The F4A51 is a 4-speed automatic transaxle featuring INVECS II technology. INVECS is intended to learn from the driver and adjust shift points accordingly.
It is used in the Third Generation Magna/Verada from E through J series. For the TH Magna, the F4A51 was available with "sports mode" or tiptronic function on sports models. The sports mode F4A51 became available on the KJ Verada before being superseeded by the F5A51 in the KJII.
Mechanically, the F4A51 is much simpler than the transmissions used in previous models. The F4A51 uses a two shaft design (instead of three in the previous series) and doesn't reverse the torque before output. On earlier Magna/Veradas the torque had to be reversed because the engine rotated in the opposite direction to the wheels.
For ease of service the F4A51 also introduces an external oil filter much the same as the engine oil filter.
Electronically, the F4A51 is much more advanced than the transmissions used previously. Instead of only monitoring road speed, the transaxle control unit also monitors the speed of the input and output shafts. The transaxle control unit also adjusts shift patterns by "learning" how the driver drives.
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