Good-Bye NP249, Hello Lower Gearing
First on the build agenda of our Project Grand Caddy was to get the vehicle running at optimum performance levels. Since the Jeep is going to shuttle us back and forth every day as a daily cab, it needed to be a more reliable and efficient form of transportation. It also had to be able to endure a beating while on remote photo shoot locations, as well as some overland trail adventures. Another unfortunate fact is that the 8-year-old ZJ will rack up more asphalt miles on Southern California's freeways than off-highway miles. In this situation, the OEM full-time four-wheel-drive transfer case isn't the best mode for this vehicle.
At the factory, the 5.2L ZJ was fit with the NP249 transfer case, which offers full-time four-wheel-drive, called Quadra-Trac. This form of traction may be OK for a person who actually needs to use it on a daily basis, but how often could that be? Aside from daily performance issues of full-time four-wheel drive, there are other inherent problems with the NP249 that make it a prime candidate for a conversion.The front driveline is constantly in motion, making it tough on moving parts and efficiency. We also checked the service records of our Grand Caddy at the dealership; it seems the front ring-and-pinion was replaced at a very low mileage. Judging from the character of the person we purchased the vehicle from and inspecting the undercarriage for trail damage, we would take bets that our Grand had never rolled across a patch of dirt or rocks.
Another problem with the NP249 is its viscous coupling. As its name implies, the VC literally couples the front and rear drivetrain. It is prone to failure and very expensive to replace. It would be more economical to pick up a completely different transfer case than purchase a new viscous coupling separately. Rather than deal with the issues of the NP49 and wanting to free the frontend from a life of enslavement, we decided to swap out the factory T-case for an NP231 transfer case. We also used a 4:1 Tera Low gear reduction for low-speed 'crawling.
The stock NP249 offers 4-Hi, 4-Lo, and Neutral. The NP231 with the Tera half case offers 2-Hi, 4-Hi, 4-Lo, 2-Lo, and Neutral. Notice the difference? How easy is the swap? Very. With the correct years, the NP231 bolts right in place of the NP249; the only difference is the shift linkage and the wiring for the drive mode indicator. This was also a great time to swap out the transfer case's slip yoke for a fixed assembly and upgraded driveshaft. Another aspect of the swap was the universal NP231 transfer case shifter assembly just released from Novak Conversions. Not only does it look cool resting in the Grand Caddy console, but it makes the shifter conversion much easier and the throw smoother.
We had the help of Vinson Pratt from Tera Manufacturing and Ben Hanks of MEPCO's Jeep and All Terrain Service Center. They found a rebuildable donor NP231J and went to work building our new T-case. Ben started the process by opening up the NP231 that had been set aside following an XJ buildup that they performed for another customer.
The scope of our buildup required replacement of the front case half, which means splitting the transfer case. Before disassembly, the outside of the case was cleaned of dirt and grease buildup. Since this T-case had been out of the donor vehicle for some time, the oil had already been drained. However, if you're taking a transfer case apart for inspection or upgrade, make sure you drain the oil first, into a suitable container for proper disposal. The front output yoke was removed and the rear factory extension housing was removed. The factory service manual has all the details if you need pointers on disassembly. The oil pump was removed and set aside for inspection and cleaning. Then the bolts holding the case halves together were removed and with a gentle tap from a plastic hammer, the case halves separated. If you have a stubborn case, there are screwdriver slots on each end of the front case half that allow you to gently work the case apart. There is no gasket used, so be careful to avoid damage to the case half surface when prying and tapping the two halves apart.
Ben removed the drivechain, the front drive output shaft, and the mainshaft as an assembly and laid them on the bench. The shift detent spring and 4WD indicator were removed from the front half of the transfer case for use in the reassembly process. The planetary reduction unit was left inside the front case half (we'll explain why in a minute). The mode fork and range fork were removed. A thorough cleaning and inspection revealed that the case internals were in great shape. Everything was set aside in preparation for reassembly with some upgrades using TeraFlex components. A Tera Low NP231 case half with its time-tested 4:1 Low-range ratio planetary was chosen to give us that super-low crawl ratio for the slowest of trail running. The input shaft length required to hook up to the Grand Cherokee automatic is the same as the input shaft length needed on the '03-and-newer TJ Wrangler applications and has a 23-spline count. The bearing retainer and seal from the old case half was installed on the Tera Low NP231 and the studs were transferred from the old case to the brand-new Low-range case half.
Reassembly was in the opposite order of disassembly, but once again we went for a couple of upgrades. The Tera Low 2-Lo range lever was installed in the new front case half to give us the option of Low range in rear-wheel-drive only. This is really useful when driving at slow speeds on tight trails where a lot of high-traction rock is under the tires. Kicking out the front drive relieves the binding that sometimes causes front axle U-joints to give up and makes turning the steering wheel much easier. You can stay in Low range, and then shift back to 4-Lo when you need to start climbing the next hill or obstacle.
On a Grand Cherokee, the driveshaft length is not so much of an issue as with a lifted Wrangler, but the 32-spline HD output shaft that is included with the 231SS gives us one less thing to worry about on the trail. Make sure to install the oil pump and pick-up tube in the correct place to ensure that all the internal components get a steady stream of cooling and lubricating oil whether you're cruising the freeway or idling along your favorite off-pavement trail.
After we received our massaged NP231J T-case, we stopped by T & J Performance Center in Orange, California, to complete the conversion. The swap is actually quite easy. We recommend starting the swap by draining the transmission and transfer case fluid. If you don't, it gets quite messy. Start by removing the driveshafts, transfer case skidplate, and factory shift linkage, then unbolt the NP249 and remove it from the vehicle. The old NP249 was removed and replaced by the new NP231J. The front driveshaft just bolted right back in place. A new CV-style rear driveshaft was made to bolt onto the new HD 231SS fixed yoke.
Powertrain Industries came very highly recommended for making our new CV driveshafts. The company provides driveshafts to some of the top desert racing teams as well as to aftermarket manufacturers. Each shaft is made to exact tolerances and thoroughly balanced before it goes out the door.
The universal shift linkage from Novak conversions is really just that - it will work with YJ and TJ Wranglers, and XJ, MJ, and ZJ Jeeps. The goal at Novak was to simplify the shifting mechanism, bypassing complex linkage systems that bind and cause the transfer case to unexpectedly pop out of the mode it was placed in. The installation is straightforward if you follow Novak's detailed and easy-to-read instructions. Before installing the NP231, we mocked up the shifter assembly to make sure we had everything in order. It's much easier to figure it with the T-case out of the vehicle if you're using the kit for the first time.
With the universal shifter conversion, you do lose the factory metal pocket that seals the cab of the vehicle from road noise and dirt. To replace, we cut the bottom off the old shifter pocket and used it as a bracket to hold down a 1/4-inch piece of split rubber sheet. Below that is a flipped universal shifter boot we picked up at the local auto parts store.
The Novak shifter kit matched perfectly with the Tera Low T-case. It now shifts between each mode smoothly and looks very clean. The shifter shaft will have to be adjusted for installation; this can be done with proper leverage in the right spot or by heating it with a torch (before installation).
For the 4WD indicator on the console of the Grand, we decided to use the 4WD indicator switch from an NP231J and hook it up to show 2WD and 4WD. It won't tell us if we are in 4-Hi or 4-Lo, but with the 4:1 Tera Low, it should be easy to tell what mode the transfer case is in.
To get the shift indicator to work properly, we mated a '93 NP249 indicator wiring harness to a '97 NP231 harness case. The wire colors may vary from year to year, and engine to engine. To modify the OEM four-pin NP249 indicator wiring to mate with the two-pin NP231 indicator wiring you will need to splice the green/orange wire (from the NP249) with the black/red (from the NP231) wire together. Splice the black wire (from the NP249) with the black wire (from the NP231) together. Cap the remaining two wires (black/pink and black/red). This will enable the dash indicator to show in the following drive positions:
2-Hi = no indication (no tires light up)
4-Hi = tires light up
4-Lo = tires light up
2-Lo = no indication (no tires light up)
This shift pattern is similar to the OEM NP231 dash indicator as found in late-model TJ Wranglers where the 4WD light is only indicated in either 4WD high and low; both 2WD high and 2WD low do not light up in the dashboard.
After the completion of the transfer case conversion, the first thing we noticed was the lighter effort it took to turn the steering wheel at speed. We haven't driven it enough to figure out the mileage but are certain we will see an increase. Quirky traits that disappeared with the NP249 transfer case were the chirping, binding, and skipping of the front wheels while completing a tight radius turn. Next on the list of things to do we will be showing you some cool engine performance tricks for the 5.2L engine and axle upgrades for rugged trail dependability.