My batt. light came on this morning on the way to school and has been on ever since. The truck starts right up with no issues, so I have a feeling it's the alternator. I'm going to stop at autozone after school tomorrow and get my battery tested just for the heck of it, even thought it's only 2 years old. I also noticed that my dash/radio lights were slightly dimmer than usual, but everything still works. Oh well we'll see how it goes.

 

You need to look at the alternator. Probably needs to be replaced.

 

A simple volt meter test will help you determine if the alt is working. Should get at least 13-14 volts. Try this with the headlights and blower on. Should see a slight drop in voltage at idle, with it picking back up with some RPM.

DO NOT unhook the battery with the truck running. It can cause a surge that can damage a bunch of stuff in you truck?

If the alt is totally dead or weak you are on borrowed time, esp when driving at night. The computer is going to experience low voltage as the battery degrades and you are going to be stranded.

 

'99 is the first year for the PATS and Digital Odometer ( did I get that correct, always have an issue with correct recall of the start MY ).

If it is a Digital Odometer, use the HEC diagnostic Mode to get the battery readings ( range I should say ).

Follow the directions to start your truck in Diag mode, and scroll through to bAtt selection.

How To :
http://www.mustangworld.com/ourpics/News/99hec.htm

Good luck, let us know what you come up with.