Duramax Alison "kick Down" Passing Gear?

Hooty · October 7, 2009

I recently purchased a used 2003 Sierra 2500HD Crew Cab 6.6 Duramax/Alison. It seems to work fine during normal driving and when towing my 6000 pound travel trailer. It has lots of power, incredible torque and it notwithstanding a few minor issues typical of a 6 year old vehicle, is a great truck.

The other day I was on the highway, unloaded, cruising at 110 kph (68 mph) and in the interest of seeing what the truck would do at those speeds, I "Mashed" the accelerator and expected a surge of power as the transmission shifted out of overdrive and into a "Passing gear" (as we used to refer to a downshift in the past).

The truck accelerated pretty well but appeared to stay in OD. There was no dramatic downshift and instantaneous feeling of being in Warp Drive. Just a smooth increase in speed.

Was this because there was no substantial load on the transmission?

Is this intended to prevent an overspeed condition for the diesel?

Did it have something to do with the engine RPMs at the time I "Mashed" it?

I'm just a bit curious.

pwall · October 7, 2009

Can't really answer your question but I'm curious.

Mine acts very similiar.

My opinion: with the high amount of torque available, it doesn't need to kick down a gear. Also, like you mention, perhaps to keep the engine from rev'ing too high.

Stevens03 · October 7, 2009

Not even having a diesel truck, I'd guess it has to do with the factory rev limiters. I'd bet if they were raised some, you'd get the result your looking for. The only way that would happen is with a programmer of some type.

born-to-fish- · October 7, 2009

I believe it has everything to do with available torque. At 68 mph you're probably close to 2000-2200 rpm. The peak torque for a duramax is in that range of rpm anyway, so a downshift would be redundant. A gas motor has a much higher torque range as far as rpm's go.

pwall · October 7, 2009

With the 6-speed, engine is turning over at approximately 1700rpm at 65mph.

Hooty · October 7, 2009

I believe it has everything to do with available torque. At 68 mph you're probably close to 2000-2200 rpm. The peak torque for a duramax is in that range of rpm anyway, so a downshift would be redundant. A gas motor has a much higher torque range as far as rpm's go.

In fact, I think the rpms at 110/68 are closer to 1600 and the peak torque is pretty close to that range. So most likely that is the reason. Still would like to hear an educated answer from someone who really knows. Not that all our guesses are wrong of course, being the smart people that we are.

Stevens03 · October 7, 2009

I went and test drove one today, and would have to bet that is your educated answer..

Hooty · October 8, 2009

I went and test drove one today, and would have to bet that is your educated answer..

It really does make sense. I retract my question,......forget I asked!

born-to-fish- · October 8, 2009

I believe it has everything to do with available torque. At 68 mph you're probably close to 2000-2200 rpm. The peak torque for a duramax is in that range of rpm anyway, so a downshift would be redundant. A gas motor has a much higher torque range as far as rpm's go.

In fact, I think the rpms at 110/68 are closer to 1600 and the peak torque is pretty close to that range. So most likely that is the reason. Still would like to hear an educated answer from someone who really knows. Not that all our guesses are wrong of course, being the smart people that we are.

Well, I was just giving my opinion based on my experience. My '05 3500 duramax has a peak torque of 610 ft/lbs. @ 2000 rpm. At 110 km/hr my tach is around 2000-2200 rpm. I have the 5 spd Allison. I don't know what the rear diff ratio is but the 16" rims seem kinda small. I am a professional "backyarder"

Stevens03 · October 8, 2009

I went and test drove one today, and would have to bet that is your educated answer..

It really does make sense. I retract my question,......forget I asked!

Not sure of what kind of answer you want, but quite frankly all the answers above are good ones. Just because a Dmax/Ally GM tech didn't answer your question, doesn't make the above answers wrong. I went to test drive one for the fun it it and tried several times to make it downshift and go... but it doesn't seem to do that due to the much lower rpms the diesel turns compared to a gas motor. It down shifted but there was no real surge of power.

Hooty · October 13, 2009

I went and test drove one today, and would have to bet that is your educated answer..

It really does make sense. I retract my question,......forget I asked!

Not sure of what kind of answer you want, but quite frankly all the answers above are good ones. Just because a Dmax/Ally GM tech didn't answer your question, doesn't make the above answers wrong. I went to test drive one for the fun it it and tried several times to make it downshift and go... but it doesn't seem to do that due to the much lower rpms the diesel turns compared to a gas motor. It down shifted but there was no real surge of power.

I completely agree with what you are saying. And upon further analysis I believe the reason is as everyone is saying.....more torque at lower RPM. I really should have done more research before I posted the question. Thanks to everyone for the responses though, it was great to get the brain working again.