Transmission hot specifically in 4th gear.

[Brian Maxwell]

6L80 transmission.  Rebuilt about 5000 miles ago and new billet TC.  PPE thermal bypass installed.

 

Day 1 of spring break trip.  Towing ~5000 lb trailer into the wind.  I was watching temps closely for the first hour of the drive and no big deal.  Happened to flip back to trans temp screen and see 255 deg.  Was driving in 4th gear.  Not shifting / hunting.

 

Slowed to 57 ish mph where the truck would stay in 5th and temps dropped down to ~220 really quick.

 

For the remainder of the drive, 4th consistently generates heat in excess of cooling capacity.  Why would that be?

 

Called the shop and was advised to be careful / mindful and finish the road trip then bring it back for them to review.

 

Why would it do this?

Photo for reference.  Was consistently at 65mph.

[rlcole321]

Just like when you remove a Tstat from an engine, it may run cooler but if you run it hard you can get into a condition called "Thermal Runaway" where the coolant doesn't stay in the radiator long enough to dissipate the heat properly so you get an overheating condition, the same can happen it your trans, I see you did a thermal bypass for the Trans Tstat, that could be what you are seeing. The new GM Trans Thermal valve has the 152-degree tstat in it which should help meter that trans fluid to keep it cooler. Justn a thought! 

[newdude]

On 3/21/2025 at 4:23 PM, Brian Maxwell said:

6L80 transmission.  Rebuilt about 5000 miles ago and new billet TC.  PPE thermal bypass installed.

 

Day 1 of spring break trip.  Towing ~5000 lb trailer into the wind.  I was watching temps closely for the first hour of the drive and no big deal.  Happened to flip back to trans temp screen and see 255 deg.  Was driving in 4th gear.  Not shifting / hunting.

 

Slowed to 57 ish mph where the truck would stay in 5th and temps dropped down to ~220 really quick.

 

For the remainder of the drive, 4th consistently generates heat in excess of cooling capacity.  Why would that be?

 

Called the shop and was advised to be careful / mindful and finish the road trip then bring it back for them to review.

 

Why would it do this?

Photo for reference.  Was consistently at 65mph.

 

 

 

Hope you've got a warranty from the rebuilder.  4-5-6 clutch pack is a known soft spot on the 6L80.  I'd bet its slipping in 4th and generating all that heat.  I wonder if they even put new 4-5-6 clutches in to begin with during the rebuild.  

 

How fast were you driving?  4th gear into the wind with a trailer is gonna put some strain for sure but I don't think you should have gotten within 10 degrees of the overheat warning.  

 

Empty driving with the PPE how hot does it get?  On a 2018 it should run rather cold/cool with the PPE or a Surecool or pill flip.  

 

 

Edited March 26, 2025 by newdude

[newdude]

2 hours ago, rlcole321 said:

Just like when you remove a Tstat from an engine, it may run cooler but if you run it hard you can get into a condition called "Thermal Runaway" where the coolant doesn't stay in the radiator long enough to dissipate the heat properly so you get an overheating condition, the same can happen it your trans, I see you did a thermal bypass for the Trans Tstat, that could be what you are seeing. The new GM Trans Thermal valve has the 152-degree tstat in it which should help meter that trans fluid to keep it cooler. Justn a thought! 

 

 

The PPE makes it a full time fluid loop from the transmission to the cooler just like the 07-13 and 99-06 and older trucks were where they had no thermal bypass at all and those never had thermal runaways.   

 

So a runaway seems like an unlikely scenario vs. OP's rebuild has an internal fault like slipping 4-5-6 clutches.  Or 4th gear into the wind going over the speed limit just worked it too hard.  

Edited March 26, 2025 by newdude

[Brian Maxwell]

19 hours ago, newdude said:

 

 

Hope you've got a warranty from the rebuilder.  4-5-6 clutch pack is a known soft spot on the 6L80.  I'd bet its slipping in 4th and generating all that heat.  I wonder if they even put new 4-5-6 clutches in to begin with during the rebuild.  

 

How fast were you driving?  4th gear into the wind with a trailer is gonna put some strain for sure but I don't think you should have gotten within 10 degrees of the overheat warning.  

 

Empty driving with the PPE how hot does it get?  On a 2018 it should run rather cold/cool with the PPE or a Surecool or pill flip.  

 

 

Just got back to some internet.

60-65 on I80 westbound.  Limit is 70.  I try to stay around 65.

 

In town it's been 145ish with the PPE.  I saw 235 deg (again in 4th) towing across TN a couple days ago.

 

Just the truck driving in Smoky Mountain NP yesterday saw 205 climbing to newfoundland gap.

 

Ive been taking photos to talk through with the shop when I get home.

[newdude]

1 hour ago, Brian Maxwell said:

In town it's been 145ish with the PPE.  I saw 235 deg (again in 4th) towing across TN a couple days ago.

 

Just the truck driving in Smoky Mountain NP yesterday saw 205 climbing to newfoundland gap.

 

Ive been taking photos to talk through with the shop when I get home.

 

 

Yea its getting hot and shouldn't be it seems.  The 145F around town is fine with the PPE on there, its the hills and towing that seem concerning being its got the PPE on.  

 

I "think" if you had a restricted cooler that you'd be 200F+ in town and not 145F.  

[rlcole321]

then it might be time to add an additional Trans Cooler system.