Then there's a loose connection somewhere then. Your fusible links might have a loose screw.
No, the light doesn't come on immediately after nor does it come off immediately after. This isn't going to be an easy find if it's a connection issue btw.

 

well i just had it check out and my mechanic put the battery on the computer and he said it was definantly the alternator.after i change the alternator everything should be just fine right?

 

well my mechanc was wrong,he daid it was the alternator so i did the test..i had the car on and unplugged the positive from the battery and the car stayed on,now if the problem is not my alternator what could it be?i know someone earlier said it might be a loose screw or something,but where should i look?