how do i fix the altonator on my vauxhall vectra 1800 from 1997?

Not really a serviceable item, your cheapest option is to get one from a breaker or scrap-yard
Can you get hold of a multimeter to check the actual output of your alternator before condemning it?
There is no point in guessing at the fault, that will usually cost more money than accurate fault finding would.

fred
November 2009