4X4NOW   TJ
Buildup
Advance Adapters
Atlas II Transfer Case

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With 35 inch tires, I knew the 4X4NOW TJ would need a much lower crawl ratio (low gear x low range x differential gearing).  The stock (2.74 x 2.72 x 3.73) = 27.8 to 1 crawl ratio just wouldn't cut it for serious rock crawling even with the automatic.  4.56 ring and pinion gears are available for the TJ and would have given me a crawl ratio of (2.74 x 2.72 x 4.56) = 34.0 to 1.  But the 4.56 gears are weaker than stock 3.73 gears and an output shaft conversion would have been necessary to beef up the stock NP 231 transfer case.  I've seen the NP 231 output shaft break during serious rock crawling.  It will ruin your whole day and is not something appropriate for a trail fix.  And I wanted an even lower crawl ratio than the weaker 4.56 gears could provide.

Another nagging problem for TJ buildups is drive line vibrations due to the extreme drive line angles serious lifts impart to the TJ's very short rear drive shaft.  I had heard of TJ buildups that required dropping the transfer case and still having drive line vibrations.  I did not want to give back any of the ground clearance my lift gained by dropping the transfer case.  So I wanted as long a rear driveshaft as possible.  I had a good start by ordering it with the automatic.  TJs with automatics start out with rear drive shafts about an inch and a half longer than the 5-speed versions.

The Advance Adapter Atlas II transfer case was the answer to my needs.   It has a 3.77 to 1 low range which gives me a crawl ratio of  (2.74 x 3.77 x 3.73) = 38.5 to 1.  Even lower than with the weaker 4.56 gears and plenty low for serious rock crawling with an automatic. 

It also leaves me with economical highway gearing.  The standard TJ has 3.07 gears.  Converting differential gear for the stock 28" tire to the 35" tires, (35/28) x 3.07 = 3.84.  This is reasonably close to the 3.73 differential gears I already have with the Dana 44 option rear axle.  This should prove economical on the highway (2,725 RPM at 75 MPH), and with redline in 2nd gear at around 90 MPH, there should be no problems on steep highway grades in the mountains.

At the same time the Atlas II gave me a full 6 ½ additional inches of critical rear driveshaft length (2 ½" more than an output shaft conversion would have given me).   After lifting and installing the Atlas II, I had a 19 ¼" long rear drive shaft (bottom of photo ) compared to the stock 12 ½" drive shaft (top of photo).   And the Atlas II has the larger 1.375" non-twisting, non-breaking 32 spline output shaft.

The Atlas is gear driven with helical-cut gears and a full compliment of bearings, in a single piece casing which along with the large rear output shaft makes if nearly "bullet proof".  The stock NP 231 (above Atlas II in photo) seems like a toy in comparison.  The Atlas II is bigger and over 50% heavier than the NP 231 (94 lbs. vs. 60 lbs. according to my wife's bathroom scales).   For fringe benefits it uses O.E.M speedometer gears, and is fully synchronized - with a manual transmission you can even shift in and out of low range without stopping (once it is broken in - its fairly "stiff" from the factory).

With all this in mind 4X4NOW enthusiastically began the first Atlas II TJ installation (outside of Advance Adapters own TJ used for development of the installation instructions).

I was surprised at how smoothly the NP 231 removal went.  First the stock linkage is disconnected and discarded.  There is a single Advance Adapters supplied rod that connects the transfer case cross link directly to the stock shifter.   To remove the transfer case the transmission was supported with jack stands and the skid plate removed.  The NP 231 was then drained and the front and rear drive shafts, NP 231 breather tube, 4WD switch wires, and the vehicle speed sensor wires were removed.  This all went smoothly.  I supported the transfer case on my makeshift "transmission jack", removed the nuts from the studs holding it to the automatic transmission adapter housing and pulled it back.  It came free with minimal maneuvering.  As the NP 231 input shaft pulled free of the seal in the back of the housing some automatic transmission fluid spilled out and then continued a slow drip.  I put a cup under the drip to catch the fluid and so that I would be able to tell how much to add back to the automatic transmission.  It turned out that even after leaving the container under the TJ overnight, I lost less than a cup (total) of automatic transmission fluid.

The next step is to switch over the stock speed-o assembly and 4WD sensing switch.  The  TJ 4WD sensing switch is simply screwed out of the NP 231 and into the Atlas II shift control housing (top left of housing).  The Transfer Case Speedometer is simply removed from the NV 231, rebuilt with the Advance Adapters supplied seal kit and replaced in the Atlas II (left side of tail housing)..  I changed speed-o gears at the same time since I would be installing 35" tires later.  AA provided a chart to determine the proper number of teeth and Jeep Speedometer Drive Part No.  The cart indicated a 31T gear would be appropriate for 35s and my 3.73 gears.  This replaced the factory 36 tooth gear shown below the speed-o assembly.

2" stud bolts are inserted in the Atlas II in one of three locations according to the "rotation" required.  TJs with automatic transmissions use shallow rotation with the studs in the locations shown.   The studs are firmed into the Atlas II transmission mounting face by temporarily using two nuts turned against each other.  The Atlas II input shaft is shorter than the NP 231 and does not reach the seal in the back of the stock automatic transmission adapter housing.   So the housing will fill with transmission fluid.  The Atlas II input shaft is sealed so the fluid can't get into the Atlas II.  But there is a "weep" hole in the bottom of the stock transmission adapter housing which would eventually drain the automatic transmission if left unplugged.  I used blue RTV and a 1/2" No. 8 sheet metal screw to plug mine.  A thin layer of blue RTV is used between the stock housing and the Atlas II to avoid leaks there.

The Atlas II bolted up surprisingly easily.  It took more time to get some slack in the wiring and transfer case breather tube so they could be attached than to bolt up the transfer case.  The wiring to the speedometer is shown on the side of the rear output shaft housing while the 4WD switch is at the top on the left side of the shift control housing.

The linkage for the Atlas II using the stock TJ shifter couldn't be simpler.  Just throw away all the old linkage and associated brackets.  All you need is your stock shift lever assembly.  The vertical bright metal in the photo is the stock shifter.   AA supplies a one piece link rod that attaches directly from the stock shifter to the Atlas II shift rails.  The hole in the stock shifter may need to be rounded out a little to accommodate the AA hardware.   This is best done before the Atlas II is bolted up.  Note the AA rod goes to the left of the stock shifter when viewed from the front of the vehicle.

The Atlas II shift pattern is different from the TJ's NP 231.  4WD low is all the way forward and down on the floor.  As you come up 2WD Low, neutral, 2WD High and 4WD high are engaged.  The throw is short and uses only about half of the stock throw distance.  By filing the detent on the shifting mechanism you can get to 2WD high just before the detent forces you to push the shifter to the right.  This makes an excellent shift pattern with all the most commonly used shift points easily found.  4WD low is as far down as the lever will go.  2WD high is as far back as the lever goes before having to push over to the right.  And 4WD high is as far back as the lever will go after pushing over to the right.  The Atlas II is also available with a twin stick shifter.

A new longer custom built rear CV driveshaft and cut down stock front drive shaft were then installed.  The skid plate was the last part to be re-installed.  To accommodate the Atlas II in an automatic equipped TJ  a portion of the rib in the top side of the skid plate must be cut away.  AA supplies a drawing showing exactly how to do this.  The first photo shows the cutout before being boxed in and painted.The second photo shows the skid plate boxed in, repainted and ready to be reinstalled.  The skid plate modifications and drive shaft work were done by Zig's 4 Wheel Drive of Grand Island, Nebraska.

The Atlas II is shipped dry so don't forget to fill it with the specified TYPE "F" automatic transmission fluid.  Mine took 3 quarts.

The Rubicon Express 4 1/2" lift control arms then had to be adjusted to align the differentials with the new drive shaft angles.  Since both were now CV shafts, the pinions needed to be nearly straight with the driveshafts.   The rear control arms had to be adjusted significantly to point the Dana 44 up at nearly the same angle as the new 19 1/4" CV driveshaft.  This took some effort since the rear arms need to be unbolted and freed on one end to make the adjustment.   The front Dana 30 had to be pointed up some also because of the lift.  This went much easier as the front upper control arms could be adjusted without unbolting and freeing one end.  I had hoped to make some measurements on the differential and driveshaft angles didn't have the appropriate device so just eyeballed it. 

Now it was time for the test.  I have yet to hear of any TJ with a 4 plus inch lift that didn't either have to drop the transfer case or live with some rear driveshaft vibrations (even if minor).  This includes even those with a tail shaft shortening kit and new CV driveshaft.  But I had none.  From 0 - 75 mph, no hint of drive shaft vibration!  Had I eyeballed it perfectly?  Probably not.  More likely the extra drive shaft length allows for a wider range of pinion angles without vibrations.

The test drive did reveal the need for a little tweaking though.  There was a the thud of metal hitting metal when I hit a good bump and when I accelerated hard.  The transfer case was bouncing up (or rotating up in the case of hard acceleration) to hit the stock shifter.  It took only a few washers to space down the skid plate just enough to avoid this problem.  It's a tight fit for the massive Atlas II inside a TJ.

With a bullet proof design and construction, a low range gearing that allows you to crawl huge boulders with your stock differential gears, and a design that allows a much longer drive shaft to eliminate those tenacious TJ drive line vibration problems, what more could you ask of a transfer case?  I'm anxious to get the Atlas II equipped 4X4NOW TJ finished and get it out on some serious rock crawling.  It will be ready for the Moab, Utah, Easter Jeep Safari this spring.  See it at the Manufacturer's Display area and/or on the trails.

Other Atlas II applications include YJs, CJs, 66-77 Broncos and applications replacing the NP 207, NP 231, NP 241, Dana 300, Dana Bronco 20 and Jeep Dana 20 transfer cases.

For more information contact:

Advance Adapters
http://www.advanceadapters.com
P.O. Box 247, 335 Santa Bella
Paso Robles, CA 93446-0247
(805) 238-7000
(800) 350-2223
Fax (805) 238-4201
e-mail: aadapters@tcsn.net

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