Transmission Temperature Hitting 220s

[DustMcGrath]

First of all sorry, I know there are a ton of transmission temperature threads...

 

My truck is a 2025 LZ0 High (10l89 10-speed) High Country with NHT max trailering package. The NHT package is supposed to have "enhanced cooling."

 

Normally the trans temp sits around 200. It seems like with any real stress it quickly jumps into the 220s (for example climbing a grade unloaded at lower speeds). I haven't towed much with this truck, but I'm concerned if it gets this hot unloaded. 

 

I've tried a few different things to keep the temps down (tow/haul mode, sport mode, low range when that makes sense) and nothing seems to affect it. 

 

I'm curious if others are the same behavior or if anyone has found techniques to keep the temps down. It seems like it just can't handle reduced airflow at lower speeds, even with relatively light loading. 

 

Thanks!

 

 

 

[diyer2]

Switch a a 100% synthetic trans fluid, Amsoil works for me.

[newdude]

2 hours ago, DustMcGrath said:

First of all sorry, I know there are a ton of transmission temperature threads...

 

My truck is a 2025 LZ0 High (10l89 10-speed) High Country with NHT max trailering package. The NHT package is supposed to have "enhanced cooling."

 

Normally the trans temp sits around 200. It seems like with any real stress it quickly jumps into the 220s (for example climbing a grade unloaded at lower speeds). I haven't towed much with this truck, but I'm concerned if it gets this hot unloaded. 

 

I've tried a few different things to keep the temps down (tow/haul mode, sport mode, low range when that makes sense) and nothing seems to affect it. 

 

I'm curious if others are the same behavior or if anyone has found techniques to keep the temps down. It seems like it just can't handle reduced airflow at lower speeds, even with relatively light loading. 

 

Thanks!

 

 

 

 

 

It is working as designed.  NHT and non NHT trucks have the same exact transmission cooling setup.  NHT just gets a larger radiator.  

 

The transmission cooler loop on these is as follows.  

 

-Transmission out goes into an oil to water heat exchanger (item #1 in the first illustration below).  This exchanger has engine coolant opposite the transmission fluid and uses engine coolant to warm and then regulate the transmission fluid temperature when its in the operating window.

 

- From the cooler it goes to a thermal bypass valve (item #11 in the 2nd illustration below).  If the ATF has not reached the temp to open the bypass (194F), it loops back.

 

- When it reaches 194F, the thermostat opens and ATF then flows out to the oil to air auxiliary cooler (item #17 in the 2nd illustration) at the front of the truck.  From there it goes back to the transmission.  

 

Your only fix would be to swap for the PPE thermal bypass delete block which will open the loop to the aux cooler all the time.  It will STILL run warm, probably 170-200F, but that's as low as you'll get it as its still flowing through the oil to water exchanger.  

 

 

[DustMcGrath]

@newdude great info, thank you!

 

So opening the thermal bypass valve early (or all the time) would cause temperature to rise more slowly, but above 194F there would be no difference from normal operation?

 

It seems like the only practical ways to bring down the peak temperature would be to improve the oil-air heat exchanger in some way or increase fluid capacity?

[newdude]

1 hour ago, DustMcGrath said:

@newdude great info, thank you!

 

So opening the thermal bypass valve early (or all the time) would cause temperature to rise more slowly, but above 194F there would be no difference from normal operation?

 

It seems like the only practical ways to bring down the peak temperature would be to improve the oil-air heat exchanger in some way or increase fluid capacity?

 

 

Rise more slowly?  Debatable.  It would run slightly cooler, yes if anything after adding the thermostat delete kit from PPE.

 

PPE makes a larger front aux cooler which they "claim" will lower temps 45 degrees but I have not used it.  

 

IMO, especially in cold states, I would not want to mess with the first cooler, the oil to water one.  Cold states, you want heat in the transmission for it to shift properly, and you don't want moisture in the ATF. 

 

Ideal temps for automatic transmissions to do their job the best is 175-220F.  Hotter than 220F you start heading towards fluid degradation and shortened fluid life/service intervals.  240F can start varnishing things like the fluid, clutches, etc.   

[PunchT37]

2 minutes ago, newdude said:

 

 

 

 

 

Ideal temps for automatic transmissions to do their job the best is 175-220F.  Hotter than 220F you start heading towards fluid degradation and shortened fluid life/service intervals.  240F can start varnishing things like the fluid, clutches, etc.   

Why do they have the gas 10l80 opening at 145?

[newdude]

7 hours ago, PunchT37 said:

Why do they have the gas 10l80 opening at 145?

 

 

No.  Idea.  

 

That's just where GM set it to.  Why?   

 

 

[PunchT37]

2 hours ago, newdude said:

 

 

No.  Idea.  

 

That's just where GM set it to.  Why?   

 

 

Good question. Most to all of what I hear in internet land is to have 212 trans temp to boil off the water.

 

That`s where my earlier comment was coming from.

[TrueBlue]

I have the 10L80 gas transmission. It rides at 145 or just a tad higher in the heat. Haven't towed with it yet. Both of my trailers are lightweight, 900 - 1500lbs empty, so I won't be pulling a whole lot of weight with it when I do pull them.

But given this statement, "Ideal temps for automatic transmissions to do their job the best is 175-220F", what does that mean for our gasser trannies? Are they not operating at ideal temp? Or do those temps only apply to the 10L90 on the HD trucks?

[Jay P]

45 minutes ago, TrueBlue said:

I have the 10L80 gas transmission. It rides at 145 or just a tad higher in the heat. Haven't towed with it yet. Both of my trailers are lightweight, 900 - 1500lbs empty, so I won't be pulling a whole lot of weight with it when I do pull them.

But given this statement, "Ideal temps for automatic transmissions to do their job the best is 175-220F", what does that mean for our gasser trannies? Are they not operating at ideal temp? Or do those temps only apply to the 10L90 on the HD trucks?

The 10L90 was used in high horsepower cars and SUVs (higher HP/torque version of 10L80). The HD trucks have the 10L1000 which was co-developed with Allison and is not related to the 10L80/90 which was co-developed with Ford. The 10L1000 replaced the 6L90 used with gas HD trucks in the current generation of the HD trucks.

Edited 7 hours ago by Jay P

[TrueBlue]

Thanks, so being a different beast altogether, anyone know what the "optimal" temp is for the 10L80/90 for proper shifting?

[Grumpy Bear]

On 7/25/2026 at 11:49 AM, PunchT37 said:

Why do they have the gas 10l80 opening at 145?

 

Most thermostatically controlled ATF systems use a 'wax' pill just like the one in your water thermostat. They work on the expansion volume of the wax and the pill is constructed in such a way as to limit the travel to remain within that range. This means it has an opening temperature often called the 'cracking temperature' and a fully open temperature. For whatever reason oil is designated by it's fully open temperature and water by it's cracking temperature. 

 

Between the two it the "Regulating Temperature" The entire range is about 20 F from closed to open. In your example the lower limit, opening point of 145 F then has a fully open temp of 165 F and a throttling temperature of 155 F. 

 

Thing is, the throttling temperature needs two things to work properly. Enough system load high enough to reach the minimum and a heat sink (cooler/exchanger) large enough to prevent any load applied from exceeding the fully open temperature. 

 

If your system exceeds the fully open temperature it is screaming MORE COOLER. 

 

Here is the silly part of the whole thermostat system where the oil is cooled/heated by the radiator. Nothing is flowing to that system until the oil temperature reaches the cracking temperature unless it is equipped with a minimum flow bypass orifice or a bypass built into the seating surface of the plunger. That system meant to get it up to temp quicker isn't doing a thing until WORK drives if from ambient to cracking temp. One a cold winter day, like zero F, it may never get hotter than about 110 F. Sometimes not even that. 

 

People forget or are young enough to not know that until this millennia transmissions were cooled by the radiator cold tank alone and without a thermostat at all.  In the 50's they weren't even cooled. OEMs learn everything the hard way. 

 

When an OEM tells you the trans "Works best" between 175 F and 220 F it means works best for THEM. The transmission thermostat is fuel play. If you watch you trans temps like I do you will learn that it may take a hundred miles if you are not towing and not driving more than say 55-60 mph on a zero F day to even reach 120 F and it might sit there all day if it's colder. Yet if functions perfectly over about 100 F. It may need to get a bit warmer for the fuel and spark maps to go without an multiplier. 

 

Worry more about too hot. Use a cooler large enough to put that thermostat in range. 

 

 

 

 

[TrueBlue]

Thanks for the detailed explanation. I'm old enough to remember the old vehicles with transmissions that were either not cooled at all, or run through the bottom of the radiator. 

Sometimes it appears that much like my latter days in my job as a computer analyst in a fortune 500 company, insofar as our vehicles are concerned testing has moved over into the production environment. And we are the testers

[PunchT37]

1 hour ago, Grumpy Bear said:

 

Most thermostatically controlled ATF systems use a 'wax' pill just like the one in your water thermostat. They work on the expansion volume of the wax and the pill is constructed in such a way as to limit the travel to remain within that range. This means it has an opening temperature often called the 'cracking temperature' and a fully open temperature. For whatever reason oil is designated by it's fully open temperature and water by it's cracking temperature. 

 

Between the two it the "Regulating Temperature" The entire range is about 20 F from closed to open. In your example the lower limit, opening point of 145 F then has a fully open temp of 165 F and a throttling temperature of 155 F. 

 

Thing is, the throttling temperature needs two things to work properly. Enough system load high enough to reach the minimum and a heat sink (cooler/exchanger) large enough to prevent any load applied from exceeding the fully open temperature. 

 

If your system exceeds the fully open temperature it is screaming MORE COOLER. 

 

Here is the silly part of the whole thermostat system where the oil is cooled/heated by the radiator. Nothing is flowing to that system until the oil temperature reaches the cracking temperature unless it is equipped with a minimum flow bypass orifice or a bypass built into the seating surface of the plunger. That system meant to get it up to temp quicker isn't doing a thing until WORK drives if from ambient to cracking temp. One a cold winter day, like zero F, it may never get hotter than about 110 F. Sometimes not even that. 

 

People forget or are young enough to not know that until this millennia transmissions were cooled by the radiator cold tank alone and without a thermostat at all.  In the 50's they weren't even cooled. OEMs learn everything the hard way. 

 

When an OEM tells you the trans "Works best" between 175 F and 220 F it means works best for THEM. The transmission thermostat is fuel play. If you watch you trans temps like I do you will learn that it may take a hundred miles if you are not towing and not driving more than say 55-60 mph on a zero F day to even reach 120 F and it might sit there all day if it's colder. Yet if functions perfectly over about 100 F. It may need to get a bit warmer for the fuel and spark maps to go without an multiplier. 

 

Worry more about too hot. Use a cooler large enough to put that thermostat in range. 

 

 

 

 

I hear ya. I remember the air cooled trans. My deal is, I`m not one to run my trans at 212 "to get the water out".  If you leave a cold beer out on the bar, will the condensation go away without boiling?

 

I don`t believe in the not hot enough to shift properly or to get the water out. A trans will fail in 2 ways. Either you break it or you roast it. How many have failed from being "too cold"?

 

Oil viscosity? Hmm. How much "thicker" does an oil get as it gets saturated with clutch particles? When it`s time for me to change my 10L80 oil, it will get LV spec oil. Lets see what happens.

Edited 1 hour ago by PunchT37