Aisin AX-15

Aisin's five-speed manual for Jeep — in the Cherokee XJ, Grand Cherokee, Wrangler YJ and TJ, and the Dodge Dakota.

It is the gearbox behind the 4.0 straight-six, which is most of its standing: a very durable engine in front of a very durable transmission, and an enormous number of them still in daily use.

Its one recurring fault is the fifth-gear synchro and its shift fork, presenting as a grind into fifth alone, or as fifth popping out under load. That is close to diagnostic — a gearbox that jumps out of one specific gear is a fork or a worn hub, not a general wear problem — and it is repairable without touching the rest.

It takes automatic transmission fluid, not gear oil, which is the trap. Filling one with a conventional 80W-90 makes it notchy when cold and hard to select second, and the symptom reads as synchro wear in a gearbox that is fine. The same mistake as the NV3500's, in a different marque.

The AX-5 is the four-cylinder version — a lighter gearbox, not a variant, and its parts do not interchange.