30 degrees is a real steep incline and I would expect any vehicle to roll backward. I can't argue that your other vehicle doesn't do that, but I can only guess it has a higher idle speed or a different transmission design. Automatic transmissions have a torque converter which is essentially an engine-driven fan that spins fluid that forces another fan to spin. That second fan is what turns the transmission. There is no solid mechanical connection between those two fans, so the second one, and the transmission are free to turn backward when the weight of the vehicle is pulling harder than the force of the engine-driven fan. Preventing rolling backward is not a function of any automatic transmission. The only issue is at what point will it allow that to happen, and that will be different between various models.
You can't get any lower engine speed than one that is stopped, and you'll see on both of your vehicles if you turn the engine off while on an incline, they're both going to roll backward. If one doesn't, there is something seriously wrong.
Also consider that a lot of changes take place every year in an attempt to get better fuel mileage, lower emissions, or lower production cost. That, along with different options and new designs is going to make vehicles from two years a lot different. There are even mid-year changes that require different service manuals, so you know those changes are significant. In this case, your vehicles could have different transfer cases. One might have a higher rolling resistance, and it could take a steeper incline to make it roll backward.
As a point of interest, there IS a solid mechanical connection in the torque converter that engages above about 40 mph when the engine is warmed up. Chrysler developed that and was the first to use it in 1977. It lowers engine speed on the highway for better fuel mileage, but that feature also has nothing to do with rolling backward.
Saturday, February 14th, 2015 AT 12:04 PM