What is this on my compresser?

On my 97 caravan le 3.3, I replaced the a/c compresser, exp vave and drier, I had some sort of sensor mounted on the compresser, it was square and its 2 wires were wired into the a/c clutch, what is it? I threw it away with the old compresser(duh!), Im getting cold air, but I hear a slight whine when ac is on, and engine rpms go over 1500, any ideas? It is the 2nd compresser to do this in the past 2 weeks. I do residential a/c for a living, so I know the correct procedures used in replacing the compresser, and I did it by the book. Thanks.

Sounds like it may have beed the diode to keep power from backfeeding from the compressor, maybe?

Well, thanks, maybe (just kidding). Is there any way that eliminating that would cause a whining sound when ac is used, Im on my second compresser this month, because the previous one whined louder.