Alternator terminals

[clyst]

Having bought a new alternator for my Perkins 4108 I notice the terminals are marked pos ,neg as would be expected but the two smaller ones both marked AC (unlike the knackered one). Speaking to the supplier he says it stands for AC current one being for the tacho and the other a spare . Doesnt sound right to me .......... However the unit works fine as the tacho is taken from the engine so both smaller terminals are unused .
can anyone exlain why 2 AC conns are required??

 

[Maine Sail]

Two AC taps are not "required" but on many Motorlola case alts they do have two stator/AC taps. Use either or if you have an AC pulse driven tach. If you don't have an alt sensed tach just ignore them.