Skip to main content
A/C System Test

Performance Test
  • Compressed air mixed with the R-134a forms a combustible vapor.
  • The vapor can burn or explode causing serious injury.
  • Never use compressed air to pressure test R-134a service equipment or vehicle air conditioning systems.

  • Air conditioning refrigerant or lubricant vapor can irritate your eyes, nose, or throat.
  • Be careful when connecting service equipment.
  • Do not breathe refrigerant or vapor.

The performance test will help determine if the air conditioner system is operating within specifications.
Use only service equipment for refrigerant HFC-134a (R-134a).
If accidental system discharge occurs, ventilate the work area before resuming service.
R-134a service equipment or vehicle air conditioning systems should not be pressure tested or leak tested with compressed air.
Additional health and safety information may be obtained from the refrigerant and lubricant manufacturers.
  1. Connect an R-134a refrigerant recovery/recycling/charging station to the high-pressure service port and the low-pressure service port, following the equipment manufacturer's instructions.
  1. Determine the relative humidity and air temperature.
  1. Insert a thermometer (A) in the center vent.
  1. Place another thermometer (B) near the blower unit's recirculation inlet duct.
  1. Test conditions:
    • Avoid direct sunlight.
    • Open hood.
    • Open front doors.
    • Set the temperature control dial to Max Cool, the mode control switch to Vent, and the recirculation control knob to Recirculate.
    • Turn the A/C switch ON and the fan switch to Max.
    • Run the engine at 1,500 rpm (min −1 ).
    • No driver or passengers in vehicle.
  1. After running the air conditioning for 10 minutes under the above test conditions, read the delivery temperature from the thermometer in the center vent, the intake temperature near the blower unit, and the high and low system pressure from the A/C gauges.

  1. To complete the vent (delivery)/ambient air (intake) temperature chart:
    • Mark the vent (delivery) temperature on the vertical line.
    • Mark the ambient air (intake) temperature on the bottom line.
    • Draw a vertical line from the ambient air (intake) temperature mark.
    • Draw a horizontal line from the vent (delivery) temperature mark until it intersects the vertical line.
    NOTE: The low side and intake temperatures should intersect in the shaded area within about 10 % of the measured humidity level. Any measurements outside the line may indicate the need for further inspection.

  1. To complete the high side (discharge pressure)/ambient air (intake) temperature chart:
    • Mark the high side (discharge pressure) temperature on the vertical line.
    • Mark the ambient air (intake) temperature on the bottom line.
    • Draw a vertical line from the high side (discharge pressure) temperature mark.
    • Draw a horizontal line from the vent (delivery) temperature mark until it intersects the vertical line.
    NOTE: The low side and intake temperatures should intersect in the shaded area within about 10 % of the measured humidity level. Any measurements outside the line may indicate the need for further inspection.

  1. To complete the low side (suction pressure)/ambient air (intake) temperature chart:
    • Mark the low side (suction pressure) temperature along the vertical line.
    • Mark the ambient air (intake) temperature along the bottom line.
    • Draw a vertical line from the ambient air (intake) temperature mark.
    • Draw a horizontal line from the vent (delivery) temperature mark until it intersects the vertical line.
    NOTE: The low side and intake temperatures should intersect in the shaded area within about 10 % of the measured humidity level. Any measurements outside the line may indicate the need for further inspection.

Pressure Test
Test results
Related symptoms
Probable cause
Remedy
Driver's and passenger's side vent temperatures vary by more than 11 °C
Poor A/C performance. Discharge (high) and suction (low) pressures are low.
Low refrigerant charge
Air mix door inoperable
Repair or replace.
Discharge (high) pressure abnormally high
After stopping A/C compressor, pressure drops about 196 kPa (2.0 kgf/cm 2 , 28 psi) quickly, and then falls gradually.
Air in system
Reduced or no airflow through A/C condenser.
  • Clogged condenser or radiator fins
  • A/C condenser or radiator fan not working properly
  • Clean.
  • Check voltage and fan rpm.
  • Check fan direction.
Line to A/C condenser is excessively hot.
Restricted flow of refrigerant in system
Restricted lines.
Discharge pressure abnormally low
High and low-pressures are balanced soon after stopping A/C compressor. Low side is higher than normal.
  • Faulty A/C compressor discharge valve
  • Faulty A/C compressor seal
Replace the A/C compressor.
Outlet of expansion valve is not frosted, low-pressure gauge indicates vacuum.
  • Faulty expansion valve
  • Moisture in system
  • Replace.
  • Recover, evacuate, and recharge with specified amount.
Suction (low) pressure abnormally low
Expansion valve is not frosted, and low-pressure line is not cold. Low-pressure gauge indicates vacuum.
  • Frozen expansion valve (Moisture in system)
  • Faulty expansion valve
  • Recover, evacuate, and recharge with specified amount.
  • Replace the expansion valve.
Discharge temperature is low, and the airflow from vents is restricted.
Frozen evaporator
Run the fan with A/C compressor off, then check evaporator temperature sensor.
Expansion valve is frosted.
Clogged expansion valve
Clean or replace.
Receiver/dryer outlet is cool, and inlet is warm (should be warm during operation).
Clogged receiver/dryer
Replace.
Suction pressure abnormally high
Low-pressure hose and service port are cooler than the temperature around evaporator.
Expansion valve open too long
Repair or replace.
Suction pressure is lowered when A/C condenser is cooled by water.
Excessive refrigerant in system
Recover, evacuate, and recharge with specified amount.
High and low-pressures are equalized as soon as the A/C compressor is stopped, and both gauges fluctuate while running.
  • Faulty gasket
  • Faulty high-pressure valve
  • Foreign particle stuck in high-pressure valve
Replace the A/C compressor.
Suction and discharge pressures abnormally high
Reduced airflow through A/C condenser.
  • Clogged A/C condenser or radiator fins
  • A/C condenser or radiator fan not working properly
  • Clean.
  • Check voltage and fan rpm.
  • Check fan direction.
Suction and discharge pressures abnormally low
Low-pressure hose and metal fittings are cooler than evaporator.
Clogged or kinked low-pressure hose parts
Repair or replace.
Temperature around expansion valve is too low compared with that around receiver/dryer.
Clogged high-pressure line
Repair or replace.
Refrigerant leaks
A/C compressor clutch is dirty.
A/C compressor shaft seal leaking
Replace the A/C compressor.
A/C compressor bolt(s) are dirty.
Leaking around bolt(s)
Tighten bolt(s) or replace A/C compressor.
A/C compressor gasket is wet with oil.
Gasket leaking
Replace the A/C compressor.
A/C fitting is dirty.
Leaking O-ring
Clean the A/C fitting and replace the O-ring.