There are several different ways mfg protect compressors from damage when a system is low on refrigerant. With modern cars it is often done with a "controller" and a pressure transducer in the liquid line. A transducer will most often have three terminal and they should NEVER BE SHORTED or jumped. They work on a 5 volt feed and the output varies with pressure. The controller takes inputs from many sensors (outside air temp, evaporator temp and even things like road speed) to decide if the system is operating within the perimeter set by the mfg. To diagnose these you will likely need a scanner and the mfg service manual to find out why it is not working.
There are several "manual" safeties that use either single action or multi action switches to protect the compressor. Without enough refrigerant, oil will not flow and the compressor will not be cooled. Both lead to major failure.
The simplest of these safeties is a low pressure switch in the high pressure line. It is normally open (meaning when not pressurized the switch does not conduct) and the switch is set to close somewhere around 35 psi. This will prevent the compressor from operating if the system is almost completely empty but will not protect against a system that is only "low". If the pressure at rest is above the switch setting, once the compressor is activated, the pressure will always rise, so unless the leak is big enough for the compressor to pump the rest of the refrigerant out, and trip the switch, no protection is provided for a low charge. There is also no protection from high pressure damage. Most systems that use a simple low pressure switch also have a some sort of pressure relief valve.
The next system uses a Binary switch. Like the above, it has a NO (normally open) low pressure switch but the same unit also houses a NC (normally closed) high pressure switch that opens when the high side pressure goes too high (often around 375 psi). This prevents the relief valve from opening and the loss of refrigerant. While it is better than the simple low pressure switch it also does not protect from a low charge, only a almost complete loss of refrigerant. What they do have going for them is that any break in the wiring will prevent the system from operating.
GM came up with their "thermal limiter" system to try and address a low charge situation. It uses a thermal probe in the compressor on the low side, once the temp in the compressor rises to the set point the switch ( aprox 140 deg F) closes and causes a "short" in the thermal limited fuse, which burns and cuts power to the clutch.
The last type uses a NC switch in the LOW pressure (suction) side. It uses a relay that once tripped, latches and prevents the compressor from operating. The low pressure switch closes below 10 psi and opens on rising pressure around 30 psi. Because a system at rest will have the pressure above the closing pressure, the relay is not activated. If the system has a full charge, the low side never approaches the switches closing point. If there is a slow leak the low side will drop with the compressor operating. Once it reaches the set value of the switch, the switch closes and pulls the relay in, a 2nd feed hold the relay closed even if the pressure switch opens when the compressor stops. Because the pressure value of the switch is above suction (positive pressure) there must be enough refrigerant in the system to move oil through the system and back to the compressor. This type will shut the compressor off much sooner that a switch in the high pressure line will, often at t point where the system performance is just starting to drop off. Like the thermal limiter described above, if the wire to the switch breaks or falls off the system is left unprotected. There is also no protection from over pressure. The low side relay will stay latched until the power is removed from the A/C circuit, then it will re-set and be ready to latch again if A/C is called for and the problem not fixed.
The best system will use both the low side suction switch and a binary switch in the receiver. With that, it has a redundant low pressure cut off and high pressure cut-off functions.
CCOT (orifice tube) systems don't really have a safety system. The clutch cycling switch will open when the low side drops below the sitting but will close when the pressure rises when the compressor stops. It will cycle more and more until the pressure drops below the setting with the compressor off, or after almost all of the refrigerant has been expelled. While the compressor will not run with no refrigerant in the system it will if there is just a small amount.
There are several "manual" safeties that use either single action or multi action switches to protect the compressor. Without enough refrigerant, oil will not flow and the compressor will not be cooled. Both lead to major failure.
The simplest of these safeties is a low pressure switch in the high pressure line. It is normally open (meaning when not pressurized the switch does not conduct) and the switch is set to close somewhere around 35 psi. This will prevent the compressor from operating if the system is almost completely empty but will not protect against a system that is only "low". If the pressure at rest is above the switch setting, once the compressor is activated, the pressure will always rise, so unless the leak is big enough for the compressor to pump the rest of the refrigerant out, and trip the switch, no protection is provided for a low charge. There is also no protection from high pressure damage. Most systems that use a simple low pressure switch also have a some sort of pressure relief valve.
The next system uses a Binary switch. Like the above, it has a NO (normally open) low pressure switch but the same unit also houses a NC (normally closed) high pressure switch that opens when the high side pressure goes too high (often around 375 psi). This prevents the relief valve from opening and the loss of refrigerant. While it is better than the simple low pressure switch it also does not protect from a low charge, only a almost complete loss of refrigerant. What they do have going for them is that any break in the wiring will prevent the system from operating.
GM came up with their "thermal limiter" system to try and address a low charge situation. It uses a thermal probe in the compressor on the low side, once the temp in the compressor rises to the set point the switch ( aprox 140 deg F) closes and causes a "short" in the thermal limited fuse, which burns and cuts power to the clutch.
The last type uses a NC switch in the LOW pressure (suction) side. It uses a relay that once tripped, latches and prevents the compressor from operating. The low pressure switch closes below 10 psi and opens on rising pressure around 30 psi. Because a system at rest will have the pressure above the closing pressure, the relay is not activated. If the system has a full charge, the low side never approaches the switches closing point. If there is a slow leak the low side will drop with the compressor operating. Once it reaches the set value of the switch, the switch closes and pulls the relay in, a 2nd feed hold the relay closed even if the pressure switch opens when the compressor stops. Because the pressure value of the switch is above suction (positive pressure) there must be enough refrigerant in the system to move oil through the system and back to the compressor. This type will shut the compressor off much sooner that a switch in the high pressure line will, often at t point where the system performance is just starting to drop off. Like the thermal limiter described above, if the wire to the switch breaks or falls off the system is left unprotected. There is also no protection from over pressure. The low side relay will stay latched until the power is removed from the A/C circuit, then it will re-set and be ready to latch again if A/C is called for and the problem not fixed.
The best system will use both the low side suction switch and a binary switch in the receiver. With that, it has a redundant low pressure cut off and high pressure cut-off functions.
CCOT (orifice tube) systems don't really have a safety system. The clutch cycling switch will open when the low side drops below the sitting but will close when the pressure rises when the compressor stops. It will cycle more and more until the pressure drops below the setting with the compressor off, or after almost all of the refrigerant has been expelled. While the compressor will not run with no refrigerant in the system it will if there is just a small amount.