Posted 25 June 2008 - 10:16 AM
Your LS uses an EPR valve to more closely regulate the boiling, evaporative rate, of the refrigerant and also to "quiet" the refrigerant evaporative process, less "noise" in the passenger cabin.
Most automotive A/C systems have the evaporator outlet completely open to the compressor inlet. Your LS has an EPR valve mounted right at the firewall. Be aware that with an EPR the low pressure in the compressor inlet line is often NOT at the same low pressure level as the evaporator outlet, outflow.
To get these pressures close enough to equal to get the guage pressure measurement system to "work" the EPR must be FULLY OPEN. To do that the evaporator cooling load must be as high as possible, HIGH blower, LOW temperature setting, FRESH intake, windows down and/or doors wide open.