I recently decided to change my thermostat. I failed to disconnect the power. And of course damaged the transformer. I replaced said transformer. The air handler now works again. But my outside unit just hums barely. No spinning of fan. What could my problem be? It worked fine before the transformer went out. Any help would be greatly appreciated.

Something else got fried when you didn't cut the power off. Sounds like a relay switch.

Check the circuit breakers and fuses outside for the unit.

I did. I'm an electrician by trade. But when it comes to relays and controls, I'm lost. Everything has the correct line voltage. My air handler works, but the outside doesn't. It has power. Most of the relays have low voltage going through them. But I haven't a clue which goes for what and where the voltage is and isn't supposed to be present. Thanks for the input.

If you have a screw driver or non metallic item press in the center of the contactor (carefully) to activate the unit. At least that way you will know if it runs at all. It could be the contactor is pulled in (hum) noise.