Hydraulic oil types - Effects on the AL4 (questions) :)

[spider]

I was really just after an article or something similar, please bear in mind I'm not a scientist so a highly technical article on fluid dynamics will melt my brain.

I'm just slightly curious as to why the correct type of ATF is critical for certain transmissions.

For instance, what *actually* happens if you drain (fully as in T.C drained too) and then refill an AL4 completely with something like Dex2 or 3 ?

Does it function normally for a while or are jumpy changes noticed straight away due to the differing pressures being applied by fluid pressure to the bands ?

I can't see it having no drive at all though.

I probably should know but I'm struggling to think what is actually different in the special oils. Normal ATF types do provide 'some' lubrication quality so items such as shaft and diff bearings would not suffer I guess. I'd suspect (I've not checked) the major differential components (crown/pinion) in the AL4 are not that far off the ones in the 4HP20 unit (the mechanical predecessor to the AL4) if not the same apart from a ratio change possibly.

Is it its viscosity when hot / cold or its reaction to different materials (brake band material types) or its behaviour under extreme pressure ? (I'm thinking in the torque converter mainly)

Just questions from the highly curious (aka: me) that is all.

[xantia_v6]

All automatic transmissions have clutches or bands that transmit power, or apply braking action. In most cases these clutches are designed to take-up gradually and slip a little to avoid jerky operation of the transmission.

To allow the correct and predictable amount of slippage in the clutches, the transmission fluid has things called friction modifiers added, which make it less slippery than pure oil. Different ATFs have different viscosity and different friction modifiers, and the clutches in the transmissions are designed for the characteristics of the ATF.

A old world example is the Borg Warner transmissions used in Jaguars of the 1960s and 70s. These were made under licence to Ford (USA) and were designed for type "F" fluid, which looks, feels and smells just like Dextron II. But Dextron II has different friction modifiers which allow the transmission bands to slip a little more, which gives smoother gear changes than the original fluid, but unfortunately causes the bands to wear out more quickly.

With modern electronically controlled transmissions, life is more complicated, the ECU will detect that the characteristics of the gear changes have changed, but the software assumes that it is due to component wear, and may not make optimal corrections.

Whether the life of a transmission is actually affected is hard to predict, but anecdotal evidence is that a number of 4HP20 transmissions that have failed have been filled with Dextron.