because there are a number of things that can affect performance. Compressor operation, evaporator valve operation, condensor blockage, etc. and other issues can be frequently pinpointed by comparing actual pressures with what is expected when system is operating properly. Plus, it's so easy to do. Think of your doctor listening to your heart with a stethescope...

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.