Aisin AF33 (AW55-50SN)

Aisin's transverse five-speed — sold to almost everyone. GM used it as the AF33 in the Equinox, Epica, Vue and Ion; Saab fitted it to the 9-3 and 9-5; Nissan used it in the Altima; and Volvo, Renault and Opel all bought the same unit under their own names.

Its failure is specific, well understood and usually caught too late. The valve body wears its bores, particularly around the solenoid that controls line pressure, and the transmission slowly loses the ability to hold a clutch pack firmly. The symptom sequence is the useful part: a slightly delayed engagement into drive or reverse when cold, then a flare on one particular change, then slipping. Nothing about the early stage feels like the start of a failure, which is why these are so often first looked at when they are already beyond a valve body.

Sleeving the worn bores is a recognised repair and considerably cheaper than a replacement, but only while the clutch packs are intact — which is the argument for acting on a delayed engagement rather than living with it.

The fluid matters more than usual because the whole failure is hydraulic: a fluid that has lost viscosity accelerates the wear that causes the pressure loss that damages the clutches.

It is not the TF-80SC. Aisin's six-speed replaced it in most of the same cars, and a Saab or Volvo of the mid-2000s may have either — the tell is the ratio count, and the two share no parts.