Get That Transmission Cooler!

sdeeter19555 · June 6, 2017

There is increased pressure and a "slower feel" , I mentioned slower shifts, etc. Not sure how many would actually "feel" it depending on drive habits and the actual temp

When this 6l90e is cold, it has very soft shifts until it actually gets remotely warm (above about 70F). 68F is also the cutoff GM uses to control TC lockup...below 68F no lockup, above 68F the TC starts locking up between gears. Maybe to facilitate faster warm up with converter/fluid shear?

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Formulabruce · June 6, 2017

When this 6l90e is cold, it has very soft shifts until it actually gets remotely warm (above about 70F). 68F is also the cutoff GM uses to control TC lockup...below 68F no lockup, above 68F the TC starts locking up between gears. Maybe to facilitate faster warm up with converter/fluid shear?

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Interesting, the only trans ( RWD) that I know of that "locked up" in any gear besides the top gear is the '89 Pontiac trans am pace car 2004R . Making this temp dependent really does make sense as the lock up solenoid is Not that big and lock up cant support a big load ( towing, cold parts resistance). I am sure in the 6L90E's case it helps the life of the transmission. I wouldnt be surprised if one day the lock up is also torque limited, which would automatically put the trans in "tow" mode ( just take out the lock up) .

sdeeter19555 · June 6, 2017

Interesting, the only trans ( RWD) that I know of that "locked up" in any gear besides the top gear is the '89 Pontiac trans am pace car 2004R . Making this temp dependent really does make sense as the lock up solenoid is Not that big and lock up cant support a big load ( towing, cold parts resistance). I am sure in the 6L90E's case it helps the life of the transmission. I wouldnt be surprised if one day the lock up is also torque limited, which would automatically put the trans in "tow" mode ( just take out the lock up) .

I have an Edge Insight that shows the gear and the TC engagement, that's how I can tell when it's in/out. Not only that, but you can feel it and that coincides with the indication on the Insight.

It even shows the trans shifting gears with the TC locked...which I think the unlock/lock just happens fast enough the Insight doesn't see it. It think by keeping the TC locked, it limits heat because the TC isn't shearing fluid. They use that shearing to their advantage when cold and the TC lockup to limit heat (also puts more power to the ground while locked up).

The 6l90e will lock any gear but 1st (and probably reverse, never checked that). And it doesn't matter if in manual mode or not, if speed determines 4th is appropriate, it will hold 4th and lock the converter. It actually makes the truck more sluggish because with the TC locked you lose your torque multiplication of the TC. If they could program the converter to lock later going up through gears or unlock and/or downshift sooner, it would probably do better?

If they could make it load dependent, it would be interesting.

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sdeeter19555 · June 6, 2017

Couple photos showing the TC locked and unlocked.

A side note, that 129F transmission temp is after 40 miles of interstate and 10 miles of backwoods driving at a 70F ambient outside air temperature.

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Formulabruce · June 6, 2017

Interesting info! The last GM school I went to was on the TH350-C ( yeah its been a while) , but I did learn about lock ups and Trans temp. They told me optimum transmission temps were at 125-150 for any trans with Lock up. I have adjusted the shift points in my 4L80E, so it actually can get into 4th and lock by around 49 mph with 60+ degrees outside and no hill, or towing. This was done to help with economy, which is probably what GM did with the "Locking" in the higher gears. The progression of all this is interesting, especially with the new cruise hooked to the transmission braking in new Duramax Trucks.