Transmission question.

[fozzi58]

As others have stated in multiple posts, I'll repeat but make it concise.

Verify with your transmission rebuilder, that they replaced the torque converter with something OTHER than the GM OEM TC. If they used the factory OEM one, look in to getting something better - Yank, Circle D, TCI, even a one off of Jegs or Summit.

Make sure they did a thermostat bypass.

Have a trans cooler added if you did not already.

Get your ECM and TCM tuned. Hopefully your transmission builder can or already did do this. If not, then BlackBearPerformance is a sponsor on this site and they do mail order tunes. Or find a local tuning shop.

 

[TheBigCheezeBall]

So we had the transmission rebuilt in our 2018 Tahoe 4x4 with the 6L80

A guy at work suggested that around town we drive with the tow haul mode on the keep line pressure up and keep from going into OD?

Any thoughts or opinions?

The following assumes data and tables pulled via HP tuners on a 2017 Tahoe 5.3 with a 6L80

By line pressure I am assuming you mean the TCC (Torque Converter Clutch) line pressure? From everything I read TCC line pressure is calculated and applied based on the actual TCC slip vs the desired TCC slip. So if it wants 5 RPM slip as an example, it is constantly varying the pressure to maintain that level of slip. I can verify this is how my Tahoe seems to operate based on scanner logs I have pulled. If the slip tables in tow haul mode are the same as they are in normal mode (mine were), then tow haul mode would actually deliver less line pressure, as a lower gear requires less torque on the TCC to produce the same output force. I can also confirm tow haul mode does not eliminate over drive, but raises the shift points as others have pointed out. Mine will drop into 5th at 55 MPH, and 6th at 74 MPH.

Now for some uneducated opinions, thoughts and theorizing: Are higher shift points (and tow haul mode) better for the transmission? If you think less torque on the TCC over time would be beneficial to a longer life then yes. If not, then no. I think it probably is beneficial based on how most describe the failures manifesting: Shudder / vibration / rpm slips when in over drive and a low RPM acceleration load (TCC under high load).

I agree with others that the most benefit would come from lowering the transmission temperature. Personally I would also prevent the TCC clutch from locking at all in 1-4, and eliminate or lower the slip via a tune.

Personally, I much prefer the tow haul mode as I feel it drives more like I would shift a manual transmission. It is also the only way I can monitor the transmission temperature, which I find much more important than the battery voltage. When cruising on the highway I usually get right above the 74 MPH 6th gear shift point so it can still drop into the final overdrive. While I do think it might help the TCC clutch, it isn't why I use it.

 

[Marky Dissod]

Are higher shift points (and tow haul mode) better for the transmission?

No better, no worse - better for the ENGINE though.
'Normal' shift table is written assuming pretty much unladen, pretty much flat road, with good MpG as the goal.
'Tow / Haul' shift table is written assuming heavily laden, hilly terrain, prevent excessive shifting, engine lugging, and to maintain better driver control.
Note that both can be improved upon, obviously, and not just the shift points. Also please note, 'Tow / Haul' is similar to but NOT the same as 'Performance'.

 

[TheBigCheezeBall]

No better, no worse - better for the ENGINE though.
'Normal' shift table is written assuming pretty much unladen, pretty much flat road, with good MpG as the goal.
'Tow / Haul' shift table is written assuming heavily laden, hilly terrain, prevent excessive shifting, engine lugging, and to maintain better driver control.
Note that both can be improved upon, obviously, and not just the shift points. Also please note, 'Tow / Haul' is similar to but NOT the same as 'Performance'.

I should have noted that I live in a very hilly area of the country. Flat roads just don’t exist within 200 miles of me.

 

[Marky Dissod]

I should have noted that I live in a very hilly area of the country. Flat roads just don’t exist within 200 miles of me.

CAFE MpG tests are mostly flat roads and light loads. If the vast majority of the CAFE MpG tests were either up- or downhill, our axles would have better gearing,
and the shift tables would upshift later and downshift with less throttle requested.

Had an LT1 Caprice wagon with a 4L60E & 2.56 long ago. Learned to tune it myself using steep & long hills. 4th was only useful on FLAT roads over 45MpH.
Tuning that shift table was the 2nd best thing I ever did for the longevity and durability of both the engine and the transmission.
Best thing was 3.42, which actually improved city MpG AND further improved powertrain durability. (From 2.56 to 3.42, tow rating improved from 3000lb to 7000lb.)