Transmission Temperature — How Hot Is Too Hot

If you own a truck, chances are that you pull a trailer of some sort. That could be a fifth-wheel camper, gooseneck utility trailer, or even a bumper pull toy hauler. Whatever you are towing, it’s a good idea to make sure that load is not creating a situation that has you sitting alongside the road. One of those unwanted situations could have everything to do with your truck’s transmission temperature.

Gone are the days of the Turbo 400, C6, or 727 transmissions. Those old-school gearboxes were — by today’s standards — simple designs. Today’s automatic transmissions have their own computers, electronic solenoids inside the case, and converters that offer lockup capability. With all the technology found in these new six, eight, and ten-speed transmissions, one thing can still destroy them, overheated automatic transmission fluid (ATF). Ask about heat and transmission fluid on any internet forum and you’ll be sure to get a myriad of replies. That’s why, in order to find out how temperature can affect this lifeblood of the gear changer, I reached out to AMSOIL to get some much-needed information.

transmission temperature

Prolonged high temperatures will cause the transmission to break down and in turn, cause the clutches and frictions to slip and eventually burn up.

Transmission Temperature: Does It Really Matter

Getting right to the point, the ideal operating fluid temperature is under 190 degrees. I know, the OEs have transmission cooling that is connected to the engine cooling that is holding a comfortable 195 to 210 degrees Fahrenheit, so your transmission is already forced to contend with a certain amount of heat. But, as fluid ages, it starts to break down and when it does, it loses its capacity to help cool the transmission and ultimately protect the inner workings.

Search the internet for information about how heat affects transmission fluid and you’ll find many articles reporting that with conventional transmission fluid, at 220 degrees, varnish will form on frictions and hard parts within the transmission, and this will allow the clutches to slip and create even more heat. Then, at 240 degrees, seals start to harden and important additives in automatic transmission fluid begin to cook. When you really get the temperature up there (260 degrees), the transmission clutch plates begin to slip because the oil is completely breaking down. Finally, at roughly 315 degrees, seals and clutches effectively burn out and carbon is formed. When carbon forms in the oil, for all intents and purposes, the transmission is junk. A typical transmission will die within 2,000 miles if subjected to 300-plus-degree heat. But, does the same hold true for synthetic fluid?

Let’s Talk About Synthetics

Finding information about how heat affects a traditional transmission fluid is easy. Finding that same information about synthetic fluid and the answers are not so easily found. We asked Mark Nyholm, a mechanical engineer at AMSOIL to give us some insight into how increased heat affects synthetic transmission fluid.

“You ask about the survival temperature of synthetics and that temperature will be higher than conventional oil. But what’s most important is what happens to that oil when those temperatures are met and exposed for long durations. That’s what separates a high-quality synthetic transmission fluid from other less-quality products like conventional transmission fluids.”

transmission temperature

It sounds like synthetic fluid can survive higher temps, and that definitely gives it a marked advantage over conventional fluid, but how? “The effectiveness of the transmission is reliant on the fluid that’s in it,” Mark affirms. “This is not only from a gear-to-gear wear protection perspective, but also from the pressure the pump creates to hold the torque converter locked and the clutches together. As the temperature goes up, typically, the viscosity (thickness) of the fluid goes down. There will be a difference in the rate of viscosity change between conventional oils and synthetics. As the temperature goes up and viscosity goes down, so does the amount of pressure the pump can create. And because pump pressure ensures the friction materials in the transmission are held together and not slipping, it is very important to ensure proper viscosity to maintain good pump pressure.”

transmission temperature

Adding a quality synthetic transmission fluid and a deeper transmission pan will help reduce the overall fluid temperatures realized.

Mark summed up his reply by saying, “conventional fluid will see a reduction of viscosity beyond synthetics due to temperature, which is a function of apparent viscosity and viscosity index. The conventional viscosity will also decrease due to shear stress that is applied by the pump and gears in the transmission — more so than synthetics. So conventional fluids suffer from viscosity loss much more than synthetics and when they do, line pressure goes down allowing a reduction of holding force putting the friction materials in the transmission at risk of slipping creating glazed discs and worn clutches.”

Both conventional and synthetic transmission fluids will have equal ability to dissipate heat under normal conditions. However, since synthetic fluids will remain more stable — and retain viscosity — at higher temperatures where petroleum oils will begin to degrade, the win definitely goes to synthetics.

Transmission Temperature Vs. Viscosity

It is no secret that viscosity is an important attribute of the fluid’s total formulation, but so is the oil’s ability to resist oxidation and varnish formation. Transmissions need to remain clean or bad things happen with respect to thermal heat transfer, shift performance due to all the small clearances in the valve body, and the interfaces of frictions and steels holding together without slipping. Lesser quality transmission fluids will typically not be designed to manage the effects that heat applies to the oil for very long and when it runs out of capability, bad things happen.

“My question to you is what data or experience is available that suggests that 220 degrees Fahrenheit is the magic number with conventional fluid,” Mark asked. “That temperature will very much be dependent on the total formulation of the transmission fluid, more so than whether it’s conventional or synthetic. I have seen our fluids manage temps at about 250 degrees without issue. In short use cases, I’ve seen our fluids manage 300 degrees without issue.” “That said, it depends on the duration in which they are being tortured that really matters.  Staying at 300 degrees for prolonged periods will not work. Additives will get torched, and the fluid will start to crumble. Temps at 250 degrees are more manageable, and 220 degrees is no issue at all. But the success at those temperatures is about the total formulation and not necessarily just about the difference between conventional and synthetic base oil.”

Being able to monitor your transmission fluid temperature is one way to help keep it from overheating. The Edge CTS monitor in Project WorkHorse has the transmission fluid temperature showing on the main screen.

Measuring Heat, Twice

We cannot discuss transmission temperature without defining the two types of heat ratings, flash and bulk. Flash temperature is a measurement of a short instance. As an example, a clutch surface might hit 500 degrees Fahrenheit, but hopefully, it only occurs for a split second.

The bulk oil temp is the measurement of the sump/pan area. This is the average temperature of the “mix” of fluids coming in from the clutch packs, bearings, torque converter, and fluid 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.

Cooling of the clutch plates and discs can only occur when oil flows in and out of the clutch material and face contact areas. The clutch disks are a porous material usually made of a cellulose composite. This porous material soaks in oil that flows during disengagement, and then some of that oil is squeezed out during engagement. But, if the fluid temperature gets high enough to create varnish, the frictions glaze over and no cooling oil can flow into or out of the porous material. Without oil flow, the discs degrade exponentially fast.

Synthetic Or Conventional Fluid

We’ve talked about transmission temperature and keeping your automatic transmission fluid cool, but we must also talk about the ATF itself. Many years ago, ATF fluid was made from crude oil and the reshaping of hydrocarbons. Various manufacturers designed fluids to meet the specifications of different vehicles. These blended crude oils were very susceptible to heat and foreign contaminants.

If you would like to add an aftermarket gauge to monitor your “bulk” temperature, most aftermarket pans have a provision for adding a temperature sensor.

When synthetic fluids were introduced, they brought with them a variety of benefits. The first benefit is the ability to lengthen between-maintenance intervals. Let’s take for instance the service life of AMSOIL Synthetic ATF. When compared to conventional transmission fluid, it is up to five times longer than the service life of conventional ATF.

In fact, it is reported that AMSOIL Synthetic ATF can be in use for up to 50,000 miles in severe service and up to 100,000 miles in normal service, or according to the vehicle manufacturer’s recommended intervals, whichever is longer. What’s more, fluid change intervals may also be extended even longer if oil analysis/testing is done.

Heat is only one factor where synthetic transmission fluid is superior, and there are many reasons the big automakers have switched to using full synthetic ATF in their vehicles, and that’s because they work better, last longer, and offer more protection.

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Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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