84 cutlass not charging

i have a 84 cutlass, 3.8, and the alt. is not charging. the charge light on the dash is on. the big red wire on the back of the alt. is hot. i can probe the brown wire and the alt. will charge, but when i cut the car off and start it again, the alt. is not charging. the plug on the alt. has a brown wire and a red wire. i had the alt. checked at a local parts store and everything checked ok. please help.