Striker2237

These are variable compressor cars so they will not act "normal", there is a test mode that has it step through various states of cooling based on exterior temp and has you confirm the pressure seen matches requested and sensors.

I'm sure it's in VCDS somewhere but I use Autel to do it as it's nearly fully automated and lets you confirm the compressor is working as it's supposed to in variable cars. For example it has you just start the car nothing on, then steps to 2k rpm, then full hot, full cold etc all while comparing pressure and temps vs what it's supposed to be.

These cars have temp probes nearly everywhere and 3 external temp probes as well as line pressure sensor and core temp sensor. If an external temp thermistor is "slow" or has a dead zone in a 10* area it may not code but it will also prevent full cooling etc. The inlet temp sensor is generally the one responsible for a slow cool error but you should check the exterior temp sensor as well.

They will code correlation if badly enough out, the computer can also see the control solenoid for the compressor as well as far as resistance etc so it can self check that as well. You will see max pressure on the high side unless it's commanding 100% PWM run state

https://www.motor.com/magazine-summary/diagnosing-variable-displacement-compressors/

LiquidRS

same here, I live in Dubai so lets just say that i might use the AC more than most people here. The AC on the A8 blows icy cold,cannot ask for more. I just set it at 22 degC auto all year round.

Andrew12

Pretty bad storms here, so only got to play for an hour or so...
I found the following.
Refrigerant pressure, believe that to be the high side or the condenser pressure. Ranged anywhere from 7 to 13 Bar depending on the load I was giving it. That all seemed normal.
Found the downstream temperature of evaporator. That ranged from 1.5 to 3 degrees C depending on the load. I assume that is normal, because you would want to be as cold as possible without forming ice.
Here is where it gets odd. I could not get any duct to drop below 10C.
So I need to understand what controls the system. Is it the downstream air temperature of the evaporater? Is this sensor reading off? Or am I mixing in warmer air somewhere?

More to come..

Striker2237

Those all look good so far but duct temp should be like 6-7 based on what mine do. Are you totally sure on charge? When mine does the test loop it doesn't go over 8.5 bar

You will not be off, there are like 12+ sensors and it's really easy to see one that doesn't agree with the others. The sensors I would worry about are the intake/exterior ones, check the blend door states as well

PaulW

I would say check the blend doors.

It's been a long time since I have played with them (the last time was on my D2), and you can do an adaption which sends them through a calibration mode and sets the stops. I remember in measuring blocks, there was a target value for the blend door (numeric value) and actual. You could see which ones were not close to the target. Sometimes the adaptation would remedy this, other times the little motorized blend door actuator had to be replaced.

I would not be surprised if you have hot air blending with the cool air.

Andrew12

I tried to figure out where and how the blend doors/flaps work. I found several. I ran an adapt end stops on VCDS. Seemed to go okay..? I forced a few to actuate. Everything seemed like it was doing what it should...
Now I can only get down to 10C on evaporator temperature and 15C on duct temperature.
So I either made the blend flaps worse or I am loosing freon...
Leaning towards freon leak.. I have a detector, so I will scan the system...

Striker2237

Dumb question but did you replace the valves when you evacuated and pulled vac on the system?

Andrew12

Not a dumb question, because I did not... which ones? are there more than just high and low side? Appears to be one on the condenser..

Striker2237

High/low and pressure sensor at the condenser and/or the other one you noticed as well. They are usually the first point that leaks and sometimes really really dislike even having a gauge put on them to check, if they are old I have had them begin leaking just from that.

Andrew12

Assume they are original. Are they anything special? Local parts store? I have a whole bag of that stuff, but it is all old...