Engine Size: 2.0T
Refrigerant Type: R134a
Ambient Temp: 90ish
Pressure Low: Varies
Pressure High: Varies
Country of Origin: Germany
AC works intermittently based on ambient temp...
Semi short version. During the hottest part of summer (usually 90+ outside ambient temps), the AC on our 2007 VW Passat 2.0T will only come on once the vehicle is up to operating temperature. However, lately, with temps into the 105+ range mid day, it won't come on at all, or only 'briefly'. Although....sometimes if you warm the car up while the ambient temp is at or below 90 (mornings, IOW), the AC will work normal all day, even with ambient temps above 90+ degrees. However, yesterday, the AC wouldn't come on at all....not even briefly.
So...this morning I hook up my gauges. Something going on I don't know how to interpret.....Weird stuff....
Just hooking up gauges, car 'cold' and off (sitting all night).
Ambient temp 90 degrees, low side reading 70, high side 150, 90 degree at vent (duh)
Car warmed up, running 2000 RPM (200 degrees on dash gauge)
Ambient temp 90 degrees, low side reading 50, high side 200, vent temp 63
then....dropped RPM to idle (~800 RPM)
Ambient temp 90, low side reading 65 (immediate) then moved to 85, high side 165-170, vent temp slowly crept up to 80
then RPM back to 2000
Ambient temp 90, low side dropped to 60, high side went to 230, vent temp back to 69
So....I see the system mostly working....but don't understand why compressor turns off at idle?
And don't understand why compressor won't turn on when ambient temp is above 90 (like what happened yesterday).
And why did I get the high readings as the compressor is 'cold' and car off, but then everything kind of works once the car is at operating temp. Sounds like a switch somewhere controlling things?
Or....combination of shot compressor and low freon?
Doing a lot of Googling and the car's system is kind of known to be 'weak', maybe even a TSB out there about the compressor....but the car doesn't have any CPO program (bought used from individual), and way out of warranty. So trying to figure this one out.
Thanks in advance!
Gauges are unfortunately not a lot of help in troubleshooting that particular AC system. The compressor is a electronically controlled variable volume model and there is no clutch, just a direct driven pulley. Very common that the electronic control valve "sticks" and won't react to signals from the AC computer. Find somebody that has a proper control valve tester such as a "CLT1". I've managed to get quite a few "bad compressors" working again by just using the control valve tester to fully open and fully close the control valve. The car AC computer never runs the valve through a full cycle, but the tester can do this. You also need to have the system emptied and refilled to the correct refrigerant amount, gauges are useless until you can be sure of the correct amount. Good luck.
BTW, engine RPM shouldn't have much effect at all on that compressor.
Auto cool is spot on.
The PXE clutchless compressor is a variable displacement/stroke swash plate comp which replaced the SDV variable displacement/stroke wobble plate clutch comp at VWAG.
VW has many electronic A/C features, e.g., to start in min. stroke (soft start to minimize torque effects on the miniaturized 4 banger engine and drivability).
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
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