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Shortly after getting the axles, gears, and lockers put in, I decided it was time to look into swapping out the transfer case. The high-pinion axle in the front certainly helped decrease the angle of my front drive shaft, but it still vibrated. The yoke going into the front of the t-case seemed a little loose, possibly indicating the front drive bearing was bad. Additionally, the rear drive shaft started vibrating considerably. After putting in a CV on the rear shaft from Rocky Mountain Drive Line, the vibrations improved dramatically. I suspect that the increased vibrations may have been due to the change in the gearing (the jump from 3.73's to 4.88's means both drive shafts spin much faster than before at any given speed). Anyway, to replace the front drive bearing I would have had to have someone take the t-case apart, which by that time you might as well just rebuild the whole thing. Additionally, nobody makes a slip yoke eliminator for the 249, so I knew I'd be getting a different t-case.

Since I didn't have the money (and wasn't quite ready to get too hard-core) I wasn't even tempted by the Atlas II. I narrowed it down to two transfer cases - the NP242 and the NP231. The advantage of the former is that it has five positions: 2 wheel drive, 4 wheel drive high (part time), 4 wheel high (full time), neutral, and 4 wheel low. For a while I was planning on doing the 242, but the guys at A-1 Transmission convinced me that I would be better off going with the sturdier 231. However, it only has 2 wheel drive, 4 wheel high (part time), neutral, and 4 wheel low. (Click here for specs on t-cases.) Anyway, I ended up going with a 231 with a slip yoke eliminator kit. The advantage of the SYE (also available from Tom Woods for the 242) is that it allows me to put in a longer drive shaft. This decreases the pinion angle, which usually decreases vibrations. I may still at some point go with a 242 and have it built up as an HD (6 planetary gears as opposed to 3 and a larger chain) if I find I'm slipping around with the 231 during daily driving in slick conditions. In the meantime, the 231 should do nicely, and I figure if I ever want to do another swap I can sell it and pretty much recoup my costs. It has a great reputation and is highly sought after by wheelers.

Above on the left is a picture looking from the rear with the heavy-duty crossmember removed, the old 249 taken out, and the transmission being held up on a tall jack stand. On the right, again looking from the rear, is the 231 t-case with SYE after it was installed. The rear drive shaft has not yet been attached.

Here are some pics of the rear drive shaft bolted into the 231 transfercase and to the Ford 8.8 axle. Note the slip yoke and CV on this beautiful piece of work from Rocky Mountain Driveline. Once again, James at Marcy's Automotive & 4X4 in Evergreen, CO helped me with the install.

After finishing up with the t-case swap, I decided to take care of re-calibrating my speedometer. It had probably been two years since it had read correctly, because I threw larger tires on even before I drove off the used car lot. It read even slower when I put 35's on it. After re-gearing from 3.73's to 4.88's, it read too fast. Tired of guessing my mph, I headed over to the dealership to pick up a new speedo gear.

First I removed the bolt holding the clamp for the speedo gear housing. A gentle tug released the housing from the t-case, revealing the speedo gear. A tech article I had read at jeepin.com recommended a 39-tooth gear when running 35's with 4.88's. The parts guy at Pro Chrysler Jeep let me put it in and drive past a few mile markers on the freeway to confirm that it was the right one before I bought it. It checked out perfectly!

Above are the new 39-tooth (green) and the old 35-tooth (orange) speedo gears side-by-side. The new gear clicked right into place. It was a quick fix (literally about 5 minutes) and it only cost $32. In addition to my speedometer reading correctly, now my odometer, trip computer, etc. are accurate again as well.

ZJ build, continued


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