Start by turning the forward most portion of the compressor while the engine is not running. It should turn smoothly only feeling compression pulses. If it won't turn or feels as if it's full of rocks, then you don't WANT the clutch to engage because the compressor is toast and will just put strain on the belt.

If the compressor DOES turn smoothly, then it is likely that it simply has leaked out the refrigerant. You need to find the leak, repair it and recharge. Before chasing a leak, however, momentarily jump 12 volts to the clutch and see if it clicks. If it does, then find the receiver drier and remove the two connectors from the low pressure switch. If it does not have continuity across the switch, then the system is low or empty.

Put manifold gauges on the system is another, more conventional approach to seeing if it is empty. It all depends on the tools and equipment you have access to.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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