2001 mercury grand Marquis the alternator light comes on and off.just put in brand new one still doing the same thing

When installing a new alternator. You MUST first disconnect the battery or you can fry a brand new alternator in the first 30 seconds of the car running. If you didn't disconnect the battery first, you might need another alternator. Take it off and take it back to the parts store that you bought it from and ask them to check it for you to see if the diodes are fried. If it's OK then you have another problem. Check the tension on the belt and check if it's worn. That could be your problem, also check for loose or bad connections on the Alternator plug or wiring. Check your battery terminals also for loose or corroded connections. Good Luck, Kenny / MrBlueOval

Everything Mr Blue says is right on, Those belt tensioners go bad and should be replaced when the belt is replaced, but not always done. I'd replace them both, just for good measure.

One more thing I forgot to mention. I'll assume that your battery isn't completely dead but I'll address this issue incase others are reading this for help with their vehicle as well. A new alternator won't recharge a dead battery so if your (or anyone else's) alternator goes out completely and your battery goes dead, you need to recharge the battery first or you will also FRY your alternator in a few seconds (not in minutes) after starting the car with a jumpstart. Jumpstarting a car with a dead battery can ALSO FRY an alternator too. Jumpstarting should ONLY be done in an emergency when you are stranded in harms way. If you have access to a battery charger, do that first. An alternator (brand new or ten years old) will FRY it's diodes trying to recharge a DEAD battery so make sure your battery is FULLY CHARGED before trying to jumpstart a car. I just thought I would throw that in for others reading this post. Kenny / MrBlueOval

Obviously you won't need a jumpstart if you have a fully charged battery but I was trying to say to charge up the battery with a charger if you can first "instead" of jumpstarting. I'm Sorry if I didn't word that correctly the first time for those who didn't understand what I was trying to say. Kenny / MrBlueoval