Holding in lower gear vs letting tow-haul mode shift

wayfun wrote:
Thanks for all of the replies. I'll probably just hold it in 4th until I get to a long level stretch and then I might let it shift to 5th.

Unless the transmission is "hunting," as in shifting frequently, this is wholly unnecessary.

One shift from 5th to 4th part way up the hill, then a shift back into 5th at the top of the hill is *NOT* hunting.

That is what an automatic transmission is meant to do. It happens when you're running unloaded on the same hill, shifting from 6th to 5th part way up, then back into 6th at the top.

You don't lock out 6th when running empty because it sometimes shifts down to 5th, do you?

The Texan wrote:
I have the same engine/transmission and my trailer weights can often be in the 8K-10K range. I use T/H mode exclusively when towing and have had no problems with heat here in the mountainous west. Also, my truck will shift to 6th while in T/H mode, if on a downhill grade. Also, I have never noticed the transmission "hunting" when in the T/H mode, downshifting to keep the RPMs up, yes, but not hunting for a gear. I am in the boat with those that say GM designed the transmission to do the work, so let it do the work for you.

I could not agree with this post more.

I have the rig in my signature with a 6-speed tranny and 3.42 rear end and loaded my trailer is at 7500# - not too far off from the OP's trailer, and I have a smaller engine than he does. It is not at the tow limit but is close to the payload limit of the truck, as is expected. What a lot of people over and over seem to forget is the efficiency of today's powertrain cannot be compared with the inefficient and less capable powertrains of the past. Simply making a judgement on towing capacity based on engine displacement is discounting too many variables that makes the tow vehicle quite well suited for the job.

I put it in tow-haul mode whenever towing, and I use criuse control. I cruise at 60 mph anywhere, all the time. Completed a 3800 mile trip to LA area and back through Arizona and New Mexico, with elevations over 7200 ft and in late August where the average temperature was in the mid 90's. No problem. Wondered what the power loss for my naturally aspirated engine would be like in the mountains - no problem, didn't notice. The tranny temperature was perfect except in 90+ degrees and in stop and go traffic where it hit maybe 220 deg briefly, once or twice.

As for what gear I was in - didn't care. The revs were between 1900 prm and 2800 rpm (most of the time it lived here), with occasional jaunts to 3400 rpm. Don't recall ever seeing 4000 rpm over the whole trip and maybe only once or twice otherwise. So the powertrain was not straining and was just singing sweetly.

Here is the best part - fuel consumption was 10.2 mpg over the whole trip.

What is your trans temp gauge telling you?

Tow/haul mode is designed solely to protect the transmission by changing shift points and keeping the torque converter locked more of the time.

Don't know about the Tahoe but my Silverado 2500 has a feature called "grade control" and it only works when the transmission is in Auto. Put in in Manual and grade control is disabled. Grade control on my truck works perfectly and unless I have a very heavy load (70% of the GCWR) I don't bother with Tow/Haul mode as this is what GM recommends for my truck.

When all else fails you can read the manual.

Thanks for all of the replies. I'll probably just hold it in 4th until I get to a long level stretch and then I might let it shift to 5th.

Dennis

The GM owners manual probably says trans tow/haul mode is not necessary unless you are towing at 80% of GCWR. However, my opinion is that towing any high walled trailer is more work than any low walled trailer and one should use tow/haul mode. Weight when towing a high walled RV at 55mph+ doesn't matter because the amount of work needed is due to pulling two sheets of plywood through the air.

6th gear is not locked out in tow/haul mode. It is just not likely to shift into it under 65mph, maybe 70mph for your 3.42 axle. My 6.0/3.73 will shift into 6th gear around 65mph in tow/haul mode.

The torque converter will lock in all but first gear in both regualr and tow.haul modes. The trans has to unlock the torque converter every time it shifts gears. Unlocked TC is what drives the trans temp up, so it is wise to keep it locked as best you can.

I use tow/haul mode and M5 and tow at 65-70mph most of the time. That's 2200-2400rpm. 2500rpm in 4th seems just right for your conditions. Usually enough power, TC locked, and good throttle response. That's what I recommend for you, tow/haul and M4.

wayfun wrote:
In 4th gear the truck doesn't struggle at all on significant hills. When holding it in 4th the tow/haul mode is still engaged. Would the torque converter be locked at those times?

Dennis

Generally yes the TC will be locked. It may unlock if looking for 3rd gear but if steady in 4th you should be fine.

Dick_B wrote:
I have a similar TV and use the `manual' mode much of the time even with Tow Haul mode to minimize shifting. IMHO the jury is still out on how long the transmissions will hold out with these little 6 L engines in tow vehicles.

I also use T/H and manual mode to keep mine from hunting too much. I guess I've spent too much time driving manual transmission vehicles. I like to control what gear it's in.

And regards to the transmission - the 6L90 in the 3/4 ton Burbs is pretty stout. I have 130K on mine, and it still runs/shifts flawlessly. I fully expect a minimum of 200k miles.

I have the same engine/transmission and my trailer weights can often be in the 8K-10K range. I use T/H mode exclusively when towing and have had no problems with heat here in the mountainous west. Also, my truck will shift to 6th while in T/H mode, if on a downhill grade. Also, I have never noticed the transmission "hunting" when in the T/H mode, downshifting to keep the RPMs up, yes, but not hunting for a gear. I am in the boat with those that say GM designed the transmission to do the work, so let it do the work for you.

In 4th gear the truck doesn't struggle at all on significant hills. When holding it in 4th the tow/haul mode is still engaged. Would the torque converter be locked at those times?

Dennis