Alternator Rev counter output

I have just replaced may alternator with a new one, the rev counter worked but bounced all over the place, then stoped and started again when it felt like it as they do when they stop charging,

I brought a new alternator now i got charge but the rev counter has stoped working totaly, I have checked and remade the terminal off and still no revcounter,

I am wonder could there be a fault on the alternator, can anyone tell me what signal/pluse/voltage comes from the termial to drive the rev counter, so i can test it i dont want to take it off and take it back if i dont have to.

Thank you in advance.

oh by the way a 97 300tdi

my rev counter didnt work when i bought mine,97 300tdi,i peeled back the insulation on the white wire on thr alt,to find it had broke about 6 inches along,so wired it back up,sorted,hope that helps mate...

DAZ