Pros and Cons of Installing Later Transmission Thermostat with Lower Opening Temperature

Grumpy Bear · March 11, 2023

1 hour ago, Another JR said:

I don’t think eliminating the thermostat is a good idea other than maybe for truck that is always operated in a hot environment. You want the operating temperature to warm up to a stable temperature and design around that.

I still am curious why GM engineers originally decided they wanted that nominal temperature up at 195F. Was it to improve fuel economy by having the ATF slightly less viscous? Another reason? They obviously later decided the 153F was a better compromise, but what was the thinking behind the original decision?

Anybody know or care to speculate?

I'm curious why after 65 years of no thermostat they thought it was even a good idea. Yea, automatics ran fine without them for that long. Won't hurt it a bit to have the pill flipped.

KARNUT · March 11, 2023

6 minutes ago, Grumpy Bear said:

I'm curious why after 65 years of no thermostat they thought it was even a good idea. Yea, automatics ran fine without them for that long. Won't hurt it a bit to have the pill flipped.

Not to be a stick in the mud. You can say that about cylinder deactivation, start stop and a few others. They want just a little more fuel economy. Doesn’t matter the toll on longevity.

Jettech1 · March 11, 2023

3 hours ago, Another JR said:

I don’t think eliminating the thermostat is a good idea other than maybe for truck that is always operated in a hot environment. You want the operating temperature to warm up to a stable temperature and design around that.

I still am curious why GM engineers originally decided they wanted that nominal temperature up at 195F. Was it to improve fuel economy by having the ATF slightly less viscous? Another reason? They obviously later decided the 153F was a better compromise, but what was the thinking behind the original decision?

Anybody know or care to speculate?

Great question. And I have no idea why they would do that. Heat is the demise of all automatic transmissions. They hate heat, it doesn't help them a bit for efficiency with the water thin fluid that we know is called ATF. So I'm at a loss here. I can't even think of an extreme example why. I'm trying....still coming up with nothing....

Jettech1 · March 11, 2023

2 hours ago, Grumpy Bear said:

I'm curious why after 65 years of no thermostat they thought it was even a good idea. Yea, automatics ran fine without them for that long. Won't hurt it a bit to have the pill flipped.

And where was the automatic transmission fluid cooled at all of those years? Yes in the radiator with 180 to 195' temps. So they survived just fine over the years at those temps. But here's where the problem comes in, and that's with heavy duty towing temps that will challenge the best of coolers. Things have gotten so complicated it's not even funny. I can see the reason to slow the fluid down with a thermostat like we have in our engines so that the engine will heat up quickly and the fluid will have time to cool down in the radiator. Perhaps the same reason applies to the trans. coolers...?? Maybe??

Jettech1 · March 11, 2023

On 3/6/2023 at 8:20 AM, Pryme said:

You already know you’re going to get a 2024 ha! I can sense it.

So yes I went on the GMC site today...I really hate you by the way....anyhow there's a 5k difference between the Pro and the SLE. The SLE gets the new upgraded infotainment package...the pro doesn't...... you are evil, I just want to make sure you know that... lol...And all these years I was thinking I was Lucifer...Nope...apparently I was wrong...I found the real one...lol...

Edited March 11, 2023 by Jettech1

Another JR · March 11, 2023

Excellent point jettech that you don’t need a thermostat to ensure a minimum operating temp when the heat sink temperature (radiator temperature) does that for you.

These trucks have a separate oil cooler, which I suspect drove the decision to include a thermostat to ensure the system warms up to a minimum temperature. I wonder if the original 195F was just trying to replicate a system that uses the engine radiator as the heat sink.

Jettech1 · March 11, 2023

2 minutes ago, Another JR said:

Excellent point jettech that you don’t need a thermostat to ensure a minimum operating temp when the heat sink temperature (radiator temperature) does that for you.

These trucks have a separate oil cooler, which I suspect drove the decision to include a thermostat to ensure the system warms up to a minimum temperature. I wonder if the original 195F was just trying to replicate a system that uses the engine radiator as the heat sink.

Probably. A better option than I could come up with.

Grumpy Bear · March 11, 2023

4 hours ago, KARNUT said:

Not to be a stick in the mud. You can say that about cylinder deactivation, start stop and a few others. They want just a little more fuel economy. Doesn’t matter the toll on longevity.

:crackup: It was a rhetorical question sir

Pryme · March 12, 2023

5 hours ago, Jettech1 said:

So yes I went on the GMC site today...I really hate you by the way....anyhow there's a 5k difference between the Pro and the SLE. The SLE gets the new upgraded infotainment package...the pro doesn't...... you are evil, I just want to make sure you know that... lol...And all these years I was thinking I was Lucifer...Nope...apparently I was wrong...I found the real one...lol...

Are you getting one haha

Another JR · March 12, 2023

On 3/5/2023 at 6:56 PM, Jettech1 said:

This topic is so weird....how in the hell are our trucks so different? Programming should have been the same...I'm so confused here...

Another data point: my April 2021 truck shifts into 6th in tow/haul mode. As someone said above, empty truck on level ground accelerating to 60 and sitting there it stays in 5th. Move up to about 65 and it goes into 6th. When I backed down to 60 it stayed in 6th. All of this was on level road.

Jettech1 · March 12, 2023

19 hours ago, Pryme said:

Are you getting one haha

Nope, not happening...

GMC4Zee · March 15, 2023

So how do I get my dealer to put the lower T stat in? I asked about it and he blew me off. I told him the trans temp goes to 215 when towing and 195 when not. He said as long as it stays under 245 I am good.

Jettech1 · March 15, 2023

27 minutes ago, GMC4Zee said:

So how do I get my dealer to put the lower T stat in? I asked about it and he blew me off. I told him the trans temp goes to 215 when towing and 195 when not. He said as long as it stays under 245 I am good.

I'm hoping someone chimes in here and tells you how to get the dealer to comply with the update because that's pretty darn hot. Last September when we picked up our 8k lb camper in Jacksonville, FL and towed it to Byron, GA it was 95' out and my trans temp never above 185'. So please keep us updated as to the outcome. Unloaded I typically run in the mid 160's when it's hot as hell out as well. When I look at your temps versus mine you have about a 20' spread loaded and unloaded. That's about what I have with the lower temp T stat in mine from the factory.

Edited March 15, 2023 by Jettech1

Pryme · March 15, 2023

18 hours ago, GMC4Zee said:

So how do I get my dealer to put the lower T stat in? I asked about it and he blew me off. I told him the trans temp goes to 215 when towing and 195 when not. He said as long as it stays under 245 I am good.

I thought someone once posted a tsb or whatever from GM on this. I’ve learned that most ppl will say some silly BS just so they don’t have to do any actual work. That guy is either blowing you off or a fool. Either way, when you get the info and proof, show him so he can be properly trained in.

mrjulian416 · March 16, 2023

You tell them you are having a hard 2-1 downshift. And then you give them the TSB mentioned in this thread which says even without reproducing the problem, they should replace the transmission thermostat.