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Old 06-17-2013, 06:35 PM   #1
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Tow/haul mode on hills with a 6.0L gasser

I have done some reading on this forum about proper technique on pulling a trailer on steep grades. Most of the advice I have read here indicates to take it slow and steady and to use the same gear on the way down as on the way up.

I have an '07 Yukon Denali with a 6.0L engine and a 6-speed transmission in it. The tow/haul mode on it seems to work very nicely on the hills (some would call them merely speed-bumps) of Indiana. It seems to understand when I am going either up or down a hill and maintains a gear as appropriate. Tapping on the brake causes it to immediately downshift a gear to help with deceleration.

My question is, on this type of vehicle how good is the tow/haul mode when going through mountains with long/steep grades? Should I just use tow/haul mode on these hills (making sure to start at the top of the hill at a moderate/slow speed), or is it better to put it in manual and shift up/down as appropriate on my own?

We are soon taking a trip that will involve a lot of mountains, and I want to be as prepared as possible. Thanks in advance for any advice on this!

Scott
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Old 06-17-2013, 07:21 PM   #2
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itscottw,
I have the 5sp allison, and always use tow/haul. The omputers in these things are a whole lot smarter than the driver. Using the tow/haul also helps keep the trans oil temperature somewhat lower than without using it. I haven't been out west with mine as of yet, but have been through virginia and west virginia hills with no problems,and I gross 23.500#.

Frank
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Old 06-17-2013, 07:49 PM   #3
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We tow a smaller 25' tt with a 5 speed tranny, 5th being over-drive. On mostly level roads I can get away with it in over-drive and stay at about 1800 rpm's at 58 to 60 mph. I'm in no hurry to get to our site because I reserve everything ahead of time. I have been averaging 12 to 13 mpg. With my new tt I tweaked the tire pressure to improve my mpg.

I don't have a Tow / haul button, but I feel the engine is not stressed or running hard. I tow about 5000 wet with a towing capacity of 7200. Coming to a uphill grade I'll manually shift down to 4th which will drive my rpm's to 2300 then if more juice is needed hit the peddle a little more to bounce the rpm's to about 3200. On down hills the Toyota has downhill brake assist, but on longer decents I'll pop it down to 4th or 3rd to not heat up the brakes

On the freeways of Western PA., WV.,OH. I'll try to not go below 54mph on the grades unless I get stuck behind a semi in the slow lane. I would assume I could drive in the 65 to 70 mph range, but could only assume my mpg would be under 10.
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Old 06-17-2013, 09:34 PM   #4
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When we drove to Alaska I learned tow/haul is not perfect. My truck is an 02 Chevy dually Duramax/Allison. Descending into Skagway I burned out the rear truck brakes and the trailer brakes were very hot too. While I was parked in a scenic outlook pull-out waiting for brakes to cool, a guy with a truck camper told be to manually downshift to 2nd gear for steep downgrades. It really does work! Next downgrade I tried it, only short taps on the brake pedal kept my speed about 35 MPH.
Try it next time you begin a steep downgrade. The Allison transmission will not allow the engine to over-rev by up-shifting itself, even if you manually downshift.
Scott, if you want a steep grade to try things out; Just West of the Brown County State Park truck/RV entrance is a 10% grade about 1/2 mile long with a curve at the bottom. Going East is uphill.
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Old 06-18-2013, 07:45 AM   #5
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We used to have a similar Yukon as you and it was a great tow vehicle. We used the tow/haul mode anytime we towed anything that weighed more than a couple thousand pounds. The 6 liter is a great tow engine. You did not mention tow weight, but at some point the aux braking system becomes real important. I have towed at least 7,500 pounds, and with that weight trailer brakes are necessary in the mountains. That 6 liter was the best GMC engine we ever had. Keep it in the tow/haul mode while towing.
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Old 06-18-2013, 07:35 PM   #6
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Thanks for the advice everyone! It still seems like people are fairly split on tow/haul vs manual. I guess I will just have to figure it out as I go. We are planning to hit Badlands NP, Yellowstone NP, then Rocky Mountains NP before returning back to Indiana, so there should be plenty of opportunities to see what works best for me!

Ray - to answer your question I am towing about ~7k lbs. I have not had this trailer to Brown County SP yet, but never had any problems there with my previous ~4.5k lbs trailer.

Vraines - I agree, the 6.0 liter engine in the Yukon coupled with a 6-speed transmission seems to be a very capable TV. There is plenty of horsepower and torque. My only beef with it (other than at the gas pump!) is with the transmission cooler. Even when I was pulling my 4500 lbs trailer it would routinely go over 200 degrees according to the dash display. On one particularly hot day (just over 100 degrees) it hit 232 as I pulled into my neighborhood.

Scott
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Old 06-24-2013, 03:49 PM   #7
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Different trucks are different, oddly enough. My Chevy diesel and Allison transmission has both a Tow Haul mode and something GM calls "grade control". Tow Haul mode helps the transmission operate in a manner that will keep the fluid cooler and keep it operating longer. The Tow Haul mode will keep the torque converter locked longer and will keep the transmission from upshifting on a downgrade. This is where it gets fuzzy.

The grade control, which only operates in Auto mode and not if the transmission is in Manual mode with the selector, will respond to my touching the brakes on a downgrade and go into a maxium engine braking mode. RPM's will go up but the engine is not getting any fuel to the cylinders so as to provide maxium braking. The diesel has an exhaust brake with accentuates the engine braking.

Check your truck's manual and see if grade control is mentioned. If so you will want to stay in the Auto mode and make use of this feature. It works fantastic on my 2011 truck and I can go down many miles of 7% downgrades without touching the brakes the entire way.
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Old 07-01-2013, 02:25 AM   #8
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Quote:
Originally Posted by itscottw View Post
Thanks for the advice everyone! It still seems like people are fairly split on tow/haul vs manual. I guess I will just have to figure it out as I go. We are planning to hit Badlands NP, Yellowstone NP, then Rocky Mountains NP before returning back to Indiana, so there should be plenty of opportunities to see what works best for me!

Ray - to answer your question I am towing about ~7k lbs. I have not had this trailer to Brown County SP yet, but never had any problems there with my previous ~4.5k lbs trailer.

Vraines - I agree, the 6.0 liter engine in the Yukon coupled with a 6-speed transmission seems to be a very capable TV. There is plenty of horsepower and torque. My only beef with it (other than at the gas pump!) is with the transmission cooler. Even when I was pulling my 4500 lbs trailer it would routinely go over 200 degrees according to the dash display. On one particularly hot day (just over 100 degrees) it hit 232 as I pulled into my neighborhood.

Scott
I'm pretty sure this is normal for the Yukon to hit those transmission temperatures. This is because much of the transmission cooling is done by the oil-to-water cooler that's in the radiator, after the out-in-the air transmission cooler does its thing. This is done to allow the radiator water to keep the transmission from running too cool in the winter. The trucks with the HD Allison transmission, I believe, use a different cooling system. 232 degrees is not particularly harmful to Dexron 6.
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Old 07-02-2013, 10:55 PM   #9
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Scott, if you visit the Allison transmission forum you will find a retired Allison Fluids Engineer posting there and volunteering to help with any related questions.
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Old 07-22-2013, 07:00 PM   #10
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Thanks to everyone that replied! I am happy to report that we just finished a 3700 mile trip - and the Yukon did an excellent job pulling our trailer. This included going over the Big Horn Mountains, going into the Rocky Mountains at Yellowstone, and into the Rocky Mountains again in Colorado.

The Tow/Haul mode (not to be confused with the OD button on some other vehicles!) really did a nice job with gear selection both going up and down the mountains. I did not have any white-knuckle moments or any areas of concern.
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