Five-Speed For Four-Banger
Got an '87-'02 four-cylinder Wrangler? Chances are you're pretty happy with it. They all came with factory fuel injection, enough power for spirited driving with moderately-sized tires, and they're cheap to buy and build. Aside from that Dana 35 rear axle, there's just one fly in the ointment: the weakling AX5 transmission. Even with stock-size tires and on-road driving, many start grinding gears after a couple of years. Throw bigger tires and hardcore off-roading into the mix and they'll go boom if you even look at them hard.
If only there was a way to easily run the bigger, stronger AX15 five-speed transmission from a 4.0L Jeep. Sure, some 2.5L Dakota pickups used the AX15, and the swap can be done if you're lucky enough to stumble onto a Dakota 2.5L AX15 bellhousing. But that's not gonna happen too often for most of you, which is probably why Advance Adapters has been doing so well with its simple AX5-to-AX15 conversion.
Advance Adapters offers two bellhousing adapter kits to mate the stock 2.5L AX5 bellhousing to the stronger AX15: PN 712563 is for the '95-up external slave cylinder bellhousing, while PN 712564 is for the '87-'94 internal throwout bearing models. Most smart '87-'94 Wrangler owners will take the tranny swap as an opportunity to rid themselves of the problematic internal throwout release bearing by swapping to a later-model bellhousing and throwout bearing fork, which is why we did the opposite and ordered up the PN 712564 internal release kit.
And naturally, as with anything we do, we had some problems. Our first issue arose when we just couldn't locate a new or used AX15 transmission anywhere. Finally, we checked Quadratec which had brand-new '95-'99 AX15 transmissions sitting on the shelf calling our name. Along with our AX15, we ordered a B&M short-throw shifter, the correct 10W30 Mopar oil for the AX15, and a new internal throwout release bearing. Which brings us to catch number two.
As it turns out, although Advance Adapter's conversion for external slave cylinder bellhousings (PN 712563) was selling like hotcakes, its conversion for the internal release bellhousing (PN 712564) was so unpopular that we received the very first pre-production unit. There were some bugs that didn't manifest themselves until after the transmission was installed into the vehicle-and began leaking all over the place from the adapter plate. With story deadlines looming large, we did what we normally do when our bacon is burning in the pan. We headed down to Mechanically Inclined Technicians (MIT) in El Cajon, California, so that they could tear our Wrangler back down and figure out why our tranny oil was all over the garage floor.
In all honesty, we thought it was just a bad input seal when we brought our Wrangler to MIT. We had no idea we were setting the technicians up for a full day of problem solving, but they're not kidding when they call themselves driveline experts. MIT deals with dozens of engine, transmission, and axle swaps every month and quickly discovered the problems with our pre-production adapter and got them fixed. Ironically, Advance Adapters discovered the same thing when conducting tests with its own pre-production unit. When we called the company with our findings, we learned that it had already implemented the fixes and ensured all of its production adapters were perfect.
So how does it drive? Other than the B&M short-throw shifter offering up firmer and much crisper shifting, we'd be hard-pressed to notice any difference from the old AX5. In the real world, the AX5 gearing of 3.93, 2.33, 1.45, 1.00, 0.85 (gears First-Fifth) differs little from the AX15's 3.83, 2.33, 1.44, 1.00, 0.79 ratios. The Jeep accelerates, downshifts, and cruises just like before, but now we've got a transmission that will last for eons behind our little 2.5L.