Man, I had a 506b for 20 years. I could have sworn the fan spun all the time. Does your steering work? I can't remember if it works off the steering pump, or was there a second pump on the steering pump that runs the fan? You got a manual?
According to the book, I think you're right on the fan turning all the time. I don't see anything about wiring or sensors to turn the fan on.
According to this picture, pump 3 sends oil to 41(cooling fan motor)
About all I can think is, once this thing is running, check and see if oil is getting to the cooling fan motor, if it's not the problem has to be pump 3. If oil is getting there, then it has to be an issue with the cooling motor itself.
That picture shows a relief valve on the cooling motor, this one does not have that. Its just one hose in, one hose out