[Richard]

Winner winner chicken dinner.

You are right on. The yellow wire is a make to ground, and that activates the relay. The relay is of course activated at low engine temps, and deactivates when the temp rises causing the fan to default to high speed.

I confirmed in two ways. I ohmed it to the ground lug, it has a bit of resistance, not not open circuit. With ignition on, if you remove the yellow wire from the lug, the solenoid deactivates.

It has to be make/break ground on that relay since the power input and power signal terminals are wired together. There would be no purpose to the relay if the ground signal was not the make or break.

I had no way of getting the engine up to temp to cause the relay to deactivate.

I do not know if the yellow wire gets it’s signal from a thermostat, another relay, or the DDEC. If you find out let us know.