The 87 has an RPM sensor on the compressor, and the Klima relay compares that to engine rpm. If the Klima detects a difference, it will disengage the clutch until the engine is shut off and restarted. This is to prevent a seized compressor from taking out the serpentine belt and leaving you stranded. So if your system is working fine for a few minutes and then shuts off, it is likely a slipping belt, faulty sensor, or Klima relay - not the PBU.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References