The Select Trac option on the Jeep Cherokee gets you the New Process NP 242 transfer case. This transfer case gives the following operating modes: 2wd, 4wd with the center differential open, 4wd with the center differential locked, neutral, and 4wd low range (center differential locked)
The addition of a center differential is the main difference between the NP 242 and the NP 231 or Command Trac transfer case. Without a differential, the 231 transfer case offers only part time four wheel drive mode.
The NP 242 transfer case Full time mode opens the center differential allowing the front and rear axles to rotate at different speeds while still being linked by the transfer case. The end with the least traction will get the most torque. While this mode may have some disadvantages off road where a wheel may loose contact with the ground, it has the advantage of being usable on all paved surfaces. Part Time mode – with the center differential locks can only be used on loose or slick surfaces with out possible damage to the transfer case.
I use full time mode on wet streets, hard packed gravel or dirt, some snow and ice situations and on dry pavement when full acceleration without wheel spin is desirable. I use part time mode on loose gravel, dirt, grass and any off road situation. I use Part time mode when RallyCross racing as well.
I use low range for more control in off road situations or on rough gravel roads when traveling downhill to avoid excessive use of the brakes. I also often use low range when positioning my trailer or other towed equipment when precision placement is needed.
The NP 242 transfer case can be switched between 2wd and either 4wd mode at any speed simply by moving the lever. The switch to low range must be made at very low speed – the Jeep manual says 3 mph.
One common operating problem with the 242 transfer case is that it will stick in 4wd even if the lever is moved to 2wd. Internally, moving the lever loads a spring which then applies force to move the selector fork. If the gears are loaded with torque, the spring will not have enough energy to move the sleeve off the gears. Once torque is removed from the gears, the spring will then move the fork and slide the sleeve of the gears.
Momentarily lifting your foot off the gas will often unload the gears enough to allow the spring pressure to move the fork. On some high mileage units, a wear groove will form on the sleeve or gear causing it to stick longer and may require a couple of changes from loaded to unloaded to get the sleeve to release form the gear.
The Jeep NP 242 is lubricated with Automatic Transmission fluid. The level is checked by removing the upper of the two large plugs on the back of the case. The fluid should be level with the upper plug.
The NP 242 has a slip yoke on the tail shaft. If the drive shaft is damaged or removed, fluid will be lost out the tail shaft. Also this seal is important to maintain in good condition to prevent fluid loss.
The NP 242 is a very versatile transfer case and is my choice for three of my five Jeeps. The full time selection gets frequent use in all of them.