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safarijim · 02-02-2007, 03:03 PM

I was just wondering if anyone has used the Tow/Haul Mode that came on my 2005 2500HD? If so, how did you employee this feature? I haven't towed with the GMC yet, so interested how it worked for others.

thanks

rickandsandi · 02-02-2007, 03:05 PM

Greeting Jim-

This was discussed once before, use the search feature, but since I'm here, whenever we are towing we always have the tow mode engaged and I keep the tranny out of Overdrive.

LI Pets · 02-02-2007, 03:43 PM

If your towing you hit the button, it changes the shift points, it does go in overdrive.

Moreover, it down shifts to help you stop

DEO · 02-02-2007, 04:00 PM

Quote:

Originally Posted by safarijim

I was just wondering if anyone has used the Tow/Haul Mode that came on my 2005 2500HD? If so, how did you employee this feature? I haven't towed with the GMC yet, so interested how it worked for others.

thanks

I find it to be excellent especially when exiting the highway or when you need to slow down...

davidz71 · 02-02-2007, 04:01 PM

Jim,
I'm guessing that you have the Allison 5 sp. auto trans with your Duramax engine. If so, you do not have to tow in 4th gear as suggested. This trans is plenty stout to tow in 5th just as I have done with my 8.1 gas engine with same trans and rear end as you.

What I have discovered is that when and if the transmission needs to downshift, in tow/haul mode, the engine rpm is not as high as it would be if this feature was not selected. I use it this way to keep engine rpm down slightly; however, I have found that the transmission does not always shift back into 5th gear as soon as I would like it to after cresting a hill. I just take it out of tow/haul mode, the engine shifts back into 5th, rpms drop and I then press the tow/haul button again. Sounds like a pain but it works for me.

The tow/haul also give you additional braking on down hill grades but I don't use the feature with my 8.1. I just work the brakes carefully making sure not to overheat them. I believe if I was doing much up and down hill driving with the diesel, I'd get an exhaust brake (jake brake) if the size/weight of my trailer indicated it would help.

**jcanavera** · 02-02-2007, 04:08 PM

The transmission that comes with the gas 6.0 liter engine also has enough muscle to tow in OD. The vehicle manual notes that it's ok. I use the tow/haul and it hangs on to the lower gears longer during acceleration, and kicks you down into a lower gear earlier if necessary. I don't see that it provides any benefit on downhill grades though. If I'm in od it stays there. To get additional braking I need to downshift to 3rd.

Bottom line read the manual and see what the manufacturer recommends. My current vehicle is the first one that ever permitted OD towing.

Jack

2airishuman · 02-02-2007, 04:11 PM

Quote:

Originally Posted by LI Pets

If your towing you hit the button, it changes the shift points, it does go in overdrive.

the best reason i've read to keep it out of overdrive!

remind me never to follow you in a caravan....

cheer
2air'

wkerfoot · 02-02-2007, 04:17 PM

Quote:

Originally Posted by 2airishuman

the best reason i've read to keep it out of overdrive!

remind me never to follow you in a caravan....

cheer
2air'

2air,

Give him a break, it is only a typo.

Bill

Cracker · 02-02-2007, 04:23 PM

I use the tow/haul mode consistently in the mountains. It does restrict the upshift to overdrive to about 58 mph. A nice feature.

Silvertwinkie · 02-02-2007, 05:03 PM

Tow haul is a great feature that GM and I believe Ford have offered in their light truck lines (vans too).

Tow haul basically uses the computer to manage shift points, locks the torque converter, etc. It is suggested it be used when towing and I've used it with great success. The GM version I don't believe actually turns off overdrive, but more control the transmissions ability to downshift as frequently/easily as it normally might without tow/haul on....but it will downshift if the need comes up. Transmission temps I've observed using tow/haul when towing are slightly cooler than without tow/haul on due to the constant downshifting that happens.

Bottom line, the suggestion to review the owner's manual is a great idea. Press the button (on GM trucks-- I assume Ford has them in the same place, but have no idea if Dodge has it or where it would be located) located on the the gear selector when ready and you too will be a happy camper.

davidz71 · 02-02-2007, 05:17 PM

Quote:

Originally Posted by Silvertwinkie

...Tow haul basically uses the computer to manage shift points, locks the torque converter, etc. ...

I get the impression that from a standing start, the shift points are higher and when the vehicle goes up a grade and the transmission shifts down, I can see the tach drop further as the torque converter locks up to limit slippage with the resulting heat associated with this slippage. Would you say that is what is actually happening? Again, this is what I am seeing with the 8.1 gas and it may be different with the diesel due to programming.

LI Pets · 02-02-2007, 06:21 PM

Quote:

Originally Posted by 2airishuman

the best reason i've read to keep it out of overdrive!

remind me never to follow you in a caravan....

cheer
2air'

What's that mean?

I have a 6 speed allison, it drops into 6 when at hwy speed??????

Silvertwinkie · 02-02-2007, 06:31 PM

Quote:

Originally Posted by davidz71

I get the impression that from a standing start, the shift points are higher and when the vehicle goes up a grade and the transmission shifts down, I can see the tach drop further as the torque converter locks up to limit slippage with the resulting heat associated with this slippage. Would you say that is what is actually happening? Again, this is what I am seeing with the 8.1 gas and it may be different with the diesel due to programming.

It's hard to say when the TC locks for sure in that situation. I know that on the flat road, if you tap the brakes, ever so slightly, you will see the TC unlock by the RPMs increasing. I would guess that if you see the RPMs drop, you may be seeing a TC lock situation. If you tap the brake pedal when that happens, every so slightly, basically making the brake lights come on and not acutally engage the brakes, and the RPMs go up that would pretty much say to me, yea, you just saw the TC lock and unlock.

An unlocked TC is one of the main sources of heat buildup, if not the biggest and tow/haul can help eliminate some (not lots, some) of that heat.

davidz71 · 02-02-2007, 07:18 PM

I love my Allison 5 speed but it must be even better as a 6 speed. LI Pets, have you driven the 5 speed and the 6 speed to get a comparison?

CanoeStream · 02-02-2007, 07:18 PM

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.

2airishuman · 02-02-2007, 07:26 PM

understanding tow/haul mode....

i'm no auto engineer but here goes....

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)

CanoeStream · 02-02-2007, 07:32 PM

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!

2airishuman · 02-02-2007, 07:46 PM

Quote:

Originally Posted by Canoe stream

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'

LI Pets · 02-02-2007, 08:10 PM

Quote:

Originally Posted by davidz71

LI Pets, have you driven the 5 speed and the 6 speed to get a comparison?

No I haven't.

I notice that if I do say 65 mph the engine i sin 5th at say 2200 rpm, if I go to 68mph, it shifts into 6th the rpms drop to about 1800 rpm.

So it is more efficient at 68........does that make sense?

If I slow down to 65 it stays in 6th at say 1700 rpm but doesn't downshift into 5th again.

safarijim · 02-02-2007, 08:30 PM

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!