On other engines the tach (grey wire) comes off the voltage regulator/rectifier module.....(uses the alternating sinewaves to count rpms). It goes directly to the tach rear terminal labeled grey, or signal, or S or something other than 12v, Ground, ground symbol or lamp or something like that.
I don't know why Merc would design a different source for that engine per se. Difference in the modules would be due to increased amperage capacity for larger engines, but no reason to source the tach signal from a different point since this one works just fine.
