For AC operation, I understand the compressor will not turn on if there's insufficient freon in the system. Will this also apply in defrost mode (leftmost button of the ACC panel depressed)? Thanks.

A'yup.

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1989 300 SEL that mostly works, but needs TLC

Thanks for that. Just want to confirm the rationale... since defrost is programmed to blow warm, it does not really need freon per se... but the compressor won't engage if there's insufficent freon, which is necessary for lubrication?

The A/C compressor runs in defrost to dehumidify the air prior to heating it and blowing it against the windshield. Yes, the compressor is disabled when the refrigerant level is too low because insufficient lubrication would destroy the compressor in short order.

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1989 300 SEL that mostly works, but needs TLC