Me personally.... I would never replace a new fan until I verified correct voltage at the fan. I would inspect the fan's electrical connections looking for evidence of overheating that could cause a weak electrical connection (a weak electrical connection could also make the voltage drop). Remember that when the cooling fan is on high it uses a lot of power (50 amp circuit). I would also run the fan directly from the battery to confirm it's actually running slow even when supplied with enough voltage. This would also involve testing the fan to confirm that the fan is capable of working on both speeds. Also confirm there's a speed difference between low and high speed. IMO it's a good idea to confirm that the HS and LS are wired correctly. Consider what if the new fan is wired backwards? Is there a back story with the "new" fan? Something why you are willing to buy another fan to replace a recently installed fan? My recommendation. Resist the urge to fall back into "parts changing" mode. Focus on the basics. Test. Then replace/repair. But if looking for some where to spend your $$, it may be better spent on a new radiator. Hard to say without looking inside.