Hi H. #38 wire has pulse width modulation of current, not voltage. I don't know voltage, my meter not good enuf. Suppose 12v is pulse voltage on wire #38: then suppose a 0.5 second on pulse, 12v. That is 50% power over a whole second, and that causes the solenoid to control the fan at 50% max rpm. Remember Monaco's pull down action? 12v @ 1 full second stops fan; 0 v @ 1 full second runs fan 100%. 0.3 sec. ON each full second yields 70% of full fan speed, and smooth action of fan at a continously adjustible speed based on engine temp. This is why the FDCA has two parts: the "brain" FDC that changes the pulse into variable voltage to the solenoid, and the solenoid itself, that switches the small tube fluid to the slide valve on the hydraulic fan motor.
Simple! Eh?