I really can't take credit for this screwup, but it's a beaut. I was called to service a two stage rooftop air conditioning unit that would not cool. The unit was old, and had had the primary compressor replaced. The AC guy swore up and down that his lines and the unit charge was perfect. The unit had a 240 volt 1Ø supply run through a capacitor bank Add-A-Phase convertor. The compressor just hummed loudly when the disconnect was turned on.

I went through the capacitors and found half of them blown. Replaced the electrolytics, turned on the disco. . .no joy, just humm.

I went through the relay ladder for the start / run capacitor bank switching. . .all good.

Finally I crawled into the compressor housing to get over behind and to the bottom of the compressor to get the model number off the black on black ID plate.

The last character of the 14 char. long ID should have been an E, not an F. . .F was for 480 volts. [Linked Image]

Al Hildenbrand