 We're all familiar with the problems of the slip yoke in the popular NP231 transfer cases. The output shaft of the transfer case is so long that it takes up precious driveshaft length. If you have a U-joint or driveshaft failure, you can't operate the vehicle with the rear driveshaft removed without losing what little ATF the transfer case contains. Third, the bushing in the tailcone wears out over time causing vibration, whether or not your angles are correct. Flawed design? You bet, at least for those of us who intend to modify and use our Jeeps off road. If you've read some of my previous articles, you know how much I despise driveshaft vibration. I simply can't stand it. I'm sure many people can put up with some vibration and go along their merry way. For me, eliminating bad vibes is nearly an obsession. We installed a Rancho 3" lift on our Jeep Cherokee a couple years ago. Being a daily driver, I'm not willing to tolerate any vibrations. We had installed a 1" transfer case lowering kit with the lift that helped minimize the vibrations, but this was just a band-aid to cover up the real problem of a flawed driveshaft design coupled with increased U-joint operating angles. After a recent 2500-mile road trip, I finally decided I'd had enough of the stock setup. The obvious answer was to install a slip yoke eliminator kit into the 231 case. This accomplishes three things: it shortens the transfer case output allowing for a longer driveshaft, it converts the slip yoke driveshaft design to a fixed yoke output that will accept a CV style driveshaft, and in our case allowed for a much heavier duty transfer case mainshaft. While there are at least a dozen companies selling SYE conversions for the 231 transfer case, there are really only two distinctly separate designs to the kits. The most common and preferred variety replaces the existing mainshaft in the transfer case with a much heavier duty and shorter piece. With this kit comes a new aluminum tailhousing and seal to accommodate the new fixed yoke. The second type simply shortens the existing mainshaft and taps a hole in the end of it to bolt on the new fixed yoke. While the latter type can be easier to install, you do not get the benefit of a much larger mainshaft and output bearing in the transfer case. We opted for the more heavy-duty unit in our XJ. We purchased ours from Advance Adapters, but many other companies sell identical kits. Shop around for the best price. Installation:
I've read many different views as to the difficulty of installation of this kit. Some suggest pulling the transfer case from the vehicle and installing the kit on a workbench, others say to leave it in and perform the work while underneath the Jeep. Trust me on this one (particularly you XJ owners), leave the transfer case in! The job is easily done while lying underneath the Jeep, there is no need to waste the extra time removing the case from the vehicle. Rather than review each step in the installation process, the installation instructions are available at Tom Wood's site. The installation instructions are easy to follow, the only additional thing that I would add is
to make sure you have a good set of snap ring pliers.
One of the major reasons for installing an SYE kit is that it allows the ability for you to run a CV style rear driveshaft. Given "normal" operating angles, a CV driveshaft tends to run much smoother than a standard shaft. There is a misconception out there that a CV shaft can operate at higher angles than a standard U-joint. This is not true; a standard CV will bind before the typical single joint will. However, under reasonable operating angles, the CV style driveshaft is the way to go. Having previously priced driveshafts through various shops, we decided to save a few bucks and purchase the shaft from a local company. After all, it was $20 cheaper than the competition, I didn't have to pay for shipping, and I could pick it up the same day. We picked it up and rushed home to install it and take the Cherokee for a test drive. That's when big problems sprang up. The Cherokee now had a violent driveshaft vibration at anything above 50 mph. I spent a week adjusting the rear pinion angle to every possible combination available (everywhere from 4 degrees low to 2 degrees high), wondering if I had a severe case of axle wrap, scratching my head, measuring, re-measuring...bolting...unbolting.... I was at wits end when I called the driveshaft shop where I had originally purchased it. They were more than happy to recheck the balance of the driveshaft. They claimed it was a "little bit out" and they would build me a new one. To make a long story short, the "new" driveshaft was only slightly better, now I could get to 55 before my teeth started to chatter. I gave them one last chance and they built me a third driveshaft...now we were down to 45 mph. Seeing that I was getting nowhere with this company, I finally did what I should have done in the first place, I called Tom Wood. Unlike SYE kits where so many are identical, driveshafts are not. The very first shaft that Tom built for me was perfect. Without making any other changes to my Cherokee at all, the Tom Wood's driveshaft ran perfectly smooth. I broke my own rule during this project. I sacrificed quality to save a few dollars on the driveshaft. Guess what happened? I ended up buying two driveshafts. Save yourself the grief that I went through and get your driveshaft from Tom in the first place. In the long run, it will be less expensive. Do it right the first time or... I'm pleased to say that 5 months later the Cherokee is running smooth. I used to dread long highway trips due to the vibration. Now I can set the cruise on 80 and there is no vibration at all. This was especially nice while driving to this year’s Easter Jeep Safari. Not only was the drive very comfortable, but I also had an extra inch of badly needed clearance under the transmission cross member once I hit the trails. If you've been hesitant to install an SYE in your Jeep, don't be. It's easy to install and the benefits are huge. Just make sure you buy the driveshaft from Tom. 
Dirt Road Magazine is sponsored by Turner 4WD Parts Co. |
