The lower one is the pressure switch which would disconnect the compressor clutch if you were to be very low on freon (protective of compressor, since oil circulates with the refrigerant).

In the eary receiver/dryer you could remove one of those switches w/o losing freon (but I can't remember which one). I can't believe your receiver/dryer is one of the eary ones (unless someone searched one out) and therefore you would have to evacuate/recover before you replaced either switch.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)