I want see if someone help me on hooking up this new Honeywell thermostat with Wifi I just purchased. I have an old ac unit and I only have 3 connections to my circuit board which does not have an option to connect my wire. Any help would be appreciated. Thanks
I think you've already figured out that older units often didn't run the C wire. Power from 'R' was switched to 'G' (fan) and/or 'W' (heat) by a simple switch in the thermostat. The older thermostats needed no power, so there wan't any point in running the common wire 'C'.
Older "smart" thermostats worked off the difference between 'R' and 'W' when the heat wasn't on, and then used batteries to ride through when it was on. That was fine before we wanted WiFi connected thermostats, but the new stuff wants more (and constant) power.
The wire labeled 'C' on the diagram is what you want. That gives the other end of the 24VAC transformer.
This site has great coverage of all this: https://smartthermostatguide.com/thermostat-c-wire-explained/