Duramax/Allison Transmission Question

hriker · May 21, 2012

I am still new to the RVing life and have a question about towing our rig. We have a Chevy Silverado 3500 with Duramax Diesel and Allison transmission. My question is regarding the tow haul mode for the transmission. Should we leave the truck in tow haul mode the entire time we are towing? The dealer indicated I could take it out of this mode when we get to highway speed. I have had a question in my mind about what is the proper way to utilize this and what is best for my truck. Any comments and recommendations would be welcome.

Thanks,
Harry

DMitch · May 21, 2012

I have basically the same truck and I always tow with the tow haul on even at highway speeds. The Allison manual recomends it also so I figure there must be a reason they do.

PhotoPete · May 21, 2012

hriker said:
I am still new to the RVing life and have a question about towing our rig. We have a Chevy Silverado 3500 with Duramax Diesel and Allison transmission. My question is regarding the tow haul mode for the transmission. Should we leave the truck in tow haul mode the entire time we are towing? The dealer indicated I could take it out of this mode when we get to highway speed. I have had a question in my mind about what is the proper way to utilize this and what is best for my truck. Any comments and recommendations would be welcome.

Thanks,
Harry

I believe the primary intent of the mode is to change shift patterns to accommodate the extra weight you are pulling. This would be seen most graphically in the start / stop nature of city and rural driving. It would also be seen in that zone of highway driving where the transmission wants to jump back and forth from higher to lower gear.

I have found that leaving the mode engaged for the rural and city helps smooth the work for the engine and transmission. I find that, at highway speeds with my light load i don't get the transmission jump so i disengage the mode at that point. I have noticed that i get better gas milage at highway speeds disengaged. and thats understandable as the truck runs at a lower rpm while disengaged.

I would suspect at your trailer weight you might fine it beneficial to keep it engaged more often than I need to.

Pete

hriker · May 21, 2012

DMitch said:
I have basically the same truck and I always tow with the tow haul on even at highway speeds. The Allison manual recomends it also so I figure there must be a reason they do.

I don't know that I have an Allison transmission manual. Is it available online?

danemayer · May 21, 2012

Here's a couple of pages from the 2011 GMC manual. Several benefits of tow mode are listed.

I leave it in tow mode the entire time I'm towing. The only exception is if I'm at 55 mph and want to force a shift to overdrive, I'll push the button to release tow mode, and then press it again after the tranny shifts.

jayc · May 21, 2012

Here are some suggestions that I found on FAQ about the Allison transmission on The Diesel Stop

Q. Tow/Haul vs. Normal Mode - What's the difference between Tow Haul and Normal Mode?
A. Tow/Haul (T/H) shift mode significantly changes the transmission's shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads. For instance:

Upshift points are raised at light to mid throttle position to use more of the engine's available power for accelerating.Downshift points are raised to enhance engine braking to help slow the vehicle.

During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking.

During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th, and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.

Back To Top

Q. Tow/Haul vs. Normal Mode - When should I use Tow/Haul?
A. T/H mode is recommended whenever you are towing or the truck is heavily loaded, for all the reasons noted in the above question.

Bob&Patty · May 21, 2012

Yes, by all means leave the T/H engaged when towing. You can however disingage it when you reach your highway speeds. That way the trans will shift into TCC lockup. It will reduce your RPM's about 150. I do this when towing and after the RPM's drop I can then reapply the T/H button. That way I dont forget to reapply it the for the trans braking. Just the way I do it and has worked fine on the last 2 D/A trucks I have owned. Ther is also 1 other function you can use. Pull the shift lever into "M" and use the paddle shifter to lock the trans into a certain gear. All the information for the trans should in your owners manual.

hriker · May 21, 2012

Thanks everyone. I had a feeling I should be using T/H mode all the time. I haven't experienced any shifting issues but the improved cooling and braking are pretty important!! I'm glad I asked. As usual everyone is very helpful!!

Hastey · May 21, 2012

The only exception is if I'm at 55 mph and want to force a shift to overdrive, I'll push the button to release tow mode, and then press it again after the tranny shifts.[/QUOTE]

You know Dan, I think that is the only complaint I have about my Allison is having to do the exact thing you are talking about.

I have the edge monitor in my truck and it allows me to see what the transmission is doing. When in T/H mode it stretches out the shift points and also allows the torque converter to start locking in 2nd gear instead of 5th only. Keeps the converter from slipping while towing.

porthole · May 21, 2012

Don't have the Allison anymore, but when I did I did the same as others, kept it in TH until I wanted to force the 6th gear shift.

The converter locking up early is a huge advantage.

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Duramax1 · May 21, 2012

I agree that forcing the transmission to shift into 6th will reduce the RPM's by 150 or so.

But reducing the RPM's does not improve the fuel mileage performance in fact, in my experience, it reduces the mileage performance.

I have a ScanGauge which is manufactured by Linear Logic. This instrument, which plugs into the trucks computer, provides many different readings on the status of the engine.

One of the gauge choices is to enable a real time fuel economy readout which I have used to validate the deterioration in mileage when shifting to 6th gear.

My experience has been that while towing on a perfectly level road at 55 mph and using cruise control in tow haul mode I will note the real time fuel economy. I will then force an upshift to 6th by disengaging the tow haul mode while continuing to use cruise control.

Immediately, the RPM's will decrease, but the real time fuel economy will decrease from what was being achieved in 5th gear.

Thus the benefit of a decrease in RPM's in this instance is being more than offset by an increase in the amount of throttle that is required to maintain the constant speed of 55 MPH in a higher gear.

Using cruise control ensures that it is a computer and not a foot movement that is changing the amount of throttle.

So in my experience, RPM's do not tell the whole story.

scottyb · Aug 1, 2012

I can't find this using the "search", so I'll ppost it here. What is considered too high for transmission temperature, when towing with the D/A. My 2007 has a factory cooler with the towing package, but I assume it is smaller than it needs to be. I took delivery on my Cyclone yesterday and on the way home, it reached 204 at one point (ambient temp was 102). I have never really monitored it before, since I had never loaded it like this before.

lwmcguir · Aug 1, 2012

Duramax1 said:
I agree that forcing the transmission to shift into 6th will reduce the RPM's by 150 or so.

But reducing the RPM's does not improve the fuel mileage performance in fact, in my experience, it reduces the mileage performance.

I have a ScanGauge which is manufactured by Linear Logic. This instrument, which plugs into the trucks computer, provides many different readings on the status of the engine.

One of the gauge choices is to enable a real time fuel economy readout which I have used to validate the deterioration in mileage when shifting to 6th gear.

My experience has been that while towing on a perfectly level road at 55 mph and using cruise control in tow haul mode I will note the real time fuel economy. I will then force an upshift to 6th by disengaging the tow haul mode while continuing to use cruise control.

Immediately, the RPM's will decrease, but the real time fuel economy will decrease from what was being achieved in 5th gear.

Thus the benefit of a decrease in RPM's in this instance is being more than offset by an increase in the amount of throttle that is required to maintain the constant speed of 55 MPH in a higher gear.

Using cruise control ensures that it is a computer and not a foot movement that is changing the amount of throttle.

So in my experience, RPM's do not tell the whole story.

We have always found this to be true when operating a tractor at near an overload condition. Shifting down a gear often lowers the fuel burn per acre hour. Lugging and engine rarely pays off.

Sandpirate69 · Aug 1, 2012

Normal operating temps for an Allison is 180F-220F. Mine usually runs about 200F when towing my rig on cruise control @70-75 mph on T/H mode. I've heard to downshift into 5th gear to get the trans oil to move faster through cooler, which in turn dissipates more heat. Personally, there's a larger tranny cooler that actually hooks up to the factory hoses. I will look for it again & post here for the Allison boys.

Westwind · Aug 1, 2012

My 2008 D/A has the 6 speed and I will leave tow haul mod on until I reach highway speed and then I will disengage it so the Allison shifts into 6 speed overdrive and then I reingage the tow haul, I find I can squeeze better mileage out of the truck and the tow haul is engaged so if the grade gets too steep the Allison will down shift to handle it.
I've done this for 5 years and haven't have any problems - I feel this is an excellent engine transmission combination for what we do.

Birchwood · Aug 1, 2012

I always follow the owners manual and never listen to a dealer .Whenever I tow my Landmark with my Duramax
the tow haul mode is on and also if its off the grade braking is off and you will wear out your truck and trailer brakes.

scottyb · Aug 1, 2012

Sandpirate69 said:
Normal operating temps for an Allison is 180F-220F. Mine usually runs about 200F when towing my rig on cruise control @70-75 mph on T/H mode. I've heard to downshift into 5th gear to get the trans oil to move faster through cooler, which in turn dissipates more heat. Personally, there's a larger tranny cooler that actually hooks up to the factory hoses. I will look for it again & post here for the Allison boys.

I talked to a guy today that owns a transmission shop in Austin and has a reputation for being an Allison guru. He confirmed what you are saying about temperatures. He said you can lower the temp some by running a synthetic oil in it. I am going to take it to him next week, to drop the pan and change the filter inside and the screw-on, and then fill it with synthetic oil. I would be interested in the larger cooler if you happen upon the name.

He also said they harly ever fail, and that I will have a hard time breaking it.

porthole · Aug 1, 2012

Personally, I would skip the additional cooler. The D/A combo has a pretty good cooling system once warmed up.

Put the bigger pan on, that adds 4 quarts and a easy access drain plug.
Once you add a deeper pan and corresponding filter and a good synthetic you may not have to change your fluid for a long time.

Just change the spin on yearly or twice a year depending on the amount if your towing.
The Allison with synthetic can handle 240+

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porthole · Aug 1, 2012

And your not going to lower the temp with synthetic

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Terry H · Aug 1, 2012

All Allison transmissions in GM vehicles come from the factory with synthetic transmission fluid.

Q. Tow/Haul vs. Normal Mode - What's the difference between Tow Haul and Normal Mode?
	A. Tow/Haul (T/H) shift mode significantly changes the transmission's shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads. For instance: Upshift points are raised at light to mid throttle position to use more of the engine's available power for accelerating.Downshift points are raised to enhance engine braking to help slow the vehicle. During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking. During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th, and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.
Back To Top

Q. Tow/Haul vs. Normal Mode - When should I use Tow/Haul?
	A. T/H mode is recommended whenever you are towing or the truck is heavily loaded, for all the reasons noted in the above question.

Duramax/Allison Transmission Question

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