How to Replace the Low Pressure Switch in a Car's A/C

The low-pressure switch in your car or light truck's air conditioning system is used to interrupt power to the compressor when the low side pressure drops below 25 psi. This prevents freeze-up of the system and possible damage to the compressor. When the switch is faulty, the symptoms of freeze-up, or no compressor function, will be present. Replacement is straightforward, but requires the removal of refrigerant from the system.

Step 1 - Recover the refrigerant in the system. The equipment to perform this task is expensive.Your local auto repair shop will be happy to recover the refrigerant for you, for a small fee. The release of air conditioning refrigerant into the atmosphere is believed to be a factor in the production of green house gases, and a possible cause of ozone layer depletion. Release of refrigerant is prohibited by section 609 of the Federal Clean Air Act.

Step 2 - Refer to your service manual to help you locate the low-pressure switch. Unscrew the old low-pressure switch, and screw the new switch onto the fitting. Replace the rubber o-ring seal with the new one supplied with the new low-pressure switch.

Step 3 - Attach the air conditioning gauge set to the service ports of the air conditioning system, and attach the vacuum pump to the service hose on the gauge set. Turn the vacuum pump on and open the service valves on the gauge set. Allow the pump to run for 1 hour to remove all moisture from the system.

Step 4 - Close the service valves on the gauge set, and disconnect the vacuum pump. Install the can tap that is supplied with the gauge set, and attach a can of refrigerant to the can tap.

Step 5 - Open the can, and open the blue service valve on the gauge set. Allow the pressure from the can to partially fill the system, and then start the vehicle and set the controls to Max A/C.

Step 6 - When the first can is empty, close the service valve, attach another can to the tap, and open the valve to install that can. Repeat this step until the system is full. Refer to your service manual for system capacity specifications.

Close all valves on the gauge set, turn off the engine and disconnect the gauge set from the vehicle.

TIPS: Use refrigerant that has dye in it. This helps find leaks if they occur.

THINGS YOU'LL NEED: A/C gauge set, Vacuum pump, Refrigerant, Low-pressure switch

WARNINGS: Never open the red high-pressure service valve on the gauge set when the engine is running. The pressure developed on that side of the system will blow the refrigerant can apart, and injury could result.

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