The AC system often uses pressure switches to sense pressure in the high side of the system as well as the low side of the system. The high side switch is located at some point in the high side of the system and is connected into the system by either threading onto a line or directly into the body of the compressor. The high side switch is typically a closed switch at pressures below about 375 psi, at which point the contacts open and cause the compressor to turn off for a few seconds to allow the pressure to drop to safer levels.
The low side switch is located at some point in the low side of the system and is connected into the system by either threading onto a line or other low side component. The low side switch is typically a closed switch at pressures above about 25 psi, at which point the contacts open, causing the compressor to turn off for a few seconds to allow the pressure to rise. The low side switch assists in controlling the temperature in the evaporator from getting too cold. This switch also causes the compressor to turn off if the system gets low on refrigerant. Below are some strategies to use when testing pressure switches.
Servicing AC Pressure Switches
1. After verifying the system has pressure, disconnect the electrical connector and connect a fused jumper wire across the terminals of the connector. This will bypass the switch to see if the compressor will turn on. Never perform this test if the AC system has lost all pressure because the compressor will possibly run without any lubricant, which will quickly destroy the unit. A small paper clip can also be used for this procedure, but care should be used to prevent terminal damage.
2. Use an ohmmeter to test the resistance across the switch contacts. The low side switch should have low resistance with the system pressure above 25 psi and the high side switch should have low resistance with the system pressure below 375 psi.
When AC pressure switches are found to be defective, then it is time to replace them with a new unit. Many of these switches are mounted to a line or component that has a check valve underneath it, so it is not necessary to recover the refrigerant prior to replacing it. These switches are made with a raised portion in the center of the switch that presses on the check valve when it is installed on the system.
These switches can simply be turned counterclockwise to remove them from their mounting location. A new O-ring should be installed with the new switch, and the unit should be tightened to specifications with a torque wrench. (There are some switches that screw onto the line or component, but there is no Schrader valve underneath.)
These systems must be recovered prior to removing the switch for service. If a switch is held in by a snap ring, then the system must be recovered prior to removing the switch for service. After replacing these switches, the AC system will need to be evacuated and then recharged with the correct amount of refrigerant.
Testing and Servicing AC Pressure Switches
1. The high side switch has two terminals and is located on a line or a component in the high side of the AC system. The connector for the switch should be closely inspected to make sure the terminals are in good shape and not corroded or
2. The low side switch has two terminals and is located on a line or a component in the low side of the AC system. This switch provides protection for the system if the pressures in the low side of the system drop below certain
3. After verifying that the AC system has pressure that indicates the presence of refrigerant, the low side switch can be bypassed to see if the compressor will engage. Using jumper wires that fit into the terminals without damaging them is a method to bypass the switch. This procedure should only be used when testing to see if the low side switch is bad and should not be done for more than a few
4. Similar to using the jumper wire, a small paper clip that fits into the terminals without damaging them is another method to bypass the switch. It is important to use a jumper device that does not cause the terminals in the connector to spread out when it is
5. An ohmmeter can be used to test the resistance of a pressure switch. The result of this test will be a very low resistance if the switch is closed and OL if the switch is open. A low-pressure switch should be closed if there is at least 25 psi of pressure in the system. A high-pressure switch should be closed if the pressure is below about 375
6. The bottom side of the low side and high side switches are made with the center portion that protrudes out. This raised portion is what engages the Schrader valve at the mounting location. AC systems with switches like this do not require the system to be recovered when replacing the switch. When this switch is removed, the Schrader valve holds the refrigerant in the
7. The low side switch is always mounted in the low side of the system and when the metering device is an orifice tube, this switch is often mounted on the accumulator dryer. This switch does have the raised center, which means that the AC system does not have to be recovered to replace this
8. The connector needs to be pinched to release the retaining clips to allow it to be disconnected. It is important to be careful when disconnecting any electrical connector to prevent breaking the latches or causing any other damage to the
9. The bottom side of the low side switch has a hex surface that will allow a wrench to be used to loosen the switch for removal. Be sure to use the correct size wrench to prevent rounding the corners off of the hex
10. After the switch is removed, the Schrader valve and O-ring should be inspected for damage. If bubbles or a hissing sound is observed, then the Schrader valve is leaking and will need to be replaced. The refrigerant will need to be recovered if the Schrader valve needs to be
11. The O-ring should always be replaced when replacing any switch or sensor on the AC system. A pick is a useful tool to use to aid in the removal of the
12. The new O-ring can be installed by hand and lubricated with refrigerant oil to aid in sealing. Make sure the correct size and thickness of O-ring is used to prevent a refrigerant
13. The new switch is installed by hand, making sure to not cross-thread the connection. The switch should be turned about three turns by hand prior to using the wrench to finish the tightening process. A wrench is used to tighten the switch back to the correct tightness and then the connector can be reattached. The switch tightness is at a very low torque and should only be turned with mild wrench pressure. A torque wrench should be used if there is doubt on the tightness level. A crow’s foot wrench can be used along with a torque wrench to attain the exact tightness. The torque specification can be found in a service manual or on an electronic
14. The electrical connector is reinstalled by carefully pushing it into place until the latches of the connector snap into
Pressure switches that do not thread onto a fitting are typically held in place with a snap ring. These switches always require the refrigerant be recovered prior to removing them from the system. The O-ring should always be replaced when servicing this type of
Pressure switches and sensors that do not have a protruding area in the center require the AC system to be recovered prior to removing them from their mounting position. These switches and sensors are not as common as the other type, and anyone who is servicing the AC system should always make sure they are aware of which type a system uses prior to performing service on these devices. The service procedures in the service manual or an electronic database will reveal whether or not the system needs to be recovered when these switches are being
