Mark P

AC failed first. I just recently got the TDI fix and Aux Heat recall done, and tried to have them check out AC at same time but they lost my phone message on it, so I picked up the car not happy with the dealership and not excited to turn around and spend money with them. Overall they're very good, just a bad situation that day when my service advisor was out.

Bob Petruska

I a way that is good as they need to recalibrate the A/C doors after the AUX heater fix.

Mark P

Yeah, I was hoping it would magically fix it

Mark P

I checked pressure and temps today on VCDS (and on S5 and A7). Temps were as expected for a condenser not turning on. Pressure was low (4 bar), others were above 10bar. It then occurred to me that one of the other cars may have the same pressure sensor. Sure enough, so I swapped the S5 and Q5 sensors and the problem remained. Glad I realized that and didn't buy a new sensor. So now I assume it is indeed just low pressure, whether or not from a problematic leak is unknown. I'll get it vacuumed and filled with R-134a to the rated 550g +/-15g and see where that gets me.

Mark P

Started with S5, just before, or right when I started logging the temps were all the same as expected:
035.1 is by evaporator
034 is in the vents




001.4 is the refrigerant pressure showing 14.4bar.
I think 001.1 and 001.2 are currents related to condenser somehow, so I think this >0 current means I had already turned AC on, set to "LO" (this is 30 seconds after first photo)



20 seconds later, evap temp down to 10C, vents down to ~17C


30 seconds later and evap is down to 5C and vents to ~10C



Checking back at pressure, I see 13.4bar

Mark P

Then I moved to the Q5 and tried the same...

Pressure at only 2.6bar, with 0amps, I don't recall if this was before or after I turned on AC, but doesn't really matter since condenser doesn't come on:



20 seconds later, no cooling:



40 secs later, still temps all around 28C



40 seconds later, pressure at only 4.4 bar, though now those currents show non-0, so perhaps not condenser itself, or maybe it is on but non-functional.



30 seconds later still no cooling



Same a minute later:



30 seconds later pressure still at 4.2 bar

Mark P

Then I checked the A7...

Pressure 7.8bar with engine off, temps around 30C:



Car on, but AC off. Still around 8bar:



AC on, pressure up now to 13.2bar



A minute later, evaporator temp down to 10C, vents as low as 16C; pressure at 16.6 bar


1 minute later, evap down to 5C, vents as low as 10C; pressure at 16.4 bar

Mark P

So the pressure ranges were: (bar)
S5: 13.4-14.4
Q5: 2.6-4.4
A7: off: 7.8-8.4, on: 13.2-16.6

I'll double check the S5 with the car off, then with the car on + hvac on + ac off and see what that low end was. It seems like pressure differences when the AC is off is more compelling now.

I've begun to look for an indy shop that will do the vacuum and refill so I can get it to the known spec:

HZ60

Sure looks like a low charge. Outside possibility being a bad compressor. When they evacuate the system (usually down to <500 microns or 24-25 "Hg if they don't have a micron gauge) have them valve everything off and watch the gauge. If it loses vacuum, you got a leak. If it holds vacuum steady for a while, I would still have them use an electronic 134a leak detector after they charge it to confirm. If it is in fact low, there's no point filling it until you find out where it went. You might be ok for a while if it's a really small, slow leak but eventually you'll be right back at square one.

MurrayA4

Valid approach HZ60.I would be surprised if the factory fill PAG oil doesn't contain a tracer by now, but I've been surprised before !
Mark P if ever you decide to become a forensic accountant you'll do just fine.Great write up.Clearly shows the depth to which VCDS will let you go and that's just scratching the surface.