A/C System Test

Pressure Test


Test results Related symptoms Probable cause Remedy 
Discharge (high) pressure abnormally high After stopping compressor, pressure drops to about 196 kPa (2.0 kgf/cm2, 28 psi) quickly, and then falls gradually. Air in system Discharge, evacuate, and recharge with specified amount. 
No bubbles in sight glass when condenser is cooled by water Excessive refrigerant in system Discharge, evacuate, and recharge with specified amount. 
Reduced or no air flow through condenser 
  • Clogged condenser or radiator fins
  • Condenser or radiator fan not working properly
 
  • Clean
  • Check voltage and fan rpm.
  • Check fan direction.
 
Line to condenser is excessively hot. Restricted flow of refrigerant in system Restricted lines 
Discharge pressure abnormally low Excessive bubbles in sight glass; condenser is not hot. Insufficient refrigerant in system 
  • Check for leak.
  • Charge system.
 
High and low pressures are balanced soon after stopping compressor. Low side is higher than normal. 
  • Faulty compressor discharge valve
  • Faulty compressor seal
 
Replace the compressor. 
Outlet of expansion valve is not frosted, low-pressure gauge indicates vacuum. 
  • Faulty expansion valve
  • Moisture in system
 
  • Replace
  • Discharge, evacuate, and recharge with specified amount.
 
Suction (low) pressure abnormally low Excessive bubbles in sight glass; condenser is not hot. Insufficient refrigerant in system 
  • Repair the leaks.
  • Discharge, evacuate, and recharge with specified amount.
  • Charge as required.
 
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
 
  • Discharge, evacuate, and recharge with specified amount.
  • Replace the expansion valve.
 
Discharge temperature is low, and the air flow from vents is restricted. Frozen evaporator Run the fan with compressor off, then check  evaporator temperature sensor. 
Expansion valve is frosted. Clogged expansion valve Clean or replace. 
Suction pressure abnormally high Low-pressure hose and check joint are cooler than the temperature around evaporator. Expansion valve open too long Repair or replace. 
Suction pressure is lowered when condenser is cooled by water. Excessive refrigerant in system Discharge, evacuate, and recharge with specified amount. 
High and low-pressure are equalized as soon as the compressor is stopped, and both gauges fluctuate while running. 
  • Faulty gasket
  • Faulty high-pressure valve
  • Foreign particle stuck in high-pressure valve
 
Replace the compressor. 
Suction and discharge pressures abnormally high Reduced air flow through condenser. 
  • Clogged condenser or radiator fins
  • Condenser or radiator fan not working properly
 
  • Clean
  • Check voltage and fan rpm.
  • Check fan direction.
 
No bubbles in sight glass when condenser is cooled by water Excessive refrigerant in system Discharge, evacuate, and recharge with specified amount. 
Suction and discharge pressure abnormally low Low-pressure hose and metal end areas 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 Compressor clutch is dirty. Compressor shaft seal leaking Replace the compressor. 
Compressor bolt(s) are dirty. Leaking around bolt(s) Tighten bolt(s) or replace compressor. 
Compressor gasket is wet with oil. Gasket leaking Replace the compressor. 

Performance Test




  • Compressed air mixed 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 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 a R-134a refrigerant recover/recycling/charging station to the high-pressure service port and the low-pressure service port, following the equipment manufacturer's instructions.

2.Insert a thermometer in the center vent.

 

3.Test conditions:

  • Avoid direct sunlight.
  • Open the hood.
  • Open the front doors.
  • Set the temperature control dial to Max Cool, the mode control dial to Vent and the recirculation control lever to Recirculate.
  • Turn the A/C switch on and the fan switch on Max.
  • Run the engine at 1,500 rpm (min-1).
  • No driver or passengers in vehicle.

4.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 behind the glove box and the high and low system pressure from the A/C gauges.

5.To complete the charts:

  • Mark the delivery temperature along the vertical line.
  • Mark the intake temperature (ambient air temperature) along the bottom line.
  • Draw a line straight up from the air temperature to the humidity.
  • Mark a point 10 % above and 10 % below the humidity level.
  • From each point, draw a horizontal line across the delivery temperature.
  • The delivery temperature should fall between the two lines.
  • Complete the low-side pressure test and high-side pressure test in the same way.
  • Any measurements outside the line may indicate the need for further inspection.