Join Date: Sep 2013
Location: Tampa Bay, FL
Vehicle: 2011 SLK300 with smart top
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It might be a little simpler than that. If the battery was left connected while changing out the alternator, the wire could have shorted to ground, blowing the fuse. That is the more likely scenario.
Just replace the blown fuse and you'll be good to go. BTW, 20 volts is not out of the question if the alternator is not connected to the battery.