Replace Alternator on a 87 Testarossa

Hi,

I had a problem with the Alternator last Thrusday, making weird sounds and the battery lights comes on when the ignition is turn off. I decided to remove the alternator with the help of some of the previous thread here. I got help from my
13 year old son removing the bolts, teaching him to be careful Not to drop any parts
or tools around the area. There is one main bolt that give us difficulty, but manage to
remove it in an hour and put in back less than an hour after getting the new/rebuilt
part from the autoelectric shop. Paid $132.00 for a 120A A/C delco as it is the same
as the one I pulled from the car. After the installation, The car runs good without any new problem. . And as a reward for my son, I let him drive the car out of the garage into the driveway. By the way, Im still having my tachometer not giving me the right
RPM, it only goes up to 2500rpm max. (not over reving it.) Thanks in advance if somebody can direct me where to look for fault.

Jr