A/C high side pressure

[Oh1-911]

I recently replaced the A/C compressor on my 996 with a brand new unit.  The system was working fine prior to that, but I was already in there doing a bunch of other work and it has 93k miles.

 

Evacuated the system for 30 min, added 34 oz of R134a as the manual says, and all is good except the high side pressure is running about 85 psi, with a normal low side of about 32 psi.  Ambient temp in my garage has been low (50-55) but I still expected the high side to be over 100 psi.

 

Air blows cold in the car, 38-40F, and everything else seems normal, compressor does not cycle off/on as it might if Freon was low.

 

Any thoughts?  May need a bit more Freon?

 

 

[Ahsai]

If air blows cold I would probably leave it alone. There's a sweet spot that the a/c will perform in great efficiency. Either too little or too much refrigerant will throw it off.

Do you know how much R-134a (in weight) you put in?

 

Our compressor has variable compression so it doesn't cycle on/off, normally at least. There are some cases it will turn OFF such as under WOT. Hope you also replenished a/c oil with the new compressor.

[Oh1-911]

I was using 12 oz cans, and it's not very accurate, but I attempted to get 34 oz. in there which is almost 3 cans.  I have heard that you need to add about 2 oz for the loss due to the charge hoses, and I didn't do that, so I could be 2-4 oz low.  I just thought if it was too low my low side pressure would be dropping to the 25-28 range.

 

On the oil- the compressor comes filled, but they want you to drain out the oil from the old compressor, measure it, and then remove oil from the new compressor to match the amount.  Keeps you from having too much oil in the system.

 

Thanks for the input.

[JFP in PA]

High side pressures are very ambient temperature dependent; Porsche actually publishes a graph that relates the high side pressures to ambient temperatures, and there is quite a swing in pressure with only a slight increase in ambient temps.  That said, the system should hold 900 grams (31.75 oz.) of R134a when full, so you are probably already there.

[Ahsai]

The high pressure graph for 996 is more or less linear. I picked the followng points. The lowest temp is 15C on the graph.

15C 9bar

40C 22.5bar

+/- 2.5bar

Low side is relatively constant at 1.4 to 1.7bar between 15C to 40C.

[Ahsai]

I would say ballpark high side pressure is approx 0.6 bar x ambient temp in Celcius.

50F = 10C so 0.6x10x14.5 = 87 PSIG. Sounds like you're very close like John said.

Edited December 23, 2015 by Ahsai

[Oh1-911]

Great thanks, I'll leave it for now and re-check when we get some warmer weather.

[Ahsai]

Good plan. Hot weather will be the true test and you can always add more R134a if needed anyway.