mad mad

By mistake my friend put the positive on the negitive terminal while trying to jump start cos my battery was dead.
A few sparks & sizzling sounds occured.
Now we put a new battery, replaced the alt fuse.
Turn the key, no dash lights or anything electrical works.
Car starts fine, still no lights or anything else.
NOW while idling, give it a small rev, & everything comes alive.
Switch off, start, same thing, rev & all starts working?
Checked all fuse, all fine, Gona take out alt & check diodes not blown.

What could be wrong?

this is just a quick guess, but it kinda looks like votage regulator got fried..

use a voltmeter and check the voltage over the battery while the engine is running, if its not a steady 13.5-14.5 v then its possibly that..

Thanks man, going to pull thr alternator out toms.

But y only when I give it a little rev, it comes alive? then everything is 100s

its hovering around 11.5v & 12.6v
Problen still not solved.
Test every fuse

measure between positive out (on alternator) and any ground/earthsrap, but disconnect the possitive out wire on alternator 1st, you should be getting a steady 13.5 voltage at a high idle, Im no expert or anything but had the same troubles when I converted my beetle to a nissan powered motor.