I have a Duramax/Allison and I would never go through the mountains again without a exhaust brake on my truck. I scared the begeebers out of myself a couple of years ago when I first took this truck into the mountains. In tow/haul mode your trany shifts down to keep your rpm's up for some stock engine braking. This is great until your rpm's start to climb because you are going down a really steep grade, suddenly you
have to hit the brakes because you are going too fast, but when you hit the brakes the truck shifts down and now your rpm's are in the red. I thought that I was going to blow the motor for 5 secs there, just long enough to shut off the tow/haul and get on the brakes hard to get myself under control. There wasn't much conversation with the wife for a while. I'm sure that more experienced drivers would have not made the same mistake but it sure scared me. I went home, spent the money and had a PacBrake put on. It has completely changed the way I feel about driving in the mountains. With my 11000 lb trailer I can coast down a 7% grade with the exhaust brake on and actually slow down as I go downhill, without touching the brake pedal. The only time I really have to pay attention is when I get into some on the 11% stuff, then you have to slow right down and take it easy. Everyone can decide if it is worth the money or not, it has sure made my holiday driving much more relaxing.