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Automated Transmission Fluids

There are certain things that improve with age, such as classic cars, cheese, and wines that don't come in boxes with plastic spigots.


There are certain things that improve with age, such as classic cars, cheese, and wines that don’t come in boxes with plastic spigots.

It shouldn’t be too surprising that transmissions don’t make the list. After all, their seals can shrink and harden over time, leading to potential leaks. Varnish can glaze gears and friction plates, and sludge will build up in the hydraulic system and pan, leading to excessive wear, rough shifts, slips and shudders.

It’s up to Automated Transmission Fluids (ATF) then to act as the first line of defence.

That’s no easy task. Modern equipment designs require chemical concoctions that can handle ever-higher power densities and operating temperatures. But while new ATF formulas have met the demands – and last longer than ever before – an unwelcome byproduct has accompanied improvements over the past 10 years.

It’s the matter of choice

Most fleet shops were able to get by with a couple of types of fluids a mere decade ago, but they now have to choose from six distinct classifications, explains Tom Hansel of ChevronTexaco. The selection is even wider when you consider engine oils that can be used in their place. Certain 10W30 and 15W40 heavy-duty engine oils will meet the Allison TES 228 (C4) standards for AT, MT, and HT Series transmissions, but they won’t meet the company’s TES 295 standard for 1000/2000 Series transit buses. Other oils won’t meet any of the standards.

It can be an intimidating choice for mixed fleets.

“It’s just getting out of hand,” a Ryder maintenance supervisor said during a recent Technology and Maintenance Council meeting on the issue, referring to the need to educate technicians to match the right bottle to the right components, and stock a variety of fluids in every shop.

A decade ago, if you had a product that would meet General Motors’ Dexron III, Mercon and Allison C4 standards, all you needed was another bottle capable of meeting Ford’s Type F requirements, Hansel admits. “Life was relatively simple.”

But the differences in modern fluids involve more than the names on the bottles.

If you expose a clutch pack designed for ATF+3 to a Dexron III fluid, the result will be harder shifts, Hansel says as an example, referring to a bump in the friction curve – a diagnostic chart that shows torque levels at specific points in time. And he questions those who claim that a simple additive will offer a one-bottle-fits-all solution.

“What you haven’t done is maintain the same capacity as the transmission was designed for,” he says. “When you start putting other products into this transmission, you’re not going to get optimum performance. You’re going to see it in the shift field, or you’re going to see it in long-term torque transmission and capacity.”

A 15 to 20% drop in torque that accompanies an easier shift can lead to excessive wear and heat, he adds. Transmission sumps run at temperatures from 175 to 220 Fahrenheit (79 to 104 Celsius) during normal duty cycles, but hotter temperatures can lead to dramatic cuts in ATF drain intervals.

Each fluid exerts a force to move transmission components, while lubricating parts and cooling the clutch, torque converter and retarder. But all things change with time. The viscosity of an ATF can thin out with use, reducing the protective thickness of the lubricating film, and leading to the damage associated with higher heats. A fluid that’s allowed to oxidize will become thicker, sticking valves and leading to a loss in clutch friction despite the variety of filters that are meant to keep everything clean and free of debris.

“The (ATF’s) friction modifiers and the additives are specific to the friction material,” adds Vasu Bala of Cognis Synlubes Technologies, referring to the tight link between fluid formulas and the components they’re engineered to protect.

The biggest difference in individual formulas usually relates to these friction characteristics.

“Allison is very particular about what fluid is recommended in its whole series of transmissions,” he says as an example. “It’s really a safeguard approved by the OEM to ensure the right quality of ATF goes into the right transmission.”

About 90% of a barrel of ATF comes from base oils ranging from mineral oils to synthetics, each of which offers its own properties – synthetics, for example, flow better at low temperatures. Then each type has its own way to battle oxidation (measured by the Total Acid Number), deal with seal compatibility, change viscosity indexes linked to different temperatures, and ensure the thickness of the protective film.

The remaining 10% of the barrel comes in the form of a performance additive to enhance the base oil and improve such things as control over wear, oxidation, rust and corrosion, minimize foam, improve flows at low temperatures, and even offer a distinctive color.

More than half of the transmissions on the market can use an ATF that meets Dexron III and Mercon standards. But about a quarter of them are met by a dizzying array of fluid requirements associated with Ford F, Allison, Mercedes-Benz, Asian-based ZF, Isuzu, Nissan, Toyota and Honda products.

It’s that wide proliferation of standards that has led to a large number of different fluids. An ATF manufacturer that needs to pay anywhere from $50,000 to $500,000 for a test to confirm that its formula meets a specific standard may not bother checking for compatibility with standards that serve smaller market segments.

“At some products, you look at your development costs and say there’s no way I can recover these in the marketplace,” Hansel explains.

So one fluid will not fit all.

The standards have undoubtedly come a long way since General Motors incorporated one of the world’s first ATF formulas in the late 1930s, using a simple mineral-based lube that was essentially a hydraulic oil mixed with antioxidants and thickeners. But despite continuing advancements, don’t hold out much hope for a truly universal ATF, Hansel adds. “Some of the specifications are actually mutually exclusive, so I wouldn’t hold my breath”.

Any hope for a universal fluid would require OEMs to share the research and development into friction materials that are used in clutch plates, Bala agrees. “If you do look at that and you factor that vein into whether one OEM is going to share that, the answer most likely is going to be a resounding ‘No’.”

While there have been discussions about whether the Society of Automotive Engineers could develop a common specification, Allison Transmission’s Tom Johnson doesn’t see that happening. “We’re trying to maintain the control of that oil just as if it was a clutch plate,” he says.

The ultimate goal is to develop a “shift-for-life” transmission that works much like a “sealed-for-life bearing,” he adds of future advancements that are more likely. “That’s kind of pie in the sky, of course, but that’s where we’d like to get to.”


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