Pre 2005 or so, the y terminal was just a pin. Nothing more than a marrett. When the thermostat called for cooling , it always has to send power to y (only the condensing unit) and g, (the furnace fan). Things are different now adays because constant fan is lower speed than a/c fan so the furnace need to decipher between a call for g and a call for g and y. But thermostats are all built the same, and always will be, a call for cooling will energize g and y. This inherent "flaw" is why you cannot jumper any furnace (r to g) with a/c to run the fan manually with no relay. It will always turn on the a/c as well.
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