Originally Posted by Gary RVRoamer
I have to disagree. My Allison 3000, in conjunction with the engine computer, keeps the RPMs right at the peak horsepower point when I leave it in Drive and push down the go-pedal to climb a grade. No need to downshift manually...
Gary, I have the exact same drive train that you do in our 2006 Mandalay. Cummins ISL 400, Allison 3000 and a 2-stage Jake Brake.
By manually shifting down to 5th at the bottom of a grade, you are ANTICIPATING
something that is about to happen and setting your drive train up for the climb. By leaving it in D and letting the cruise control handle it, or just mashing the pedal down in D as you get into the grade, you are REACTING
to something that has ALREADY
happened. There is a huge difference.
By shifting down to 5th at the bottom of the hill while still at 65 mph, you don't lose any speed before the downshift. 65 mph is 5th gear yields 2,000 RPM, right smack dab on the HP peak for the ISL. This means that as you start to climb the grade, you are ahead of the game by having your RPM up at the HP peak. This is what Freightliner recommends, by the way.
If you leave it in Drive, speed will drop a few MPH before the transmission downshifts. When it finally does downshift, you have slowed enough that even in 5th gear, you are below the horsepower peak. Because of that slight delay, your are now behind the 8-ball for the climb, lugging the engine at an RPM below its HP peak. This can lead to overheating, or perhaps another downshift to 4th as your speed drops even more because of the late initial shift to 5th.
As I said above, these coaches are NOT cars. You cannot just leave the transmission in D and expect the Allison to make all your driving decisions for you. There are many times when your
decision-making can be better than the Allison programming.