Q: four wheel drive transfer sensor on 2000 Ford Expedition

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i own a 2000 Ford Expedition Eddie Bauer edition...bought it brand problem is the truck will suddenly switch from 2 wheel drive to 4 wheel even on dry flat surface...this incident happened so many times in 10 years. I actually brought it back to my original dealer, they kept it for few days and didn't find anything wrong with. I brought it to another dealer near by and same thing. Now the truck is constantly making a rubbing noise (like when you first switch it to 4 wheel) someone told me it might be the 4 wheel drive transfer sensor.
(1) Answer
I had a very similiar noise, thought it was going to cost me big time, found out from a very trustworthy auto shop that it was low on a "friction modifier fluid" for the 4 wheel drive, these guys could have taken me to the cleaners, but $22 later, I was out the door. hope this helps.