So my AC isn't working. Not the cap. Inside fan is not coming on so nothing is starting. thermostat seems to be working. I'm thinking it might be the heat relay? Seems to be consistent with the symptoms and the old owner had marked the juction box where it's mounted in the attic with Sharpie "Heat Relay", so must have been replaced before. The whole system is probably from the 70's or 80's, it's a Carrier in the attic.
Any advice or wild guesses welcome, going to try to test the relay tomorrow. Tonight I'll drink beer to cool down.