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Depending on the actual vehicle and the compressor being installed, it’s very common for A/C compressors to include one or more pressure sensing switches in the rear of the compressor. This is most common on GM compressors which are used on a variety of vehicle applications.
Each of the switches performs a specific function and helps protect the compressor. As an example, one switch may be a high pressure cut out; that would shut the A/C System down should high pressures rise too high. Another switch may be a low pressure cut out; that’s a safety so that the A/C compressor doesn’t operate when the refrigerant charge is too low. There are a variety of switches, each with their own function. The switches will change from vehicle to vehicle, depending on the overall electrical circuit design.
Typically, replacement compressors will come without the switches. That’s simply because for some compressor designs, the compressor itself remains the same; the switch or combination of switches change. The supplier will ship the replacement compressor with plugs.
In cases where the original compressor does not use any of the switch ports (usually in the rear head of the compressor), then leave the plugs in place. They are sealed on an o-ring and will make no difference to the operation of the A/C system. Where the original compressor does have one or two switches, it’s important to remove those switches from the old compressor and install them into the replacement. Use care to assure that each switch is installed into the proper port. Typically, one port is sensing high side pressure while the second one senses low side pressures.
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