Automatic Transmission Fluid (ATF) performs many functions in the transmission.
1. Hydrodynamic torque transfer is transmitted through the viscosity of the ATF. The viscosity
(or density) must be maintained for proper torque converter operation. This means that ATF must
have the proper viscosity index with a high shear strength.
2. Hydraulic pressure (250 psi) is required to engage drive clutches, operate bands, and to
open shift valves. ATF is pressurized and carefully regulated to provide this pressure. This means
the ATF is used as a hydraulic medium and must have qualities that prevent compressibility.
3. Lubrication is essential for bearing and gear life. Proper lubrication accounts for 20%-30% of
gear life and flow is directed at incoming mesh. This means ATF must have adequate anti-wear
qualities.
4. ATF must be able to carry and transfer heat for proper transmission operation. Proper
cooling accounts for 70%-80% of gear life, and flow is usually directed at output mesh. Clutches and
bands are cooled by ATF as well. Torque converter operation creates excess heat, which needs to
be dissipated through the cooler. This means ATF must have a sufficient anti-oxidation/anti-foam
package.
5. There is almost a zero tolerance for deposit buildup in the transmission for proper operation.
Deposits in the valve body or fluid ports can affect hydraulic functions, proper cooling and the overall
shift ability. This means ATF must have sufficient detergent-dispersant additives.
6. Friction response at the clutch materials is vital for proper shift quality and torque converter
lock-up operations. Special friction modifier additives are required to maintain a proper coefficient of
friction at all of these clutches.
7. One last property of ATF is the ability to change color and odor when additive levels have
been depleted. This is what explains the red color.
It is the multi-function properties in ATF that makes it special for automatic transmissions.
Each function requires a specific additive, which is why ATF is about 40% additives. Engineers have
told me that there can be as many as six different friction modifiers in ATF. Complicating everything
is the long drain life. This gives you an idea of how difficult ATF is to develop.
Question #2: A. Why do different manufacturers require OEM fluid?
B. What is the primary difference between the various ATF’s?
Auto manufacturer’s specific ATF’s and various aftermarket ATF’s are classified by frictional
properties or frictional response. Automatic transmissions/transaxles have three specific areas the
clutch/frictional material is present:
1. The direct clutches
2. The band brakes/clutches
3. The torque converter lock-up clutch
(When you think of clutch material, think of a brake pad’s friction material. The material will
eventually wear out and can have various friction coefficients with a rotor when a certain amount of
hydraulic pressure (brake pedal) is applied)
When ATF is engineered for a particular manufacturer, the friction modifiers are
developed for the unique friction material used in their transmissions, bands and clutches.
(Again think of brake pads. Ford uses Ford brakes; Nissan uses Nissan brakes, etc. They can be
completely different friction materials. The same is true for transmission clutches. The primary
difference is the roughness of the surface and the surface area, which can change the coefficient.)
The fluid is designed to keep a specific clutch at a specific coefficient of friction, at a specific
temperature. If the specifications are off too much, the shift quality can be affected. Too much
additive and the clutches can slip during engagement. Not enough and a hard shift can result.
(Back to the brakes. If the pads were coated with oil, the pad would slip before it could build up
enough heat to grab the rotor. The exact opposite would be true if the pads were coated with
something that locks the brake rotor.)
Multiple friction modifiers in a specific ATF are a result of a transmission having multiple types of
friction material in the three specific areas. Other additive packages (anti-oxidant, detergent,
dispersant, etc.) are important but usually not the decisive difference.
Multi-purpose ATF’s are engineered with friction modifiers that are compatible with all kinds of friction
material surfaces. These “universal fluids” help to simplify the increased diversity of ATF
specifications.
As far as quality is concerned for a particular fluid, longevity seems to be the key. The life of the
bearing, gears, clutches, bands, valves and torque converter can usually be extended if the fluid can
maintain a “consistent” coefficient of friction, ability to carry and transfer heat and ability to perform
hydrodynamic and hydraulic function.
Question # 3: Why are there no fill-tubes or dipsticks in some newer vehicles?
In certain new GM and Ford vehicles (there are others) there is no way to check the fluid level or add
new fluid without now raising the vehicle and finding the fill plug, which is usually positioned at the
level the transmission is filled to. Some newer Ford Explorers even require a special tool to add fluid
through the transmission oil pan.
This is an additional step for these manufacturers to press their “fill-for-life” 100,000 fluid
agenda. The idea is after the 100,000 miles both transmission and fluid are closer to failure.
This is primary to reduce costs by reducing the number of transmissions replaced under warranty.
Additional costs can be reduced by eliminating the fill-tube assembly. This also makes it harder for
quick-lube type places and independents to service the transmission.
Question #4: Why is ATF temperature being regulated so precisely?
ATF is designed to operate at a specific temperature (175o–190oF approximately). In certain
vehicles (mostly Fords) the ATF temperature regulator is designed to permit flow to the cooler at this
optimum operating temperature. This helps eliminate cold weather/cold shifting problems.
Question # 5: What causes hard shift/soft shift?
Hydraulic pressure needed to engage clutch pack piston or servos to engage band brakes is
generated by a fluid pump in the transmission. The fluid pump (which is driven by the torque
converter) usually produces adequate pressure to perform hydraulic function around 800-rpm
engine speed. At 4000-rpm engine speed, the pump produces way too much pressure and requires
regulation down to 250 psi. This pressure is constantly adjusted so there are times when
(approximately) 200-300 psi is present. Any excess pressure is routed back to the fluid pan.
During a shift, hydraulic pressure is routed to a specific shift valve, which opens that valve. When
the valve opens, ATF ports are opened and the pressurized fluid is allowed to flow to a specific direct
clutch, engaging that clutch. When the pressure is released off the shift valve, a valve spring returns
the valve back to its original position. This closes the port again closing off flow and rerouted to
another shift valve or back to the pan. The valve springs can vary in size and spring rate, requiring
various amounts of pressure to open them. Additional regulation is required for that purpose.
It is the amount of pressure that is ported to a specific clutch, when the shift valve is
opened, that determines shift quality or shift feel. Since this pressure is constantly being
regulated, approximately any pressure over 250 psi is leaning toward a hard shift engagement and
under 250 psi is leaning toward a soft shift engagement.
It is this variation in engagement pressure that allows a firmer shift to be engaged when a vehicle is
under heavy load, high rpm or WOT operation. When a vehicle is under light load, low rpm or partial
throttle operation a softer shift is required.
The accuracy of the pressure directed to the clutches under various engine conditions is not always
optimum and shift quality can be affected. This effect can be amplified when ATF starts to lose its
friction response properties.
From a component longevity perspective, a firm shift puts less wear/stress on the clutch packs. This
is because a quicker engagement creates less heat.
When the clutch material wears, the clutches will usually start to slip and eventually will be unable to
transmit power to the output shaft.
Question #6: What is torque converter shudder?
The torque converter replaces a clutch in a manual transmission and connects the engine to the
input shaft of the transmission. There is no mechanical link between the two. Torque is transferred
through the ATF by hydrodynamic force.
(Think of two fans facing each other about a ¼ inch apart. One fan is spinning under power and the
other is not. The spinning fan creates a force to spin the other fan even though the fans are not
connected. That is a simple form of torque transfer. If the two “fans” are encased and filled with
fluid, the torque would transfer through the fluid.)
The impeller is the name for the “fan” connected to the engine. The turbine is the name for the “fan”
connected to the input shaft. When a vehicle is moving in gear under load, the turbine is not
spinning as fast as the impeller because the two are not connected. This results in a loss of
efficiency.
A torque converter lock-up clutch “locks” the turbine to the engine during specific driving conditions.
This increases efficiency by allowing the impeller and turbine to rotate at a 1 to 1 ratio. It is usually
only engaged under low torque conditions. The clutch would slip under high torque conditions.
Shudder or vibration can be caused by harsh engagement or disengagement of the lock-
up clutch. This can be the result of a clutch engagement/disengagement under the incorrect
operating conditions or incorrect hydraulic pressure. ATF without the proper friction response
properties can wear clutch material, which can also cause shudder.
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