A Little History
With the spirit of the Jeep Universal Truck still very much intact, the Jeep TJ has done Jeep aficionados generally proud by retaining its off road heritage, and doing so with a touch more panache and refinement than ever before.
The Jeep TJ Wrangler was introduced in 1997 after skipping a model year after the 1995 YJ.
It is back to round headlights with the TJ, but much more than that. A very major redesign - even more so than from the CJ to the YJ - the TJ got some dramatic changes that include a full coil and track arm suspension system, a low profile hood, and a more refined interior. However, what changed little was the powertrain. From the axles to the engine, the TJ relied on the same systems as were found in its YJ predecessor, which makes for an easier engine conversion than one might expect.
Factory Engines
The TJ Jeep has two engine options. The Powertech 2.5L four-cylinder returned in the Jeep, carrying its 1984 AMC heritage though with horsepower tweaked up slightly.
The 4.0L is a good motor but hardly anything to write home about. This 4.0 has the Mopar engine management system. Fuel economy for the best rendition of the TJ 4.0L I6 is 15 city/18 highway MPG.
Factory Transmissions
Six transmissions are available for the Jeep TJ. The AX5 five-speed manual transmission is standard with the 2.5L four-cylinder engine throughout the span of the TJ's production. This transmission is light-duty. They are known to fail even behind the anemic 2.5 I4. We neither recommend nor make any conversion to retain this transmission. The automatic transmission available with the 2.5L through is the Torqueflite TF904. This transmission has a better history than the AX5, but we still recommend replacing it when upgrading to any motor that is significantly stronger. These gearboxes that were available with the 2.5L are rated and proven as light-duty.
From 1997 the AX15 is the five-speed manual transmission as carried over from the Jeep YJ. This medium-duty gearbox remained as the standard shift transmission in the six-cylinder Wrangler through 1999.
For the 2000 model year, Jeep introduced the NV3550 five-speed manual as a replacement for the AX15. It is similar in appearance, strength and compatibility.
Automatics for the six-cylinder TJ is primarily the 32RH. These transmissions are adequate, but nothing to get too excited about, especially in comparison to the GM automatic transmissions. GM automatics are superior enough that it is a clear choice to make one of them part of the swap, and adapt them to the very worthwhile Jeep transfer cases.
Transfer Cases
The TJ inherited the NP-231 or the New Venture Gear NVG 231) part-time transfer case from the YJ line. This transfer case is the de facto reliable transfer case with all four and six-cylinder TJ's throughout their span.
With the introduction of the TJ Rubicon, we discovered the NP241OR transfer case. This version is even stronger than the 231 and sports a 4:1 low-range.
Both transfer cases are fully adaptable to GM conversion transmissions and each has the strength to very well transfer V8 power. We encourage your additional reading about your transfer case, by following the above links.