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Every A/C compressor requires 12 volts of power in order to engage the clutch. Is the problem that there’s no power or that there is power and it’s not coming on. Those are two different problems. When there’s no power at the compressor, you have to start with a basic check of the system and electrical components.
For openers, you should check the compressor coil itself. Using an ohm meter you can check for continuity. In cases where the clutch has become overheated, the coil windings (internal to the coil) can break. That means the coil can not create the magnetic field required in order to engage the clutch.
Assuming the coil is ok, you can jump 12 volts to the compressor from the battery. Doing that should engage the clutch and also confirm that the clutch and coil are working properly.
Knowing that the coil and clutch are engaging, the problem lies in the electrical circuit and/or switches that control the operation of the compressor. The most common failure is that the cycling switch (usually located on the low side accumulator) is not closing, allowing the electrical current to flow to the compressor. If you remove the wiring harness and check for and find voltage on one side of the switch, then you either have a faulty cycling switch or it’s just doing it’s job. It’s determined that refrigerant pressures are too low and the switch is acting as a safety to keep the compressor from coming on. If that’s the case, you will require the system to be leak tested and recharged. That should solve the problem.
Additionally reasons for not having power at the A/C compressor include:
- Faulty connections at cycling switch or other electrical components
- Faulty control head
- Faulty safety switches that act to help prevent the compressor from starting if there’s a problem.
If the ‘no power’ problem continues after these basic checks, you will require the specific wiring diagram for the vehicle you’re working on in order to troubleshoot the electrical circuit.
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