For my '06 D/A, the manual says: "Tow/Haul mode adjusts the transmission shift pattern to reduce shift cycling, providing increased performance, vehicle control, and transmission cooling when towing or hauling heavy loads." Having had two vehicles now with this function I would agree with Craig that it raises the shift points so that you stay in the lower gear a bit longer in a given RPM range for increased torque application and less shifting when the loads are high.
More importantly -- a direct, continuous quote from the manual without editing:
"Tow/Haul is designed to be most effective when the vehicle and trailer combined weight is at least 75 percent of the vehicle's Gross Combined Weight Rating (GCWR). See "Weight of the Trailer" later in the section. Tow/Haul is most useful under the following driving conditions:
When pulling a heavy trailer or larger or heavy load through rolling terrain.
When pulling a heavy trailer or a large or heavy load in stop and go traffic.
When pulling a heavy trailer or larger or heavy load in busy parking lots where improved low speed control of the vehicle is desired.
"Operating the vehicle in Tow/Haul when lightly loaded or with no trailer at all will not cause damage. However there is no benefit to the selection of Tow/Haul when the vehicle is unloaded. Such a selection when unloaded may result in unpleasant engine and transmission driving characteristics and reduced fuel economy. Tow/Haul is recommended only when pulling a heavy trailer or a large heavy load."
The manual's GCWR for any extended/crew cab C-2500 with 6.6L V8 Diesel short or long box (all are 3.73 rear end) is 22,000 pounds and the max trailer weight is 12,000 pounds (yes, all are identical). The underlined variables will change the equation of TV curb weight but one could roughly estimate that a trailer has to be around 8,000 pounds before Tow/Haul would be required ... except in the bulleted conditions in blue above. Exactly like Cracker said 6 posts ago about towing in the mountains.
__________________
Bob
Last edited by CanoeStream; 02-02-2007 at 09:04 PM.
folks think tow/haul modes do more than they actually do...
basically, tow/haul is a 2nd tranny program
1. shift points are raised so a given gear is held longer, improving acceration under load.
2. brake application (at a measured force) also downshifts the transmission...
3. that lowered gear is held indefinitely until the go pedal is touched....
4. downshifts happen automatically on steeper down grades for example at 6 or 7% unless the go pedal is touched.
that really is all the tow/haul does...
older versions may have kept the tranny out of overdrive but that isn't a feature of newer programming or with the more powerfull drivetrains...
similiar tranny shift programs are available on many cars for 'sport mode' or 'boulevard mode' or 'economy mode' but without the 'inclinometer' feature.
most/all modern automatic transmissions only allow torque converter slippage in 1st gear, for smooth take off...
then a locking clutch reduces or eliminates virtually all slippage except for a spilt second at shift points...
now that lock up clutches are controlled electronically, torque converter slippage is reduced even further....in 2nd gear and beyond.
so heat from 'slippage' should not be an issue on any of these newer transmissions...
whereas heat from the tranny running at higher rpms in the lower gears, does happen, and with or without tow/haul engaged.
so tranny temps may increase going downhill with tow/haul or uphill without for very very long stretches...
as i understand it.
cheers
2air'
i think the original question is related to a 2005 truck, not a 71 beetle or 74 honda or 85 buick, or any vintage domestic car used for towing and mountain drives...
so the answer to that question is tow/haul works very nicely to improve acceleration under load and for slowing a load, but doesn't engage the trailer brakes. some folks like to use the trailer brakes too on long down hill stretches or when the engine rpms get too high (especially on the diesels)
__________________ all of the true things that i am about to tell you are shameless lies. l.b.johnson
we are here on earth to fart around. don't let anybody tell you any different. k.v.
Mechanics on dieselstop.com & dieselplace.com speak about a lot of activity in the Allison tranny -- I'm not sure how much is mechanically vs. chip controlled; suspect it's a combo of both. An interesting observation of mine while towing: If I crest a hill with the throttle on, the overdrive runs free and I will pick up speed. If I crest a hill with the throttle off (or activate the brakes while descending ... I think) -- anyway, the tranny will downshift so that the TV-trailer combo will not pick up speed the entire way down the hill! I haven't proved this to my full satisfaction on grades as high as 7 degrees but it is an intriguing feature!
anyway, the tranny will downshift so that the TV-trailer combo will not pick up speed the entire way down the hill! I haven't proved this to my full satisfaction on grades as high as 7 degrees but it is an intriguing feature!
hi canoe stream...
yes that is what i experience too...
however the tachometer will show the engine rpms continue to limb....
so at about 3500-4000 rpms, and if the downgrade continued for miles...
i usually apply ALL of the brakes for a few seconds until the tach drops then let it run up again....
10 disc brakes engaged slow the whole mass nicely
and i'd rather wear brake pads some that run the diesel at red line...
on the fords you get 2 downshifts...one from the hill and one from the brakes...
so that allows 2 gears to run it up again.....
tow/haul is a very nice feature and especially going downhill...
cheer
2air'
__________________ all of the true things that i am about to tell you are shameless lies. l.b.johnson
we are here on earth to fart around. don't let anybody tell you any different. k.v.
I appreciate all the feed back on the question of the Tow/Haul Mode. It has made it a lot more clear than the manual and I will read it again before I start the trip. I weighed the truck with everything I could think of, full load of fuel, and two people and came out at 7500 lbs. the AS is rated at 7300 lbs.
Thanks again!
__________________
Safari Jim
N6QYV
GMC 2500HD Diesel
"The Older I Get - The Better I Was..."
I don't see that it provides any benefit on downhill grades though. If I'm in od it stays there.
Jack
Only the Allisonon has the downshift feature.
In response to Silvertwinkie, with the Allison, I see absolutely no need for an exhaust brake. I have descended steep switchback mountains and needed practically no brake.
In little ups and downs, as posted earlier, it sometimes stays in the lower gear too long. As mentioned, I simply hit the button to cancel tow/haul and, a few seconds later after it is in 5th again, hit the button to go back into tow/haul mode. This hasn't been a hassle.
__________________
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2004 Silverado 2500HD Duramax/Allison
WBCCI Region 9 Webmaster, #9632
Hey guys, my bad, I was talking about my exp with a 4L80e. I drove a diesel based Allison once and it was a night and day difference in terms of transmission functions.
__________________
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 1991
We've got another thread going right now on cold weather diesel operation. BillTex posted a link about the engine coolant heater. I went there, poked around a bit and found this! Karma BillTex!! The engine mentioned isn't the LBZ or LBY of 2006s but I'll bet the system is quite similar.
In response to Silvertwinkie, with the Allison, I see absolutely no need for an exhaust brake. I have descended steep switchback mountains and needed practically no brake.
In little ups and downs, as posted earlier, it sometimes stays in the lower gear too long. As mentioned, I simply hit the button to cancel tow/haul and, a few seconds later after it is in 5th again, hit the button to go back into tow/haul mode. This hasn't been a hassle.
Ditto
Mostly I just drive without it engaged, it does save on brakes going down hills, at exits, and coming up to stops. Otherwise I just keep it in drive.
__________________ Let those who can play, let those who cann't rule.
With respect to "red-lining" the Duramax on long downhill grades, this question came up in the diesel forum and, as I recall, a direct quote from GM implied that no harm would be done. Apparently "red-lining" when not under load is considerably different than "red-lining" under load. I know that it happens all the time for me when I use the the Allison to hold my speed on long downhill grades. With the 2003 GMC, tapping the brake causes the downshift. I understand that this procedure has been modified on the later model GMC. How does that work? Does the tranny respond to the cruise control?
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