Cant find the info I crave

AZJim · July 27, 2022

So my tranny is messing up during hot days and towing my travel trailer. On cooler days towing the same trailer no problem. It revs and chugs and I have to stop and let it rest for a few minutes and it works. Not all the time but it gets me to my destination. Where the transmission thermal bypass should be is just a block with the lines going into it. No factory installed bypass. What am I missing here? I'm pulling what little hair I have left. There is no markings like "Allison" was able to see a D-13 and thats about it. Can I install a thermal bypass block? The block that is in it is not up and down, it's horizontal. Any help would be reall appreciated.

CamGTP · July 27, 2022

That block has the thermostat inside it if it's a half ton truck.

The 3/4 and 1 ton trucks use a 6L90e transmission in the gas models and those should not have a thermostat at all.

Torque converter failures are pretty common on 6L90e transmission, it can lead to fluid over heating and chugging if the converter starts slipping alot.

Edited July 27, 2022 by CamGTP

AZJim · July 28, 2022

is there a mod to fix this mess.

slj2008 · July 28, 2022

have your local shop check it out , then go from there

newdude · July 28, 2022

22 hours ago, AZJim said:

So my tranny is messing up during hot days and towing my travel trailer. On cooler days towing the same trailer no problem. It revs and chugs and I have to stop and let it rest for a few minutes and it works. Not all the time but it gets me to my destination. Where the transmission thermal bypass should be is just a block with the lines going into it. No factory installed bypass. What am I missing here? I'm pulling what little hair I have left. There is no markings like "Allison" was able to see a D-13 and thats about it. Can I install a thermal bypass block? The block that is in it is not up and down, it's horizontal. Any help would be reall appreciated.

Gas HD trucks up to 2019 do not have the thermostat. Its open flow all the time. Transmission is the 6L90.

You can try a fluid change. Might have to do it a couple times to swap out to all new fresh fluid. Fluid additive package can degrade and lead to some strange driving characteristics such as converter fishbite.

If changing the fluid doesn't help, time to pull the transmission and see what is going on. It's likely at a minimum you will need a new torque converter. Best way to determine that the converter is gone is when you drop the pan, inspect the magnet. If it's loaded up like an etch-a-sketch, converter is toast.

6L80/90 also like to eat 4-5-6 clutches which can lead to poor or loss of 4th, 5th or 6th gears.

Edited July 28, 2022 by newdude