Hello,

Have a 2010 A6 coupe with some strange A/C behavior...

Problem

Last summer it would occasionally take 1-2 minutes to start cooling after work.

This summer the delays started up again only 5+ minutes and is 100% consistent. The RH side vents will show signs of cool air about 20-30 seconds before the LH vents. This issue occurs after the vehicle has sat for an extended period of time - at least 4-ish hours. Less time and it works fine right off. No variation in the HVAC head unit settings seems to have any affect on the issue, auto/manual, recirc, dual mode on/off, cycling compressor on/off, etc... Fuses checked, relay pulled/connectors cleaned, condensor doesn't have obvious debris blocking it, pressure switch connector pulled/cleaned. Displays about 14V at DIC. Car does not run hot.

When it occurs at startup the compressor IS engaged and spinning but the lo side line is not cold to the touch. At about 5 minutes the lo side line gets cool to the touch. Same time the air starts cooling at the vents. This tells me there's probably nothing wrong w/ the air blending box.

Once it starts cooling - it is VERY cold. Going home in the afternoon on a 105F day, outlet temps can hit 40 or even lower.

Diagnostically - here's what I've observed so far...

Static pressure w/o engine running, lo side: 75; hi side 125 at about 105F and over 50% humidity.

When initially turned on after 4+ hours sitting and condition occuring...

Lo side moves up and down between approx 65 and 75 (compressor NOT cycling - just spinning continuously), Hi side climbs immediately to about 150 and that's it.... hot air is coming out of outlets and no cooling can be felt on the lo side line...

Wait and watch for about 4 or 5 minutes and things start to change...

Hi side starts creeping up steadily albeit slowly... an electric fan can be heard to start up front ... the low side stops moving back and forth and stabilized at about 65...

Then in few more seconds cool air can be felt inside at the RH vent and the lo line was felt to be cold to touch. (within another 20 seconds the LH vents will start blowing cold too)

The lo side drops to about 48-50. The high side climbs up to about 275 or so. But doesn't stop there - it keeps climbing to 300, 350 and even 400 and some change and the fan is roaring up front.

The hi side seems way too high. But I have a cheap gauge from many years ago - could be innaccurate (remember the static pressure difference above).

Relevant service history: @ 2 years old in 2012, dealer treated HVAC for odor problem and replaced the HVAC head unit shortly after that. Receipt mentions installing 'Frigi Quiet' as well.

Questions

There is one final wrinkle to muddy the waters... Through experimentation I figured out that in the afternoon if I turn the car on, pop the hood, disconnect and reconnect the pressure switch, by the time I drop the hood and get back in car - a hint of cool air can be sensed from the RH vents. That little trick dramatically shortens the usual 5+ minute wait I've been enduring. But why? (cycling the A/C switch on the head unit doesn't have the same effect either)

Since discovering that trick - I've since gone in and cleaned the contacts at the compressor clutch plug and the ground connection on the RH frame next to the hood strut.

Blockage? Overcharge? Contaminant? Electrical gremlins? Combination of multiple issues? Ideas...?

How many pounds/ounces does the FSB spec for refrigerant for 2010 models? Or doe they spec pressures still?

Anyway I can readily determine if the dealer applied the TSB software modification to the HVAC to have the fan do the 'run on' or 'after run' feature or whatever to mitigate odors?

Thanks!