The NP231 transfer case is a rugged, reliable, and durable unit that has been used in a variety of vehicles since 1988. Most commonly found in Jeep Wrangler YJ, TJ, and XJ Cherokees, the NP231 is fitted with a slip-yoke style rear output from the factory. This portion of the transfer case is fine in stock form, but a well-known weak link in lifted applications.
In short wheelbase rigs, such as the Wrangler, the driveline angle becomes too great once the Jeep is fitted with a suspension lift over 2 inches. This increased angle creates a drivetrain vibration and can even damage the transfer case and driveshaft. In rigs like the Jeep Cherokee XJ, axlewrap can cause the slip portion of the driveshaft to rapidly plunge in and out of the case, which can destroy the output. Lowering the transfer case can sometimes mend part of the problem, but defeats the purpose of raising the vehicle for greater ground clearance.
The easy fix is to upgrade the rear of the transfer case with a slip-yoke eliminator (SYE kit for short). Aside from reducing the likelihood of a broken output shaft, an SYE comes with additional benefits. First, the fixed-yoke allows you to use a longer driveshaft. This decreases the driveshaft angle and reduces the stress on the output shaft. Using a fixed output also allows you to use a CV (constant velocity) yoke and Double-Cardan U-joint at the transfer case. This joint type is better equipped to handle the increased operating angles, thus eliminating driveline binding and vibration. Essentially, the SYE conversion moves the compression and extension of the driveshaft from the output shaft, and applies it to the function of the driveshaft.
Installing an SYE on the NP231 is a common upgrade. In fact, several aftermarket manufacturers offer kits. In most cases, this upgrade can be performed with the transfer case still in the Jeep. If you’ve got a set of snap ring pliers, common hand tools, and a little knowhow, you’ll probably be able to handle the installation yourself.
We installed our Advance Adapters SYE kit and Tom Woods driveline on our 1995 Jeep Cherokee XJ with the help of SoCal Super Trucks in San Bernardino, California. Our XJ is equipped with a 4½-inch BDS Suspension long-arm kit, and absolutely needed the SYE conversion. The install only took a couple of hours, and to date, we haven’t experienced any problems or vibes with our new setup.
The NP231 transfer case has an output shaft cover that houses the shaft seal. Behind it, are another cover for the speedometer gear, and finally, a cover for the transfer case. Once you drain the fluid from the case, you can begin the breakdown process. We chose to keep the transfer case in our ’95 Jeep Cherokee XJ for the conversion.
Once the speedo gear is removed, use a set of snap ring pliers to take off the output shaft’s snap ring.
With the snap ring removed, unbolt the second section of the stock output housing. You may need to pry it off, depending on how well the RTV silicone is set.
At this point, you are ready to unbolt the transfer case’s back half. Given that it is made of aluminum, it isn’t very heavy or difficult to handle.
The stock shaft slides out easily.
The Advanced Adapter’s output shaft uses the latest NP231 design (introduced in 1997). The AA upgrade can be used on older model transfer cases, as well.
The Advanced Adapters Fixed Yoke kit comes with a 13/8-inch diameter, 32-spline rear output shaft, 356-T6 heat-treated aluminum-alloy tailhousing, and a 1310 constant-velocity yoke.
After thoroughly cleaning the case cover, the chain and upgraded output shaft assembly is installed.
We applied a fresh bead of RTV silicone after cleaning the back half of the case and attached the rear cover. Once installed, we bolted the new Advance Adapters output housing into place.
We opted for a new driveline from Tom Woods. The heavy-duty driveshaft is designed to handle the rigors of ’wheeling and provides 4½ inches of spline engagement.
At the transfer case end, we installed the 1310 CV-style yoke, which virtually eliminates any driveline vibration or binding.
At the differential, we retained our Jeep’s stock 1310 yoke, which is plenty strong for our XJ ’wheeler.
The NP231 uses regular ATF fluid for cooling. In this instance, we added two and a half quarts of synthetic Royal Purple ATF to fill the transfer case up to the drain plug.
