What happens to Transmission Fluid When It Gets Hot?

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I always wondered why there is a cold and hot markings on transmission fluid dipsticks.
What happens to the fluid when it gets hot? Does something happen to the properties of the fluid itself, or does the fluid just get moved from the pan to other internal components when hot after running and settle to the pan when it stops running, thus changing the level of the reading?
 
but the basic answer is transmission fluid expands, as mentioned above.

some vehicles you check with engine running, and others with it shut off.
 
The fluid expands and you have to account for the fluid not being settled in the bottom of the pan. It will fill the torque converter and the case. Also you are supposed to shift through all of the gears to allow the valve body, if applicable, to fill.
 
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.
 
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.

How would that happen? The specific heat capacity is an intrinsic material property of the hydrocarbon, I don't know how that would change.

it would make more sense that the internal surfaces get deposits of some sort which inhibit heat transfer, but that wouldn't necessarily be reversed with new fluid.
 
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.

Which is why I've always thought it was a good idea to keep transmission fluid in good working condition. Normal wear and tear will eventually kill any transmission, but I don't want fluid to be the cause.
 
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.

Is that from the old school recommendations or from new fluids? My truck will bring the ATF to 195F without doing anything other than gentile highway driving. Get the TC unlocked and pulling hills, and I've seen 230F if I let it (usually I kick down a gear to prevent this). That's convertor outlet, so what goes to bearings and whatnot is cooler--but this is the temp my fluid would see if I just drove it as Toyota intended (towing and leave it in drive and letting it pick a gear).

Many transmissions have a loop in the radiator so as to bring ATF and coolant to around the same temperature.
 
That's the old school number, I know ATF+4 (a full synthetic) starts to degrade above 280F, that's when the warning light on my Ram comes on. I've never seen it above 220F on the Scangauge, even when towing in hot weather (no big loads nor mountains, though). Cooler is always better, IMHO.
 
Silverado 1500 has a 180 degree trans thermostat that bypasses the cooler. Idea is to get the temp up to operating more quickly probably for .1 mpg fuel savings. some people mod the thermostat Open all the time. The known problems of the 8 speed TC seem to be resolved by new fluid flush. This to me indicates the heat is causing some breakdown to occur. the 2500 and 3500 do not have the trans t-stat.
 
Apart from the breakdown of oil from heat, its also affecting other components in the transmission if its faced on daily basis. Old school or not even with fully synthetic ATF I have always found shift quality to be far better and smoother if oil temps are kept below 180 especially in traffic or uphill climbs on hot days were shift quality begins to deteriorate with the oil heating up.
 
We have group II on up base oils these days . The old temp charts are so out dated.
 
Originally Posted by Kawiguy454
Silverado 1500 has a 180 degree trans thermostat that bypasses the cooler. Idea is to get the temp up to operating more quickly probably for .1 mpg fuel savings. some people mod the thermostat Open all the time. The known problems of the 8 speed TC seem to be resolved by new fluid flush. This to me indicates the heat is causing some breakdown to occur. the 2500 and 3500 do not have the trans t-stat.


But there's no way Dex VI is breaking down at 180-200F operating temps unless these vehicles have somehow accumulated 6 figure mileage.
 
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.

Quote
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
 
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My 2008 Silverado 3500HD is usually around 150 when warmed up during the summer and has never gone over 180 when towing. In the winter it may stay under 130. Probably why it's gone over 200K so far.
 
You could measure the fluid either hot or cold on the dip stick with the engine operating. The cold operation will be lower on the stick.

You can also measure your fluid level dead cold with the engine off. On my vehicle it's much higher up on the dip stick - well above the "normal" range. I have it marked on the word "Cross" of "cross hatched area." And it's always at that same point with the engine off and cold. So that's a quick way to check level when you don't want to go through the time of a full heat up and cycling the gears.

I've done it all 3 ways and have a pretty good feel when it's dead on. Same transmission type (AX4N) on 3 different vehicles for past 18 yrs.
 
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Originally Posted by Kawiguy454
Silverado 1500 has a 180 degree trans thermostat that bypasses the cooler. Idea is to get the temp up to operating more quickly probably for .1 mpg fuel savings. some people mod the thermostat Open all the time. The known problems of the 8 speed TC seem to be resolved by new fluid flush. This to me indicates the heat is causing some breakdown to occur. the 2500 and 3500 do not have the trans t-stat.

My 2017 3500 HD has a trans thermostat.
 
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 (flash) higher temperature spikes.

Quote
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





Thats why the AISIN brochure even for their synthetic ATW+ is recommending full flush at 20000 rather than the usual lifetime.
 
Due To Low Transmission Fluid, it Becomes hot, Hot Transmission Fluid Not acceptable, It causes fails the transmission gearbox, Vehicle Speed Becoms Slower, Abnormal Noise Comes trough the Transmission Gerar Box.
 
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