These externally controlled variable displacement compressor's dont have a clutch and the internals are apparently always moving.

Whats to stop (does) it just damaging/destroying itself if the system leaks all its gas and is still driven ?

Would there not be oil just sitting in the pump to lubricate it ?

If you are replacing a pump (comes with oil in it) at home, you may not be able to drive it to shop for gassing ?

1 Answer 1


GM uses internally controlled variable displacement compressors (VDC) with a clutch. My understanding of vehicle ac systems to protect against loss of refrigerant is the combination low/high pressure sensor monitoring the high side. GM uses pressure sensors. Below 40 psi, the pressure sensor sends a signal to the ecm to disable compressor power to the clutch. Normal standby pressures are close to outside temps; 75F/75psi, 85F/85 psi, etc. High side operating pressures vary from 150-250+psi. A leak releases refrigerant until pressure drops below approximately 40 psi when the ecm disables power to the clutch. Refrigerant moves lubricating oil throughout the system as a mist, returning to the suction side to provide continuous lubrication. Once refrigerant is lost, oil can't circulate. Without a pressure sensor, the compressor would continue to run, without lubricating oil and quickly self destruct. I don't know if there are vehicle ac systems without clutches to disconnect compressor operation and if they exist, there must be a way to disable compressor running when refrigerant leaks out whether from wear and tear or damage resulting from a crash, rupturing any part of the system. All refrigeration systems are under pressure whether running or not.

Here's a link to Denso's site; https://www.denso-am.eu/products/ac-engine-cooling/ac-compressor

Here's a link explaining clutch less ac compressors; http://www.mechanexpert.com/compressors-without-magnetic-clutches/

  • Hi, Im well aware of traditional clutched systems. I want details about clutch-less such as Denso 7SEU17C Nov 30, 2022 at 9:28

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