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Refrigerant System Tests Diagnostic Table

  1. NOTE: The following table is used to guide diagnosis of the refrigerant system if operating pressures are outside normal limits.

    Refer to the chart below.

High (Discharge) Pressure Low (Suction) Pressure Component - Causes
High Normal to High
  • Condenser - inadequate airflow.
  • Engine - overheating.
Normal to High Normal
  • Refrigerant overcharge - air in refrigerant.
Normal to Low High
  • A/C  Compressor - low performance.
Normal to Low Normal to High
  • A/C  suction line - partially restricted or plugged. (1)
Normal to Low Low
  • Low refrigerant charge - leak in system.
  • A/C  suction line - partially restricted or plugged. (2)
Erratic Operation or Compressor Not Running
  • Ambient Air Temperature (AAT) sensor - poor connection
  • A/C  pressure transducer - poor connection.
  • Evaporator temperature sensor - poor connection.
  • Low refrigerant charge - leak in system.
Additional Possible Components or Causes Associated With Inadequate Compressor Operation 
  • Control Assembly Switch - dirty contacts or sticking open
  • Compressor Operation Interrupted by Engine Computer
Additional Possible Components or Causes Associated With a Damaged Compressor 
  • Suction Accumulator - refrigerant oil bleed hose plugged
  • Refrigerant Leaks
(1) Low pressure reading will be normal to high if restriction is downstream of service access valve.
(2) Low pressure reading will be low if restriction is upstream of service access valve.
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