Category: Air conditioning refrigerant

ac hoses and lines on site repair and installation

AC compressor has blown 15 minutes after a regas/recharge.

There is a high chance the system was working with a low refrigerant charge. It was a system performing poorly, but depending on weather conditions the driver wouldn’t notice a problem with the A/C system.
After the system was charged up to full capacity a problem with some restriction on the system or a fan clutch/radiator fan assembly would be more noticeable and would have enough refrigerant running in the system to open the high pressure relief valve and vent some part of the refrigerant.
An overcharged air conditioning system won’t hold more than a couple minutes before the pressure inside the system are high enough to open the relief valve and vent the refrigerant to the atmosphere.

A/C Refrigerant pressure controls

High Pressure Relief Valves
The high pressure relief valve (HPRV) is a blow off valve, designed to release refrigerant into the atmosphere if pressures get dangerously high. In an effort to keep refrigerants contained more efficiently, many manufactures have introduced controls designed to prevent pressures from getting high enough for the pressure relief valve to blow off. Single contact pressure switches simply shut the compressor down if pressures get too high.
The dual contact pressure switches have replaced most HPRV to prevent refrigerant venting to the atmosphere.
Fusible Plug
The fusible plug is installed on the top of the receiver/drier to serve as a safety device. The fusible plug has a special solder filled hole. If the condenser ventilation becomes poor and the cooling load becomes excessively large, the pressure in the high-pressure side of the condenser and receiver will become abnormally large and create a danger of bursting the equipment.
When the temperature and pressure in the high-pressure side reaches 427 psi and 200 to 212 °F the fusible plug melts and allows the refrigerant to discharge into the atmosphere thus preventing damage to the equipment (This will add to ozone layer problems)
Dual pressure switch
The dual pressure switch is fitted between the TXV and the receiver-drier. It detects pressure on the high side of the system and switches on the cooling fan(s) to high speeds and turns off the magnetic clutch shutting down the compressor when the pressure is abnormal, thus preventing damage to the system.
Low Side Pressure Relief Valves
Some R-134a systems use a pressure relief valve on the low side of the system. This valve is designed to prevent evaporator damage during system shutdown, when pressures equalize. The higher pressures in R-134a systems could cause low side pressure to rise beyond safe limits during system shutdown.
Low pressure cutoff switch
The low Pressure cutoff switch shuts the compressor down when refrigerant levels drop too low. Since the A/C systems use the refrigerant to carry lubrication, running the system without an adequate charge can cause compressor damage. This switch prevents the compressor from running when refrigerant levels are too low to provide enough lubrication.
High Pressure Cutoff Switch
Some vehicles have high pressure cutoff switch designed to shut the system off if pressures get too high. Once pressures drop back to normal, the switch will allow the compressor to operate as designed. Look for this switch on the discharge muffler’s service port, or mounted on the back of the compressor.

What causes an air compressor to lock up?


  1. Lack of lubrication
  2. Cooling system poor performance
  3. Condenser fan not working or performing poorly
  4. A/C system overcharged
  5. Incorrect type of oil
  6. Low refrigerant charge
  7. A/C system contamination(A/C refrigerant cross contamination (Charging R-134a refrigerant into R-1234YF refrigerant.)
  8. Inefficient condenser
  9. Dirty evaporator core (In the past, not much attention was paid to dirty evaporators. Technicians are now beginning to realize that low evaporator pressures cause high compression ratios.)


How to find an automotive compressor problem, before is too late?

Compression ratios are used to compare pumping conditions for a compressor. When compression ratios become too high, the refrigerant gas temperature leaving the compressor rises to the point that oil for lubrication may become overheated. Overheated oil may turn to carbon and create acid in the system shortening the life of the Automotive A/C system.
Ambient temperature
High Side Pressure
Low Side Pressure
90 °F/32°C      250-270 psi /1724-1862 kPa   45-55 psi / 310-379 kPa
Calculation example:
Compression ratio = Absolute Discharge
Absolute Suction
CR= 250 psig + 14.7 atmosphere
45 psig + 14.7 atmosphere
CR = 264.7
CR = 4.43:1

Understanding the A/C compressor compression ratio results.

A compression ratio of 4.43:1 would indicate to a service technician that the absolute or true discharge pressure is 4.43 times as great as the absolute suction pressure. Either an increase in head pressure or a decrease in suction pressure will cause higher compression ratios.

Symptoms of a Bad or Failing AC Compressor

Growling noise with compressor disengaged
Whistling noise
Grinding noise
Squealing noise
Rattling noise
Growling noise with compressor engaged
Banging noise
Knocking noise

Freon recharge.

To charge a system with the engine running, the refrigerant should be added to the low side of the system.

Safety precaution.

Never open the high-side hand valve with the system operating and a refrigerant can connected to the center hose. The refrigerant will flow out of the system under high pressure in to the can. High-side pressure is between 150 and 300 psi and will cause the refrigerant tank to burst.

Recovery, recycling, reclaims.

The only occasion for opening both hand valves at the same time would be when evacuating the system or when recovering the refrigerant with the system off.