I had this Exact problem with my truck not to long ago. Alternator is done. Pull it out and pull the voltage regulator out and check the brushes to see if they make contact. I had this happen, replaced regulator and brushes, put it back in, worked fine and only cost about $50~$60. Only problem was about 2 months later, the problem came back. Checked the brushes again and they were ground down again not making contact at higher RPM's. I ended up replaceing the whole Alternator. Its really simple to swap out. I did it in advanceds parking lot.

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1997 Ranger XLT
4.0 4X4 Lotta Mods.

2001 Ford Crown Victoria Police Interceptor. 4.6 V8