Since the ac system is located mostly inside a car engine and the car is warmed up prior to adding freon, and the temp/humidity in engine bay is different from ambient temperature//humidity, and all of the above is done with car in parked position, why is ambient temperature used to measure pressure instead of temps closer to engine area with hood open.

  • Welcome to Motor Vehicle Maintenance & Repair! Feb 14, 2019 at 23:03
  • Because part of the a/c system is also in the car cabin and not under the hood.
    – Solar Mike
    Feb 15, 2019 at 6:23
  • Okay. But then why is the car cabin the least common denominator and not the car engine when measuring pressure.
    – StackUser
    Feb 15, 2019 at 19:34
  • Think about the a/c circuit - the compressor causes the pressure rise and the throttle or expansion valve reduces it.... where are those components and which components are downstream of them....
    – Solar Mike
    Feb 16, 2019 at 8:31
  • Try plotting the cycle on an H-S diagram and evaluating the presdures around the circuit based on the temperatures...
    – Solar Mike
    Feb 16, 2019 at 8:33


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