Transmission Fluid Change After Flipping the Thermostat

Byrds8 · February 17, 2022

Reading the fluid level does not change. There used to not be a thermostat on the transmission.

HighSierraAT · February 19, 2022

Do you guys that are running lower transmission temps now notice any difference in fuel mileage? Yeah I know its a truck not a prius and all that jive, and I know it'd take a lot of fuel savings to pay for a new $5k transmission install, but my truck did pretty good before and now it's went to hell in a hand basket. I will continue to monitor, I know it's a terrible time of year for good mpgs but according to dic it's tanked doing the same trips on the same fuel. Had the updated thermostat valve (part number 86774933) installed last week and dropped the pan changed fluid and trans filter. That's the only change in the truck. Trans runs 145-150 degrees now. 14 sierra 5.3 6 speed btw.

Grumpy Bear · February 19, 2022

35 minutes ago, HighSierraAT said:

Do you guys that are running lower transmission temps now notice any difference in fuel mileage? Yeah I know its a truck not a prius and all that jive, and I know it'd take a lot of fuel savings to pay for a new $5k transmission install, but my truck did pretty good before and now it's went to hell in a hand basket. I will continue to monitor, I know it's a terrible time of year for good mpgs but according to dic it's tanked doing the same trips on the same fuel. Had the updated thermostat valve (part number 86774933) installed last week and dropped the pan changed fluid and trans filter. That's the only change in the truck. Trans runs 145-150 degrees now. 14 sierra 5.3 6 speed btw.

None. In point of fact is has been steady in rise since I bought it.

Byrds8 · February 19, 2022

When I had my 2016, I had a slight drop off in mileage but I changed my engine T-stat at the same time. So I contributed it to both. It was roughly about .5mpg that I had lost give or take a tenth. With that being said, when my warranty is up on the new truck, Ill flip it on the 8sp lol.

Edited February 19, 2022 by Byrds8

HighSierraAT · February 19, 2022

Thanks for the input guys. I'd say its either this time of year, or the fact that i do short commutes throughout the day so maybe the fluid never has a chance to get warm? Who knows, will keep an eye on it

dieselfan1 · February 19, 2022

5 hours ago, Grumpy Bear said:

None. In point of fact is has been steady in rise since I bought it.

Your truck is a freak of nature.

Gagliano7 · March 17, 2022

Read them the same. As the fluid gets hot it expands and if you have to much it will leak out.

Grumpy Bear · March 17, 2022

On 2/17/2022 at 9:59 AM, Dan411 said:

I had the same question as i just did mine. Also when you check the fluid on dipstick do you still read the cold and hot the same? Or should you read at the hot even when cold

Read the cold cold and the hot when fully hot.

16LT4 · March 18, 2022

23 hours ago, Grumpy Bear said:

Read the cold cold and the hot when fully hot.

In the interest of being the devil's advocate, the quantitative definition of "fully hot" has now changed by 40-50*F, leaving less thermal expansion and room to add more ATF. Would this bother the transmission? Probably not. Does the original service manual state a specific temperature to check the fluid (thereby making the temperature ceiling irrelevant), or simply say "at operating temperature. The GM 6L45 in my E83 needs the fluid level checked at 30-50*C, yet operating temperature is ~80*C, so checking that transmissions fluid level (by the fill plug; no dipstick) would result in an underfilled transmission.

Edited March 18, 2022 by 16LT4

Grumpy Bear · March 18, 2022

52 minutes ago, 16LT4 said:

In the interest of being the devil's advocate, the quantitative definition of "fully hot" has now changed by 40-50*F, leaving less thermal expansion and room to add more ATF. Would this bother the transmission? Probably not. Does the original service manual state a specific temperature to check the fluid (thereby making the temperature ceiling irrelevant), or simply say "at operating temperature. The GM 6L45 in my E83 needs the fluid level checked at 30-50*C, yet operating temperature is ~80*C, so checking that transmissions fluid level (by the fill plug; no dipstick) would result in an underfilled transmission.

Good question and one worth a look into the book for.

Here's a thought. I replaced my 6 quart pan with an 8 quart pan. I add two quarts to reach full. What does that tell you about the fluid IN the transmission? It's a 12.6 quart system stock. Now 14.6 and yet when you drain the pan you get only 8 quarts out. Use to be 6. What you are checking is the level in the pan, not the transmission volume. Transmission is a hydraulic system that only works when there is no air in the system. The cold check is an initial fill that is high enough to satisfy the system being void of air. The pan volume assures no air ingress. Remember these checks are done with the transmission pump running and after all ranges selected on level ground to draw that level to it's lowest point....when hot.

In your 6L45 (same as the box in my Buick) running warmer than the hot check does not give a 'overfull' Just a higher level that hurts nothing. In the 6L80E they want that temperature to exceed the TBV setting to assure the cooler and it's lines are full. 6L45 has no TBV so they don't mind the hot check being lower.

Now...it this is a sticking point there is a simple solution. If you've installed the 70C thermostat and remain concerned then check the fluid at the old temperature 195 F. It will get there sitting still. Takes a while but it will get there.

Before all this TBV nonsense we filled it to the cold mark and a splash then drove an hour. Parked and checked it hot. Adjust. Done.

16LT4 · March 18, 2022

24 minutes ago, Grumpy Bear said:

Good question and one worth a look into the book for.

Here's a thought. I replaced my 6 quart pan with an 8 quart pan. I add two quarts to reach full. What does that tell you about the fluid IN the transmission? It's a 12.6 quart system stock. Now 14.6 and yet when you drain the pan you get only 8 quarts out. Use to be 6. What you are checking is the level in the pan, not the transmission volume. Transmission is a hydraulic system that only works when there is no air in the system. The cold check is an initial fill that is high enough to satisfy the system being void of air. The pan volume assures no air ingress. Remember these checks are done with the transmission pump running and after all ranges selected on level ground to draw that level to it's lowest point....when hot.

In your 6L45 (same as the box in my Buick) running warmer than the hot check does not give a 'overfull' Just a higher level that hurts nothing. In the 6L80E they want that temperature to exceed the TBV setting to assure the cooler and it's lines are full. 6L45 has no TBV so they don't mind the hot check being lower.

Now...it this is a sticking point there is a simple solution. If you've installed the 70C thermostat and remain concerned then check the fluid at the old temperature 195 F. It will get there sitting still. Takes a while but it will get there.

Before all this TBV nonsense we filled it to the cold mark and a splash then drove an hour. Parked and checked it hot. Adjust. Done.

Not an overfill, but an underfill. If the fill plug is removed at operating temperature of 80-90*C fluid will pour out. If left to cool back down to 30*-50*C, one would be able to add more ATF (in my experience servicing those transmissions, up to 0.5 quart). If that level as per the fill hole is perfect at 30*C, and then checked again at 80*C, fluid will be lost. The 6L45 in my application DOES have a TBV/thermostat... I've replaced them because they typically shatter when replacing the expansion tanks. They DO behave differently based on the half quart difference, and improper filling temperature is a common cause of low fluid levels in that application.

So, all else being equal, the fluid level in the pan rises and falls with temperature due to thermal expansion of the ATF. That's why the dipstick has (or had) cold and hot mark lines. The larger pan doesn't mean anything, as that just lowers the sea floor... sea level remains the same when the volume difference is added.

I doubt the new TBV makes any difference with these transmissions as possibly impacted by slight deviations in ATF level, as they aren't entirely made of glass, and this could all be a thought exercise. If anything, checking at 130-150* allows for slightly more fluid to be added prior to a level so high as to induce aeration. Raising the fill plug in the 6L45 slightly to read operating temperature would take the guesswork out of that operation, but it probably is as it is to avoid service possibly resulting in being rained on by near 200* ATF.

Edited March 18, 2022 by 16LT4

Grumpy Bear · March 18, 2022

10 minutes ago, 16LT4 said:

Not an underfill, but an overfill. If the fill plug is removed at operating temperature, fluid will pour out. If left to cool back down to 30*-50*C, one would be able to add more ATF (in my experience servicing those transmissions, up to 0.5 quart). If that level as per the fill hole is perfect at 30*C, and then checked again at 80*C, fluid will be lost. The 6L45 in my application DOES have a TBV/thermostat... I've replacedthem because they typically shatter when replacing the expansion tanks. They DO behave differently based on the half quart difference.

So, all else being equal, the fluid level in the pan rises and falls with temperature due to thermal expansion of the ATF. That's why the dipstick has (or had) cold and hot mark lines. The larger pan doesn't mean anything, as that just lowers the sea floor... sea level remains the same when the volume difference is added.

I doubt the new TBV makes any difference with these transmissions as possibly impacted by slight deviations in ATF level, as they aren't entirely made of glass, and this could all be a thought exercise. If anything, checking at 130-150* allows for slightly more fluid to be added prior to a level so high as to induce aeration. Raising the fill plug in the 6L45 slightly to read operating temperature would take the guesswork out of that operation, but it probably is as it is to avoid service possibly resulting in being rained on by near 200* ATF.

Okay so that brings it full circle. What does the book say the conditions for the check should be? Do that.

My point about the larger pan was that the more than half of the volume that doesn't drain remains constant and unaffected by the pans level UNLESS it uncovers the pump suction. The level checks are to prevent that from happening. That's all.

Your 45 has a TBV. Interesting. Now I'm going to have to look next time I service mine. I love learning new things. Thanks!

Edited March 18, 2022 by Grumpy Bear