I own a 2013 Ford Fusion with automatic transmission, All wheel drive, traction control, and of course ABS. I live on top of a hill in Fairbanks, Alaska so have to deal with a steep icy road 6 months of the year, such that I either have to downshift or brake to keep from going too fast. Years ago, I learned on a 2WD manual car, and so learned of course the importance of keeping the front wheels turning at all costs by limiting braking and downshifting before beginning the descent in that type of vehicle. I know NOT to downshift in a front wheel drive only car for similar reasons. However, with all the computerized functions of a modern car and all wheel drive, I just can't wrap my head around whether downshifting prior to starting down helps or harms my control over leaving it in drive and using brakes with ABS for backup, with the goal being to have as much drag as possible on the rear wheels to minimize the chance of front wheel lockup. Does traction control do anything if the engine is exerting drag and not propulsion to the wheels? Is drag evenly distributed to all 4 wheels in all wheel drive, or in the 75%/25% rear to front ratio that propulsion is distributed? I should mention that I have differing opinions from Ford's help line, the automotive instructors at the local technical college, and a large driving school. I'm hoping someplace out there is a real engineer who can offer facts based on actual test data.
And yes, I know to go slowly, have good winter tires, etc. etc.