Looking at a used 2009 Expedition El with 20k miles. Took it for a test ride and put it in 4x4 high and turning a corner it started to feel like the transmission going fall out from under the truck. Lots of thumping and clunking. The dealer told me yes it will do that on dry pavement in 4x4. Is this true?
Pretty much what you did to that truck should never be done to a vehicle. In 4x4, the wheels have to turn the same amount, so while turning, the inside wheel doesn't have to turn as much as the outside wheel, thus the axle will start to twist as the traveling distance grows further apart.
The reason you can turn on mud/snow/ice/gravel is that the tires are not glued to the surface and can turn more freely, so if the axle starts to twist, it can give off enough power to the wheels to make them break free of the surface and spin.
Edit-Now I feel like a jackass, because I did not realize until just now that the new expeditions don't even have solid axles, they have control arms. I do not know how turning in 4x4 on dry pavement works with control arms, but just don't do it haha
If I were the dealer and found out that you did that I would have made you buy it. Anyone who knows anything about 4x4 would never ever put a vehicle into 4 wheel drive on pavement, you could have and may have hurt that truck.
It's called axle windup. When it gets too tight the tires slip -- bang! -- hopefully just the tires. 4x4 is only good on loose surfaces or snow, never dry pavement. 4x4 Auto is a different animal and even that is rarely needed.
You just test that the 4x4 system was working. I did that to all of the Jeep wranglers I test drove before buying them. I just did it enough to feel the initial hop to make sure the wheels engaged. I did not want to tear up anyone's vehicles. Of course I did find a few that never engaged, so I was glad I did the test.
BVut I do agree, 4x4 on dry pavement is a bad thing.