The tow haul will downshift automatically to a safe lower gear if you hit the brakes. Using the brakes tells it you want additional slowing. Otherwise it will downshift on it's own when the rpm' get too high but by then additional braking may be needed anyway. When we see a steep downgrade coming we slow down with the brakes and engage tow haul just before braking and then it will usually take care of things by itself. On very steep downgrades or if you did not slow down enough at the top of the downgrade additional braking may be needed.
One additional caution about mountain driving is when you have just climbed a pass and expect a long downgrade and will not be using the accelerator pedal, pull over at the top of the pass and put it into park and let the engine idle for a few minutes. Climbing long passes heats the exhaust manifolds up to sometimes cherry red. Going down a long grade at high rpm pumps massive amounts of cool air and little burned fuel to raise the exhaust temp. The result can be a broken cast iron manifold from rapid cooling.
You will often see truck pull over places at the top of passes just to prevent this problem. One other solution is to install stainless steel headers, but that costs $$$.
George 20 yr USAF & Lesley
