Aisin K-series CVT (K41 / 773K)

Aisin's K-series belt CVTs — in the Toyota Aygo, the Scion iQ, the Yaris and the small cars around them.

⚑ These are not Toyota's Direct Shift CVT, which adds a mechanical first gear and is a different design with its own behaviour. A reader comparing one to the other will find the K-series slower to move off and conclude it is faulty; it is not, it simply has no launch gear.

A belt CVT's health is the belt and the pulleys, and both are decided almost entirely by fluid. Aisin's CVT fluid is specified and is not ATF — the substitution is behind most reported juddering and slipping, and it is not always reversible by the time the symptom appears.

Shudder under light throttle at low speed is the standing complaint, and on a healthy one it is usually the torque converter lock-up rather than the variator.

They are fitted to very small cars and are correspondingly lightly built; towing or sustained heavy load is outside what they were designed for.