Let's say you have an 03 GMC Sierra 2500 HD.
What all happens when you push the tow/haul button?
I know the shift points are changed and some other tranny trickery goes on but does it mess with the timing and fuel also?
Let's say you have an 03 GMC Sierra 2500 HD.
What all happens when you push the tow/haul button?
I know the shift points are changed and some other tranny trickery goes on but does it mess with the timing and fuel also?
Magic
What I have seen on a last gen Dak, it runs at a higher RPM, I think only using the first of the 2 OD gears.
9 times out of 10 it just locks you out of overdrive. Sometimes shift points are delayed. Really only needed if your towing something sizeable, I still argue with my dad who always presses the tow button even when towing only 500lbs worth of duck boat
I pull a trailer loaded with tools and usually a couple thousand pounds of granite.
The reason I asked was because my boss asked if I had been using it. I said no, (the gauge cluster is out so I can't tell if it's on or not anyway) and he kind of jumped on my ass about it. He rambled on for a while but I heard him say something about how it adjusts the fueling. That didn't sound right to me but I'm not going to argue with him about it.
Honestly, aside from a little firmer shifts and a higher shift point I can't really tell a difference.
On my Ford, it locks out overdrive.
On the bosses GMC 3500 with Duramax and Allison trans, it does all kinds of great stuff. Firms up the shifts, holds a gear longer, senses braking and downshifts to provide engine braking.
TCM technology is getting way better.
Yeah, it does a bunch of transmission trickery on my F-150, including downshifting on downhill runs to provide some engine braking. One thing it doesn't do is lock out overdrive, actually. Depending on my speed and the grade, it will shift into 6th quite often.
In reply to Tom_Spangler:
Must have a new fangled ford. Dad's 2000 expedition wont shift into od ever with the button clicked...quite exciting running over 3k rpm in a 300,000 mile vehicle.
Usually it adjusts line pressures and shift points/load points.
Some of the older models may have locked out OD due to band-aiding weak OD clutch packs, but a modern day system will typically try to get into the highest TC locked gear possible to minimize heat.
The dmax really likes to stretch its legs to 3200rpm when in towhaul mode!
On an 03 it probably doesn't do much.
On newer cars it can turn on lots of other features. But none will reduce fuel usage.
on the 02 Silverado HD2500 that i had way back when, the only thing it did was raise the shift points. it still went into overdrive when pulling a loaded 20 foot car trailer, it just did it later..
honestly, it is a button that doesn't need to exist. the ecm can sense when the trans needs to shift differently and adjust accordingly. it can all be programmed in if they want: they only put it there so the driver can feel like they are contributing to the operation of the truck in some meaningful way.
Nick_Comstock wrote: Let's say you have an 03 GMC Sierra 2500 HD. What all happens when you push the tow/haul button? I know the shift points are changed and some other tranny trickery goes on but does it mess with the timing and fuel also?
Darned if I know. Friend has this on his Chevy, and I don't find it to do a darn thing really. Digging through the owners manual reveals nothing about what it's supposed to do. Same with digging through GMs web page, the forums and the repair manuals.
On some trucks, it can prevent the higher gear or prevent torque converter lockup. Can't say I'm enamored of that myself. Tends to generate more heat, not less. And you burn a whole lot more fuel. Digging through their resources usually shows vague claims of increased engine braking ability as the motivation.
If it's his truck and he wants you to use it. Fine, do so. If it's your truck, I don't see any real reason to bother with it.
Nick_Comstock wrote: I pull a trailer loaded with tools and usually a couple thousand pounds of granite. The reason I asked was because my boss asked if I had been using it. I said no, (the gauge cluster is out so I can't tell if it's on or not anyway) and he kind of jumped on my ass about it. He rambled on for a while but I heard him say something about how it adjusts the fueling. That didn't sound right to me but I'm not going to argue with him about it. Honestly, aside from a little firmer shifts and a higher shift point I can't really tell a difference.
So everyone has answered the transmission part, of what actually happens.
And here's why the fueling isn't changed- when certifying a car, one must demonstrate that alternate running options do NOT alter emissions. For all require tests. As a matter of fact, since the altered shift schedule can change gas emissions on some tests- it does have to be proven that the vehicle will still meet the emissions requirements.
Now I know your boss is more harping about fuel economy vs. emissions- but the point here is that the a/f used isn't changed, as that will change the gas emissions. So fuel economy should not change due to the engine running richer because of this alteration of how the car runs.
Locks out Overdrive (highest gear), holds lower gears longer, downshifts sooner for increased engine braking, and probably does a little bit with varying the torque converter lockup.
Not relevant to your truck, but an '08 F350 here. From what I can see, it holds a gear for longer accelerating (especially going into OD), and it keeps the torque converter locked on decel and downshifts for engine braking. It's useful, especially the decel features.
My truck is always in Tow/Haul mode. I love manual transmissions.. 
What motor/transmission is in the truck? If it's the Duramax or 8.1 with the Allison, then the tow/haul button's effects are huge, very apparent, and not subtle at all. It does all the things people have mentioned above, and generally makes towing a big heavy trailer a lot easier, especially up and down hills.
If it's the 6.0 with the 4L80, then I think it does a lot less.
In reply to alfadriver:
Thank you, that answered my main question. I didn't think it altered anything other than transmission functions but wasn't sure.
I went to him because I was concerned that over the last few weeks I have been getting a shudder through the drivetrain pulling away from a stop that has been progressively getting worse. His immediate response, without even thinking about it was that it's because I haven't been using the tow/haul mode. And that's when he started rambling on about timing a fueling.
It feels like possibly u-joints or something in the rear end to me. It's only noticable when pulling away from a stop through first gear. After the change into second it smooths out.
It's a 6.0 with the 4L80 with about 114K on it.
Sounds like it could be something transmission related. When's the last time the fluid was changed?
In reply to Tom Suddard:
That I don't know.
I do know that they are pretty good about staying on top of maintenance on their equipment, but I really couldn't say.
I checked the fluid last week and it was still redish and didn't smell burnt.
FWIW, when I bought my truck, it had a really weird shudder when pulling away from a stop and shifting 1-2. One shop looked at it and pronounced it a torque converter problem, wanted $800 to pull the tranny out for further diagnosis. I said no, took it to the Chevy dealer, they diagnosed it as a broken driveshaft center support bearing. Swapped that out ($400 or so, I think) and the problem was fixed.
codrus wrote: FWIW, when I bought my truck, it had a really weird shudder when pulling away from a stop and shifting 1-2. One shop looked at it and pronounced it a torque converter problem, wanted $800 to pull the tranny out for further diagnosis. I said no, took it to the Chevy dealer, they diagnosed it as a broken driveshaft center support bearing. Swapped that out ($400 or so, I think) and the problem was fixed.
Big +1. The rubber around them gets soft. I've replaced several of them over the years.
Toyman01 wrote:codrus wrote: FWIW, when I bought my truck, it had a really weird shudder when pulling away from a stop and shifting 1-2. One shop looked at it and pronounced it a torque converter problem, wanted $800 to pull the tranny out for further diagnosis. I said no, took it to the Chevy dealer, they diagnosed it as a broken driveshaft center support bearing. Swapped that out ($400 or so, I think) and the problem was fixed.Big +1. The rubber around them gets soft. I've replaced several of them over the years.
That fits with what I'm feeling. Whats the procedure for diagnosing this?
If I can get under there and wiggle the driveshaft showing them that it is an actual issue and not just me being a pain in the ass it would go a long way towards getting it fixed.
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