Jump to content

Transmission hot specifically in 4th gear.

Brian Maxwell
 Share

Recommended Posts

Brian Maxwell Enthusiast

Brian Maxwell

Posted
Posted

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.

20250321_104752.jpg

rlcole321 Enthusiast

rlcole321

Posted
Posted

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! 

  • Like 1
newdude Senior Enthusiast

newdude

Posted
Posted (edited)
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 by newdude
  • Like 1
newdude Senior Enthusiast

newdude

Posted
Posted (edited)
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 by newdude
  • Like 1
Brian Maxwell Enthusiast

Brian Maxwell

Posted
  • Author
Posted
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.

  • Like 1
newdude Senior Enthusiast

newdude

Posted
Posted
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.  

  • Like 1
  • 4 weeks later...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Latest Articles

  • Posts

    • Grumpy Bear
      Transmission Temperature Hitting 220s

      By Grumpy Bear · Posted

      After I wrote that reply I went for a ride. 85 F out. So trans started at 85 F and it shifted the same as it did an hour latter at 168 F. I've never actually recorded the to cold to shift point but it is under 32 F before they start hanging on to a lower gear or leaving the converter unlocked. Both intended to heat fluid.    Yes, brake or bake. Absolutely.      Your welcome. True research ended decades ago. Field trials are still in play.    There is a thing called oxidation onset temperature. This is the heat related oxidation not the O2 exposure kind. For mineral oil that starts at 140 F. So what happens after 140 F? Each 20 F cuts fluid life in half! Does using a Full Synthetic improve that? Yes! By how much? Depends on the base oil type and a few other factors. But enough to notice. Some esters are even higher. There are even synergies between types the will raise it higher than any one component is alone.    Then there is "Hindered" oils. Antioxidant additives. Hindered Polyol Esters can sit at 500 F for years in a turbine.   When a blender brags his oil will 'take 500 F" in service he doesn't specify for how long. Water will dance on a 500 F hot plate for several seconds. @newdude put up a chart showing the relationship between fluid temperature and it's life in an automatic transmission. Here's a version of that chart for Group III based oils. These temperatures are those in the pan as observed as 'normal'. that is you drive as you do for as long as you do under what load you ask of it and whatever that temperature is this graph applies to. Flash temperatures of a few minutes isn't what his means.    Note that fluids held at a constant 240 F have a service life of only 20K miles. 220 F explains the factory exchange limit, eh? My equipment I keep under 180 F and I'm good if the cooler will hold it as low as 140 F. But that's me.   
    • Greg3160
      Actual Full Screen CarPlay on Refreshed Silverado/Sierras.

      By Greg3160 · Posted

      Im in the same boat. Carlink adapter comes tomorrow and dev account is in process. Would you he able to send the new instructions my way also? Question, do you have to change code and republish app everytime developer updates? Thanks for your assistance, truly the only thing i cant stand about my truck is the carplay limitation 
    • newdude
      2002 GMC Yukon

      By newdude · Posted

      Parts department or service department at the dealer.
    • Coastie05
      Future GMC Jimmy

      By Coastie05 · Posted

      I’ll take the blue one!
    • rav3
      Weight limit in bed

      By rav3 · Posted

      And Google AI says......... Yes, your truck can technically handle it, but you will be right at or near its maximum limit. A 2016 Chevrolet Silverado 1500 High Country with a 6.5-foot bed has a maximum payload capacity of approximately 1,770 pounds.Weight RealitiesLoad weight: Half a cord of dry hardwood weighs roughly 1,100 to 1,500 pounds depending on the wood type (like oak or hickory).Payload limit: Payload includes the weight of the wood, you, any passengers, and a full tank of gas.The math: If you weigh 200 pounds and have a full tank of gas, a 1,500-pound load of wood puts you right at that 1,770-pound ceiling.Safety TipsCheck the yellow and white sticker on your driver-side door jamb for your truck's exact payload number, as high-end luxury trims with extra features have slightly lower capacities than basic trucks.Expect the rear end to sag noticeably.Drive carefully, avoid sudden turns or stops, and keep speeds low if you have to travel a distance.

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...