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Aisin AX manuals (AX-4, AX-5, AX-15)

The Cherokee's Japanese manual family

The Aisin AX family was the backbone of Jeep's manual gearboxes through the 1980s and 1990s. Three members with the same Japanese blood: the four-speed AX-4 (rare, from the early years), the five-speed AX-5 for the small engines, and the beefed-up five-speed AX-15, the one paired with the 4.0. Strong and pleasant to shift, they're the manuals of choice for anyone building an XJ in earnest.

The AX family at a glance

Aisin —the Japanese house within the Toyota group— supplied Jeep with three related manuals. The AX-4 was a short-lived, low-volume four-speed, fitted in the early years behind the 2.5 and the GM 2.8 V6. The AX-5 added a tall fifth and stayed on as the four-cylinder's manual for nearly two decades. The AX-15, physically larger and stronger inside, is the one that earned the reputation: the straight-six's manual.

Reading the table closely: the AX-15 has a slightly taller first (3.83) and, above all, a shorter fifth (0.79 versus the AX-5's 0.85), tuned for the six's torque. The AX-5 stretches taller up top because the 2.5 needs revs.

AX-15 vs. AX-5: which is which

The XJ ran both five-speeds side by side. The lighter AX-5 sat behind the 2.5; the beefier AX-15 behind the 4.0. If you want an XJ to build on, the AX-15 is the one: it takes far more abuse. And watch out —they're easy to mix up. A few ways to tell them apart:

• Center (mid) plate: the AX-15 uses aluminum; the AX-5, cast iron. A magnet sticks to the AX-5 plate but not the AX-15.
• Size and ribbing: the AX-15 case is physically larger with heavier ribbing, especially around the main body and tailhousing.
• Input shaft: the AX-15's is larger in diameter.
• Application: if it came from the factory behind a 4.0, it's almost certainly an AX-15.

Toward the end of the XJ run (2000–2001) the NV3550 replaced the AX-15, but the Aisin left a reputation that's hard to match.

Aisin lineage and what it swaps with

Here's the most-repeated myth and its truth: «the AX-15 is a Toyota gearbox». It's an Aisin(-Warner) design —and Aisin is part of the Toyota group—, from the «AR» family: the same base Toyota used for its R-series (the R150F/R151F in the Hilux, Pickup and 4Runner, and the R154 in the Supra Turbo), and that GM rebadged MA5 and Isuzu AR5. They share the case architecture and the bellhousing-to-body bolt pattern. But they're not interchangeable twins: the Toyota input shaft is too short for the AX-15 bellhousing, and helix angles and ratios differ. Cousins from the same family, not the same part.

What does it actually interchange with? The same AX-15 is fitted to the Dodge Dakota with the 3.9 Magnum V6 (2WD ~1992-99, 4x4 ~1994-99). And the NV3550 that replaced it in the TJ and the 2000 XJ is a near-direct swap: they share the nine bellhousing bolts, the clutch fork and throwout bearing, the input-shaft length and the 0.750" pilot snout. Within Jeep the AX-15 was everywhere: XJ, MJ Comanche, YJ, TJ and the ZJ (1993-95), always behind a straight-six (the 4.2 and the 4.0).

A fact to bust another myth: the AX-15's bellhousing behind the 4.0 uses the AMC straight-six pattern —the same as the 258 and the AMC V8s— and it didn't change from 1987 to 2006, not even when Chrysler took over. There's no separate «Chrysler bellhousing» for the 4.0. What did change was the clutch: an internal hydraulic throwout bearing (1988–1993) and a conventional external slave (1994 onward). Don't cross boxes between engines: the 2.5's AX-4/AX-5 use the GM 2.8 V6 pattern, different from the straight-six's.

Strengths, faults and maintenance

The AX-15 is the tough one of the family: well maintained, it handles considerably more torque than the 4.0 makes stock, which is why it's sought after even for swaps. The AX-5, tied to the 2.5, is more fragile: it doesn't like abuse or big tires, and worn second-gear synchros and bearing noise are common on high-mileage units. The four-speed AX-4 is more of a collector's oddity than anything else.

Key maintenance is simple and cheap: the right fluid, on schedule. The AX boxes want a specific lubricant (Dexron-type ATF or the manual-transmission fluid the manual calls out for the year —not just any heavy gear oil); the wrong fluid shortens synchro life. Keep the clutch and slave cylinder healthy and that's it: cared for, these gearboxes last decades.

Specifications