When upgrading to a smart thermostat (Ecobee Lite3), I discovered that the original installers of the A/C system must have been lazy. Smart thermostats require a common/ground, but regular old-school ones don't - so why bother connecting that wire at the air handler? Ugh.

Turns out my system was fully capable of supporting the common wire, but the wire simply wasn't connected inside the air handler. One trip to the attic later, and problem solved.

Here is my wiring. Yours may be different. I am not your A/C guy.