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So the fan runs without thermostat leads connected right?
If you take wire 6 off the relay does the motor stop?
It should be numbered right on the relay for you to find.
I have a lot of experience with them, but I will be offline for awhile, let me open this up to see if another expert can asisst
You have a relay that the green thermostat wire eventually lands on. This is listed as a K3 relay and is known as Part P-8-10283. See Here
The board is for time delay on and off for the heat cycle.
The K3 is for fan on and cooling function.
You would find that black blower motor wire attaches to this relay. Main power comes in one side of this relay and just sits there waiting. Once G terminal from thermostat sends 24v back on the green wire..it closes this relay.
In you case this relay could be stuck.
If you pull the black motor wire off and motor stops..then it is cooling K3 relay stuck.
If you pull black and it still runs, then it is a heat board/wire issue. Heat and cool use 2 different speeds.
Pull the black wire, confirm it stops and if so...put black back on...pull the green wire off that comes back from board. If it still runs replace K3.
Just let me know as go along troubleshooting.
Any questions just ask.
That is the culprit.
and you have tested correctly to find a bad relay. Well done!
You are very welcome. Just return if any issues at all