To raise pressure, there must be a restriction in the air conditioning system. The restriction divides the system into the high side and the low side. The terms high side and low side refer to the high pressure and low pressure within the system.
The flow control can be either an orifice tube or a metering device called an expansion valve. A thermostatic expansion valve controls the amount of refrigerant that is allowed to flow to the evaporator. A temperature sensing bulb on the evaporator inlet controls the expansion valve.
- A bad expansion valve will cause low readings on both the low and high side.
- Failure of the sensing bulb is the most common problem.
- Sometimes the valve can become plugged with debris.
- If the sensing capillary tube is warm, there is high pressure that pushes on the diaphragm.
- The sensing bulb checks the temperature of the outlet line of the evaporator.
- If it is warm, it opens the valve.
- Pressure on top of the diaphragm tries to push the valve open.
- The operation of the expansion valve can be checked.
- Some expansion valves can be cleaned, repaired or adjusted.
This system uses an expansion valve as it's flow control device.
Some expansion valves are wrapped with insulation. When the sensing bulb is in the evaporator case, it does not need to be wrapped because it stays cold. When an expansion valve is packed in that type of system, it is only to silence the noise that the evaporator makes.
Only EPA-certified technicians can purchase R-12 refrigerant, since proper recovery and recycling techniques are required to prevent damaging the environment. Improper disposal of R-12 refrigerant can result in a fine.