MY FORD EXPEDITION 1999 IT TURN ON THE 4WD LIGHT TODAY, WHY IT HAPPEND?

Some more info would be nice. Like what were you doing when it came on? Has it since gone off? Is the vehicle actually in 4wd?

It is an automatic system and switches automatically when it darn well wants to: from a thread-Yes, these modern day "automatic" AWD systems will often ALWAYS make use, by default, of 4 wheel drive mode at times when Loss of traction, wheelspin/slip, is most likely to result should the roadbed traction coefficient be marginal

i was driving and the light turn on alone the 4wd was off

i tryed to power the switch and turned off but the light still blinking

Ah so it is blinking. Take it and get the dtcs of the 4x4 and abs module

What should i do? Clean it? Replace it?

And if u know where is it please, thank u

Well it depends of what codes it has alot of times its 4x4 motor on the transfer case. But i have put modules in before i cant tell you what to do until i know the dtcs