28 - 30 psi is proper - Saab 9-5 Bulletin Board - Saabnet.com
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28 - 30 psi is proper
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Posted by 2000AERO [Email] (more from 2000AERO) on Tue, 24 Apr 2007 23:28:16 Share Post by Email
In Reply to: A/C low-side psi?, jarrald, Tue, 24 Apr 2007 08:03:39
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The AC system in the 9-5 has a variable capacity compressor with a TXV (thermal expansion valve). The TXV maintains a constant superheat (temperature and thus pressure on the suction (low) line. The compressor also ramps up and down to help maintain the constant pressure.

Thus you should see a constant 28 - 30 psi on the suction line. You CANNOT use the pressure on the low side to guage the amount of refrigerant in the system. You can get close, but it will not be fully correct. The only way to charge variable capicity systems is by weight. The 9-5 takes 875 grams of R-134. (You can also use the subcooling method, however this requires the temperature and pressure of the high side. Charging by weight is much more accurate.)

Try this when your guage is connected the the low side. Start with the ACC on LO, and the cabin fan on low speed. The pressure will be 30psi. Raise the cabin fan up to high speed and watch what the guage does. It will start to climb (and the temperature of the pipe will rise) momentarily, then it will drop back down to 30 psi. You are seeing the delay of the compressor ramping up to meet the new load on the system. Now drop the cabin fan back down to low speed. You will see the pressure drop (and the temperature of the pipe will fall) The compressor may even cycle off. After a few moment, the pressure will again stabilize around 30 psi. The compressor just ramped down to a lower capacity.

Michael

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