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I can't detect any difference with the engine brake/tow-haul mode on or off. The downhill downshift (or whatever it's called) seems to do exactly the same thing. I don't detect any difference between the shift points (RPM) either. There's supposed to be an exhaust brake. While I don't expect it to sound like a Jake Brake, I hear nothing. I was towing 4,000 lbs BTW. What's your experience?
 

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the truck has a variable vain turbo (excuse my spelling if its wrong). down shifting manually and when the truck does it on its own are very different. if you downshift, it is strictly the engine slowing the truck, but if it is in tow haul, the turbo will create back pressure which will be like an engine brake on steroids. may be hard to notice at times but if you try both while going on a slight decline, you will notice a pretty big difference.
also, if you have a lead foot, you will likely never notice a difference in the shift points. take the truck out in tow/haul and get it up to 60-65mph. then turn the tow/haul off, you will notice that the truck wont go into 6th gear while tow/haul is on until you get to a pretty high speed (70+)
hope i helped
 

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The engine brake is computerized... downshifting doesn't seems to get it to kick in, it's the brakes that seem to trigger it. Just make sure you're in tow mode and hold the brakes a bit at highway speed. It should be very obvious that it's engaging. It can get embarrassingly loud when stopping a 6500 lb boat.
 

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From page 46 of the duramax supplemental manual. Referencing downshifts vs how long you hold brakes

"Downshifts may be automatically
selected to increase engine speed,
which increases the effectiveness of
the exhaust brake. The number of
downshifts selected is determined
by the length of time the brakes are
applied and the rate the vehicle is
slowing. The system delivers the
correct amount of braking to assist
in vehicle control. The heavier the
vehicle load, the more active the
engine exhaust brake will be. Use of
the exhaust brake will help maintain
vehicle speed when used with
cruise control. See Cruise Control
in the owner manual."
 

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I noticed that just letting off of the accelerator while on a rather curvy county highway at about 50-55 mph, not sure what gear it was or how high my rpm's were (I figure they were kind of higher as, like you said, the higher the rpms, the more you can tell it is working), but I could tell it was working to slow me and my 12x6 trailer with 2 atv's down.
 
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