There are two different directions one could travel:
* The fan is bad (the tach circuit shorted out within it)
* The speed regulation logic on the logic board is damaged
I would try the easier fix, replacing the fan to see if that fixes things. Otherwise you’ll need to scare up the schematics and board views to trace out the logic if you have the skills to read them as well as the needed tools to test and replace a damaged SMT component or two. That last option is just replacing the logic board.