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Old 08-13-2015, 09:22 PM   #11 (permalink)
skmct
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I have a 35 ft fifth wheel dry weight just over 10k, gvw 13k I have not weighed it yet but with all the crap we have in it (we spend 5 to 6 months at a time in it) so I am pretty sure we are pushing the gvw. This trip we left south fl and traveled to upper state NY pulling some pretty good grades and had no problems with any of the grades. We averaged about 12 mpg for the 1700 plus miles to upper state NY. My truck is an 07 classic so pretty much the same as IGO's . I think you have to find that sweet spot for your truck. I find the same thing as IGO's about 1800 RPM but I use tow haul and cruise to keep it in that RPM range My truck wants to shift into sixth at about 65 so I set the speed to 63/64 MPH to keep it in fifth. If it shifts into sixth it seams to lug down and I see the exhaust temp rise. I think you need to find what works best for you and your truck.

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Old 08-16-2015, 06:53 PM   #12 (permalink)
dayle1
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My fiver is very similar, 34 ft, 9.2k lbs dry and 12k lbs loaded. Just finished a 2-1/2 month trip, so we were really, really loaded. I tow in M, w/o using T/H but EB on and had no problems on any grade. I use 6th gear as much as possible and will set my cruise accordingly. But, I don't usually try to maintain speed on steep grades, instead, downshifting as needed while keeping RPMs around 2k. But always had the power to go faster if I wanted to. Going over Monarch Pass I had to make three dead stops for road construction and effortlessly got back up to speed. Hand calculated mileage was 12.5 mpg for the first 4500 miles (which included all the steep grades in CO, UT, OR and CA), but dropped to 12.1 mpg for the 1500 mile run in 100 degree heat from LA to Austin.

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Last edited by dayle1; 08-16-2015 at 07:01 PM.
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Old 08-20-2015, 08:29 PM   #13 (permalink)
6686L
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I am new to diesel trucks - and to GMC/Chev. This 'tow-haul" position. I understand it locks the torque convertor (presumably to reduce heat generated by the torque convertor's slippage".). I note on the same "stalk" it also has a button to "lock out" the overdrive.

Do I understand correctly, that I can leave it in the tow-hall position ALL the time I am towing, and that it will ONLY lock up the torque convertor - it will NOT lock the transmission out of overdrive ?
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Old 08-20-2015, 11:44 PM   #14 (permalink)
stanleyrock
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Yes, just push the button quickly for tow/haul or hold it in for a couple of seconds to lock out OD.
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Old 08-21-2015, 01:35 AM   #15 (permalink)
jake111
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Tow/haul will:
1) raise the shift points when accelerating.
2) lock the torque converter around 25-30 mph instead of almost 50 mph.
3) more aggressively downshift when braking.
4) be more reluctant to downshift under load (but still will with just a little more throttle).

T/H doesn't lock out 5th (overdrive).

Edit: added #4 observation.
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Last edited by jake111; 08-21-2015 at 01:37 AM.
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Old 08-21-2015, 08:38 AM   #16 (permalink)
6686L
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thanks - guys - nice to be in a technical forum where you guys actually know what you are talking about !
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Old 08-21-2015, 08:30 PM   #17 (permalink)
dayle1
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Quote:
Originally Posted by jake111 View Post
Tow/haul will:
1) raise the shift points when accelerating.
2) lock the torque converter around 25-30 mph instead of almost 50 mph.
3) more aggressively downshift when braking.
4) be more reluctant to downshift under load (but still will with just a little more throttle).

T/H doesn't lock out 5th (overdrive).

Edit: added #4 observation.
#4 should be more reluctant to upshift under load
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Old 08-22-2015, 03:00 AM   #18 (permalink)
jake111
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No, I meant the upshifts are delayed (reluctant) in #1. But in addition, it doesn't downshift as early in T/H as it does normally (#4).

For example, say at 55, it takes more throttle in T/H to get a downshift than not-in-T/H. I will sometimes take it out of T/H for a short time to get a more gentle downshift with less throttle when I want to speed up slightly.

I'll also do that for an earlier upshift above 50 into 5th (we're just 5 speed in the LLYs such as 6686L who asked about it)...above 50 because I don't the torque converter to unlock.

If you can go smoothly in/out of T/H without causing any jerking, then it does no harm. (I guess that's my opinion.) I'll probably wear out the switch, though.
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Old 08-26-2015, 01:09 AM   #19 (permalink)
terryk
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The TH programming is design to not make the shifting overly busy and reduces the heating of the trans fluid. You will also notice the TC will lock up and stay locked up earlier. This too reduces the heating of the fluid.
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Old 08-26-2015, 06:45 AM   #20 (permalink)
IGO1320
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T/H will lock the TC from second up. When T/H is not engaged TC will not lock up till around 50mph.

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