Yesterday night I was driving home and the battery light comes on !!!

As soon I parked i check the batterie cables and were tight. I turn on the engine and the battery light turned on again.
With the engine running I disconect the negative battery terminal and the engine turns off!!!

Can this be a bad alternator? Is there any fuse I can check? It is easy to replace the alternator? What can be the price of it?

1999 F-150 4x4
5.4L Flareside

 

On the newer vehicles unfortunatley they will die when you disconnect the battery. To test the alternator now we have to have a battery/alternator tester. Most auto parts stores will test both batt./alt. for free in your vehicle for you. It could be a floating short in your battery where plate has come loos and is only making contact part of the time. Take it to the parts store and have them put the tester on it first then go from there.

 

Check fuse #11 (20A) under the hood. This is the alternater field fuse.

 

Sounds like you need to replace the alternator. It takes a good 30 minutes to remove and replace the alternator. Easy job!