What happens to Transmission Fluid When It Gets Hot?

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Originally Posted by MolaKule
In transmission parlance we use two temperature definitions and one is "flash" temperature and the other is "Bulk" oil temperature in the sump.

A flash temperature is one that occurs for a very short time at clutch surfaces and can hit 500F, but this temperature only lasts for milliseconds because of continual cooling.

The bulk oil temp in the sump is essentially the average temperature of the "mix" of fluids coming in from the clutch packs, bearings, Torque Converter, and the heat exchanger (cooler).

When the ATF becomes degraded from being in the sump too long, or due to extreme heat cycling (insufficient temperature control), the fluid oxidizes, the dynamic coefficients of friction change, and the fluid may start leaving varnish deposits on the clutch faces. Hence, my recommendation for ATF OCI's of ~ 30-35k.

Now remember, cooling of the clutch plates and discs is accomplished only when oil flows into and out of the clutch material and faces. The clutch disks are faced with a porous material of a cellulose composite. That porous material allows oil to flow into it during disengagement, and squeezes some oil out during engagement.

Once varnish deposits are allowed to form a "glaze," no cooling oil can flow into or out of the porous material and it and the oil degrades even faster because of the higher (flash) temperature spikes.

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Oil flow is supplied to the clearance between the clutch discs and clutch plates through radial pathways on the hub for cooling and lubrication purposes.

During a clutch engagement, a piston compresses the plates. The oil between the plates is squeezed out of the interface and permeates into and or out of the pores in the friction material. Engagement is completed when no speed differential exists between clutch discs and clutch plates.


Internal Heat Generation in ATs


Entertaining presentation, Mola-so you're basically saying there's no such thing as "too big" when it comes to transmission coolers & sump pans, and even in arctic temps-cooler ATF is ALWAYS better?
 
Originally Posted by bullwinkle


Entertaining presentation, Mola-so you're basically saying there's no such thing as "too big" when it comes to transmission coolers & sump pans, and even in arctic temps-cooler ATF is ALWAYS better?
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No, I never said that nor was I inferring that.


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When the ATF becomes degraded from being in the sump too long, or due to extreme heat cycling (insufficient temperature control), the fluid oxidizes, the dynamic coefficients of friction change, and the fluid may start leaving varnish deposits on the clutch faces. Hence, my recommendation for ATF OCI's of ~ 30-35k.
 
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Originally Posted by MolaKule
Originally Posted by bullwinkle


Entertaining presentation, Mola-so you're basically saying there's no such thing as "too big" when it comes to transmission coolers & sump pans, and even in arctic temps-cooler ATF is ALWAYS better?


No, I never said that nor was I inferring that.


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When the ATF becomes degraded from being in the sump too long, or due to extreme heat cycling (insufficient temperature control), the fluid oxidizes, the dynamic coefficients of friction change, and the fluid may start leaving varnish deposits on the clutch faces. Hence, my recommendation for ATF OCI's of ~ 30-35k.
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Couldn't get a good front view but this is my 2 large transmission coolers mounted in front of the condenser on my 84 Cutlass. The transmission has about 270k miles on it and shifts perfectly still. I bought it at 115k miles and it towed a 3500 pound boat for the previous owner. I took it to the drag races whenever I could and drove it really hard for the first 10 plus years I had it.

I've started to drive it easier because I feel like my luck will run out eventually. I've broken a couple rear ends behind this transmission but the trans hangs in there.

30k mile flushes plus filter and the coolers are likely the only reason it made it this far.

IMG_20190729_203928.jpg
 
Originally Posted by oldhp
The ideal fluid temperature is under 175 degrees, but as fluid ages it starts to break down and lose its capacity to cool down the transmission. This is when transmission overheating occurs. At 220 degrees, varnish forms. At 240 degrees, seals start to harden. Anything higher fluid starts to burn, clutch plates warp, spools in valve bodies stick. You know, all kinds of bad $$$$$$ things.



Your statements are likely true if your talking about DEXRON III type fluids. Today many AT's the operating temps are 200F.
 
On my 80s toyota. When its cold the level is way beyond the top notch last mark, the 4th mark. When its hot and engine is running, ran through all gears slowly and put into P, its 3/4 up from 3rd notch, close to the last 4th notch, where its supposed to be (in between 3rd and 4th notch is the HOT reading).
 
But, just because the overall temp is XX degrees, that doesn't mean that in certain areas the trans fluid isn't hitting hotter temperatures, correct? Hot Spots.
 
Originally Posted by Gasbuggy
But, just because the overall temp is XX degrees, that doesn't mean that in certain areas the trans fluid isn't hitting hotter temperatures, correct? Hot Spots.



Thats the reason torque converter always reports higher temps than the pan temps.
 
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