The other day I was checking out an overheated compressor. After putting the red and black wires back on the single pole contactor, the compressor would hum then stop. A service tech came out, and said that I had put the black an red wires on wrong sides of contactor. My question is, does it matter which side the wires are installed?? I always thought that 120 volts to each wire, either way, wouldn't make a difference.

A peek at the electrical diagram for the unit will answer that question. Usually, it does matter.