The diesels use a completely different gearbox. The GM 5AT used in the US petrol models can't handle the torque of the diesels.
Generally speaking, there is a gearbox program for hills. Upon recognition of a up or downhill, it is supposed to hold revs and block out upshifts, dependent on the gradient and vehicle load. For example, when heading up a 15% gradient (on some road in SW PA) at about 60mph, the gearbox detects that there isn't enough power in 5th, or 4th for that matter. So thus, it kicks down to 3rd gear @ 5250rpm and holds that gear for the 3 mile stretch. Yes, it might seem excessive, but I just let the thing rev its heart out. Plus, now there's sufficient output for passing.
On downgrades, generally the gearbox does not do much engine braking. It does NOT behave like Toyota or Honda automatics, which actually kick down to a predetermined gear (usually 3rd). Instead, it locks up the torque converter in that given gear, which can be any gear. For instance, when coasting down a mountain behind our campus, I do not need to brake. When I let go of the throttle in 3rd gear, the revs continue to rise as the engine does the necessary braking. If I accelerate, it will upshift to 4th, and upon letting go, it will hold 4th.
It would be cool if I went ahead and recorded all of this. Perhaps I will, just for the guidance of everyone here.
Last edited by AzNMpower32; 04-24-2008 at 01:04 PM.