Ok folks here wots happening it driving me insane !

Mk2 orion, converted to turbo efi !

Car starts no problem at all, but wont charge until i rev it above 2500 revs, then when it charges it usually stays charging, occasionally at light etc it jumps back donw to 11.4 ish volts (approx) and when its reved again it charges fine.

I replaced the alternator, battery with brand new items, have tested earth and positive cables for resistance and all is fine, however the small blue wire that comes from alternator does not seem to go anywhere. I have done a continuity test on the blue wire to the earth terminal on the battery, and the same with the positive and nothing. (open circuit)
Also no battery light on dash. My question to you all is does this small terminal on the alternator with the blue wire control charging at idle revs? what should i be testing for next?

thanks Paul