travel (in my car) alot...and the appointement has to fit my schedule too. So, we need an estimated block of two weeks that works for BOTH of us...so far that hasn't happened. However, my travels should be about done for this year. So, now I just their go ahead to bring the car in.

In regards to the "engagement" at 2500+ rpm. I mean this: When I get in the car of a morning. I start the car and immediately press the ECON button thereby commanding the compressor OFF. The reason, since it is not blowing cold, I figure damage might occur if I continued to let it run for 10-15 minutes. So, I drive my car w/o the A/C (ECON LIGHT ON). After the engine temperature has stabilzed, I wait until I am on a stretch of raod that will allow me to get to a sustained speed with and RPM over 2500. Then, I simply reach down and press the ECON button, thereby commanding the A/C compressor to function. Without fail the system immediately begins to blow cold air. It will blow cold all day long after that point...so long as you do not let the car sit for more than 6-8 hours. I have driven the car 500 miles straight through and the A/C worked as if there was no problem whatsoever. For what is is worth, last night I found several accounts of AUDI owners with "variable ouput" A/C compressors which had this problem..."Delayed A/C Function". No codes, and after the engine warmed up it would work. There were very few follow-ups with a diagnosis, but a coupe of comments that a new compressor seemed to fix the problem. These variable compressor do not have a clutch, they are pulse width modulated (PWM) with square wave signal that gives them a duty cycle between 20-70%. I think the actuator that tilts the "swash" plate is mechanically bound...once things heat up, the clearances increase and the actuator is able to move the "Swash" plate increasing the output to a point that the compressor works again. Anyway, that is based on my underatanding of how these new compressors work. FWIW, it is a Sanden PXE16 comressor...fairly new technology to already on production vehicles.

 

[Staz]

Hmm didn't know they didn't use a clutch any more. I'll have to look into how they work fully. Why only up to 70% duty cycle?