92 Buick Regal Air Conditioning

My 92 Buick Regal Air Conditioning is not working. Specifically, the A/C relay is not getting a signal to turn on the relay, which energizes the
compressor clutch. The compressor and clutch are working fine - when I remove the relay and jumper across the pins for the relay contacts, the clutch engages, and the A/C works.
I see on the wiring diagram that the A/C relay is supposed to get its signal from pin 7 of the ECV, so I would like to measure directly on pin 7 of the ECV - but where the heck is the ECV, and how do I tell which is pin 7?
Also, what signals are given to the ECV to have it determine the A/C relay signal, i.e. whether to engage the compressor clutch? Are there sensors somewhere that should be checked?
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