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I rebuilt an AX 15 in a 99 Wrangler.  It went back together fine, it shifts tight, only 1 problem.  Shifting from 2nd to 3rd is fine and from 3rd to 4th is good.  I just can't get it to go from 4th to third.  I am guessing it may be a ball or shift plug out of place in the rail or intermediate plate.  Any thoughts?

 

Thanks

 

Tim

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Hi, Tim...The shift balls, springs or plug theory could be happening, parts out of order; however, before you tear a freshly built unit down, try feeling the detents for each gear.  If the third gear and fourth gear detents make a distinct "click" into place, the detents might be okay...

 

This could be a binding or cocked shift fork or sleeve.  Did you replace brass blocking rings, bearings, and so forth?  Third and fourth gears have a much better engagement on the AX15 than 1st/2nd does.  The shift sleeve for 3rd/4th moves thoroughly over the gear cogs, which the 1st/2nd sleeve does not in some AX15 applications.

 

Is there gear clash?  That would be more symptomatic of plugs out of place or missing, as the shifter would be trying to catch two gears at one time.  3rd/4th is on the same rail and should not cause a sticking problem unless there is cocking or binding, something like the synchro hub springs and keys not indexing properly.  This would allow a key to raise out of its notch in the hub, which is a real possibility with what you're experiencing. Properly positioning the synchro hub springs and keys is essential.

 

Let me know more details, we'll kick this around...Moses

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  • 2 weeks later...

MOSES, I WONDER IF TIM'S SHIFTING PROBLEM  FROM 4TH TO THIRD COULD BE RELATED TO THE CHANGE THAT WAS MADE—LIKE ON THE AX15 I REBUILT. IN MY PHOTOS, SEE THE ANGLED SYNCRO TEETH ONLY ON 3RD GEAR, SLEEVE AND RING.

 

MAYBE TIM REPLACED THE ANGLED TOOTH SYNCRO RING WITH THE STRAIGHT/ARROW CUT VERSION THAT COMES IN A REBUILD KIT? MAYBE THIS CAUSED THE  GEAR AND SYNCHRO RING TO HAVE GOOD BRAKING ACTION, BUT IT IS UNABLE TO MATCH TEETH ENGAGEMENT FROM THE SLEEVE TO GEAR...JUST A THOUGHT...

 

GARYT

 

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Gary, welcome to the forums!  Thanks for providing AX15 parts photos...Tim talks about a 3rd gear downshift problem.  Your photos suggest that Aisin may have addressed a downshift to 3rd gear problem.  The angle cut you describe and share in your photos would engage the shift sleeve differently. This would allow easier engagement on the downshift from 4th to 3rd.
 
From a practical standpoint, either ring would work, as they each engage the sleeve troughs accurately (taking a straight-on view).  The difference is that the angled ring teeth would engage the sleeve with a different pressure and slight rotation of the ring.  This is the readily identifiable concern...

 

Think about the third gear engagement:  Shifting up, there is the lag in the shifter’s neutral gate and a “slower” engagement of 3rd gear.  Shifting down, however, would be a direct, quick movement of the shift level and fork.  Consequently, the sleeve would want to quickly rush over the brass ring teeth—without providing smooth or effective braking action at the gear hub.  This would cause harsher gear engagement.  Apparently, the slight angle makes the downshift to 3rd gear smoother, with better braking action and less risk of gear clash. 
 
Probably, either would work if all drivers shifted up and down smoothly—without taxing the synchronizers.  However, American drivers (and other Jeep 4x4 owners) want to affect a quick downshift to 3rd gear.  Another consideration is driving in low range of the transfer case—the 3rd gear downshift problem would be even more exaggerated!
 
If nothing else, this is a testimonial to the detailed design of Aisin transmissions, more like European types than traditional U.S. gear products.  The aim with a wedge ramp instead of arrow-shaped teeth would be the shift “timing” and precise angle of engagement.  This reflects the AX15's overall design quality and close-tolerance fitment.  Aisin and European transmissions are more complex units for a reason...  
 
Does this make sense, considering the parts layout?  As for the tooth spacing and tooth offset on the brass rings, this would keep the synchronizer blocking ring square as it moves onto the gear hub.

 

Great to have you at the forums, Gary!  Looking forward to your sharing...Moses
 

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MOSES I THINK IT WOULD BE GOOD TO SHOW THE FULL ENGAGEMENT OF THIRD GEAR AND NOT SO FULL ENGAGEMENT OF FIRST AND SECOND ON THE AX-15 TO SEE IF ANY OTHER BUILDERS HAVE NOTICED OR SEEN THE SAME ISSUE DURING THEIR REBUILD.

 

GARYT

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Agreed, Gary...See the YouTube video:

 

 

In this video (above), an instructor at Weber State University shares the gear functions of a manual transmission and synchronizers.  For the benefit of AX15 builders, the demonstration unit just happens to be a Toyota version (A150) of the AX15 transmission...As Gary notes, and you all will see within this instructor's 24-minute video, the synchronizer sleeve for 1st/2nd gear does not engage the 1st and 2nd gear cogs as deeply as the 3rd/4th gear synchronizer sleeve engages the 3rd/4th gear cogs during shifts. Although the view is not as clear, the 5th gear sleeve engagement also appears shorter than 3rd or 4th gear.

 

Note: To save time, view the shift sequence and parts alignments from approximately 7:30 to 23:00 minutes.  The entire video is helpful if you want to understand the functions of a synchromesh transmission and how gear speed ratios work...As an automotive technology instructor, I would like to thank Weber State University and the instructor for making this video available as a "public" YouTube download.

 

The unit in this video is a 1992 version.  The unit I step-by-step build at the magazine article is also from the 1990-91 era.  Note the shape of gear cog teeth from that period.  Cog teeth are all arrow shaped, none are the angle shape found on the later 3rd gear in Gary's photos from a 1999 AX15.

 

Our concern here is whether this short sleeve engagement is inadequate for 1st or 2nd gears (possibly 5th, too).  Gary noted the short sleeve throw while rebuilding a 1999 TJ Wrangler AX15 transmission: The 1st/2nd gear shift sleeve does not engage the 1st and 2nd gear cogs to the same degree as the 3rd and 4th gear sleeve engages the 3rd and 4th speed gears.

 

Has anyone else made this observation during an AX15 rebuild?  Both Gary and I question this short throw of the 1st/2nd gear synchronizer sleeve when engaging either 1st or 2nd gear.  Does this lead to trouble?  In particular, can this cause the transmission to jump out of 1st, 2nd or even 5th gear?   

 

Moses

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Moses

Thanks for your suggestions.  Also thanks to Gary.  I am able to smoothly go through the gears with the engine off.  I think that validates the detents being ok.  I drove it again today and it will not go from 4th to 3rd.  If I slow down to 15 mph and double clutch, it slips in.  I replaced bearings and syncros when I had it apart.  You mentioned hub springs and keys, but would it still shift into 3rd and from 3rd to 4th if that was the issue.  I guess there is something that will not let it slide one way at high speed from the 4th to 3rd (front to back) direction.  I will probably pull it apart in the near future because it is only an annoyance at the present, but it would be nice to have an idea of what needs to be changed to fix it. Could I have reversed the hub?

 

Thanks

 

Tim

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Tim, we're each being thoughtful about the AX15. The aim is to make rebuilds perform as new—perhaps better. Gary and I have batted ideas back and forth about improvements to the 1st/2nd synchro and sleeve assembly. 3rd/4th has presented less of an issue, though there is the puzzling Aisin changeover to the angled 3rd gear cog ramps and bronze synchro ring tooth shape...This indicates something inherently challenging about the 3rd gear shift.

You are spot on with your detent analysis if each gear position produces a shift lever feel of "holding" when the lever is in the selected gear position. The 15 mph with a double clutch downshift indicates that the problem is synchronizer related. Essentially, at that road speed and with a double clutch effort, you can synchronize the input and output shaft speeds to the speed of 3rd gear.

As a segue, I once owned a '51 F3 Ford truck with a "spur gear" four-speed transmission (totally non-synchromesh on any gear with rugged straight-cut gear teeth). I taught myself to use the clutch pedal only at complete vehicle stops. Once moving at all, I could shift up or down through every forward speed without depressing the clutch pedal—and without clashing gears. While this may sound unique, it really wasn't.

Older truck and car drivers (well into the 1930s) downshifted straight-cut, non-synchromesh transmissions by either braking the vehicle or accelerating enough in neutral to sync the engine and output shaft speeds for a desired gear's ratio. At the correct vehicle speed, engine speed and output shaft speed, the clutch pedal was dipped, shifter moved to neutral, then the clutch pedal released momentarily in neutral. Only if necessary, the throttle was blipped to change the engine speed slightly to match the output shaft speed for the next gear down. The clutch pedal was dipped again (the "double-clutch" effect), and the driver moved the shifter into the downshift gear before smoothly releasing the clutch pedal.

For a non-synchromesh gear, the trick is to have engine speed match the desired gear speed and the output shaft speed. In good time, you can learn to shift flawlessly and "know" when to affect a shift to each gear—braking and accelerating as necessary to "synchronize" shaft and gear speeds.

Along came synchronizers—for Ford light truck buyers, the solution came with the 1952 introduction of the T98 four-speed transmission. I'm forever grateful for that '51 Ford F3 "learning experience" and have instructed four-wheelers on how to get home from the backcountry with "clutch-less" shifting—like when the clutch mechanical or hydraulic linkage fails...In your case, Tim, you discovered the "sweet spot" for third gear downshifting when the synchronizer proved ineffective.

Your problem is definitely related to the 3rd/4th synchronizer assembly. Unless you have the wrong lubricant in the unit (please share details so we can determine whether that's a factor), the problem will be the hub springs, keys/plates or a thrust washer out of position or missing. In the case of the hub springs, they must be staged properly.

Click here to the magazine page on the AX15 rebuild. Read Illus. AX15-104 to AX15-113 carefully. I describe the alignment of the plates (keys) and springs in detail for both the 1st/2nd and 3rd/4th synchronizers. Throughout the assembly discussion, I outline the direction that the gears and synchronizer hubs face—plus the location of thrusts and critical thrust endplay measurements.

In the Weber State University video, the instructor talks about bronze ring gap measurements as a determinant for ring wear. There is also the issue of placing the correct ring at the correct position. The brass rings in an AX15 can look very similar; however, when placed in the wrong position, the ring will mate with the gear hub incorrectly and not create the needed braking action for "synchronizing" the gear's speed with the hub/shaft.

I am confident that you will find a distinct problem with either the 3rd/4th synchromesh assembly or the 3rd gear/brass ring fit-up. When you do, please share with us, as your comments could be beneficial to others.

Do not hesitate to post questions. The AX15 is not a simple transmission to rebuild. When I decided to share rebuild steps, I knew this unit was complex and took a "textbook" or "blueprint" rebuild strategy. Anyone tackling an AX15 rebuild will be challenged. Everything has to fit correctly and precisely, clearances are critical, and parts alignment is only correct if every single step falls into place properly...

I have been rebuilding manual and automatic transmissions professionally since 1969, methodical and painstaking when rebuilding transmissions with close tolerances. The AX15 is much more a European kind of gearbox than a historical Jeep unit. Tolerances are closer, the cases will only fit with all parts aligned—and that's just the beginning. The AX15 has a steep learning curve...

Moses

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Postscript on GARYT's comment...If the 3rd gear has one kind of ramp design, and the synchronizer ring has another, this could cause an issue as Gary suggests...Tim, do your remember the design of your AX15's 3rd gear cog teeth: an angled ramp or arrow-shaped?  Do you recall the design of the 3rd gear bronze ring teeth: either angled teeth or arrow-shaped teeth? 

 

Moses

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Hi, Tim...There are two photos here, each looks like an original, worn brass ring.  The one at the right shows the plates/keys cutting away at the brass.  The damaged ring was rotating considerably.  What was the condition of the plates/keys at the 3rd/4th gear synchronizer? 

 

The ring at the right has straight "arrow" shaped teeth; the ring at left has offset/angled teeth.  The ring at the right should be 4th gear for your later, 1999 AX15 unit.  Correct?

 

Since I cannot see the new ring (please upload the third photo), I'm not able to determine whether the new bronze ring has straight arrow shaped teeth or offset teeth.  As Gary suggests, on these later AX15 units, 3rd gear has a unique cog tooth arrangement (angled ramp shape, not arrow shaped).  This fits with a unique brass ring design (which has offset crowns on the teeth). 

 

The generic aftermarket rebuild kits apparently do not have offset teeth at the 3rd gear bronze synchro ring.  Aftermarket kits often pattern a specific year(s) design.  The earlier AX15 has arrow-shaped 3rd gear cog teeth and arrow-shaped teeth on the bronze ring—not the offset or ramping design.  Both my magazine rebuild article and the Weber State University video demonstrate an earlier version of the AX15: 1990-92 vintage.

 

This 3rd gear synchro issue came up in JayDLogan's AX15 build, which is also a later Wrangler application.  He and GaryT discovered that Chrysler/Mopar offers a unique part number for the 3rd gear bronze synchro ring plus a unique 3rd speed gear for these later AX15 applications.  (I can furnish Mopar part numbers for the '99 3rd speed gear and the 3rd gear bronze synchro ring if you need these numbers, Tim.)  In this topic's threads, GaryT uploaded photos of the later 3rd gear design found in his '99 TJ Wrangler AX15 rebuild.

 

GaryT, Jason (JayD) Logan and I have each raised questions about the bronze ring for a later AX15 3rd speed gear.  The angled/ramping gear cog teeth may require the offset-point bronze ring teeth like in your photo at left (presumed to be the 3rd gear bronze ring?).  Please clarify which ring fit where in your AX15, Tim.  4th gear appears to be the badly damaged ring at right with straight-arrow shaped teeth.

 

Aftermarket suppliers may need to offer an "early" and "late" rebuild kit—or specific "early" and "later" 3rd gear bronze synchro rings to accommodate the 3rd gear cog/teeth design change.

 

For some solid insight here, I started a brand new thread (click here) on the 3rd/4th synchronizer design change.  Jason Logan (our fellow forum member JayDLogan) and I began an exchange just before these forums launched...I revisited that Q&A and added our exchange as a new thread. 

 

At the new thread, you will find some insightful photos by Jason plus a valuable clue: the later AX15 transmission uses a different 3rd/4th gear synchronizer shift sleeve.  If installed backward, especially with the wrong synchronizer ring match, this sleeve would raise all kinds of trouble with shifts between gears! 

 

Take time to read and review the photos at the new thread, Tim.  Our discussion should resonate...

 

Moses

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Hello guys! I don't know if this helps but it is possible that your synchro rings (blocker rings) for 3rd and 4th are not indexing properly. Meaning that the synchro ring teeth are not meshing with the slider. This can be caused by bad keys and springs. Also, the key slots in the synchs could be worn or not in spec. I got in contact with Aisin with regards to 3rd and 4th synchs. Prior to 1999 the third and fourth synchros where the same, straight cut (house). In 1999, they changed third synchro to an angle cut and fourth synchro to a straight cut. My slider had a angle cut on one side with a corresponding angle cut synchro and on the other side was a straight cut (house) with a corresponding straight cut synchro. The AX15 I have been working on is from a 1999 TJ. If you have an AX15 transmission after 1999, I can give you OEM part numbers for third and fourth synchros Note that Aisin changed the straight cut synchro (house) as well after 1998. There is a slight change in the teeth pattern. Hope this helps!

Jason

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Hey guys! I found that the aftermarket synchros for third and fourth are angled cut and are identical for THIRD gear synchro for 1999 and later. I spoke to Aisin and I was able to get some information. If you opt to use OEM, then for third gear use part number 4897051aa (angle cut) and for fourth use 4897052aa (straight or house). These part numbers match up identical to Aisin specs for 1999 and later. I have done a considerable amount research on this issue and it has been very exhausting. The best thing to do is pay attention to the design of third and fourth and if falls within this variation I have found then these part numbers should be useful. Aisin has told me that prior to 1999 the third and synchros should be identical (straight cut). I have noticed that it is possible that the synchros should be matched up with same cut as the slider. Do not go by the factory OEM part breakdown! Maybe Moses can add to that!

Jason

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Jason did a thorough job of identifying the features of a late (1999) Aisin AX15 transmission used in the Jeep Wrangler.  Jason, your photos are very detailed and helpful, and I posted them at the new topic I started yesterday, discussing the 3rd-4th gear synchronizer differences.  Click here to see the new topic post and Jason's photos.

 

My two cents worth: If the transmission has never been apart before and shifted normally to this point (aside from wear factors requiring the current rebuild) then match up the synchronizer rings, the sleeve and the gears to exactly the OEM layout.  Do so by taking the transmission apart carefully and laying out all parts in the factory orientation.  Match up original parts and new parts to duplicate the original designs and layout.

 

According to Jason's comments, it sounds like the change year is 1999.  Not surprising that we have a rash of issues, and all of the transmissions in question are 1999 AX15 types.  Apparently, Aisin made the 3rd/4th synchronizer change to solve a shifting issue.  The aftermarket has yet to identify these parts differences, and based on Jason's experience, Mopar had difficulty cataloging them properly.  That would leave only one solution for transmission builders: Match new parts to the OEM design—hunt down parts until they match the original equipment (OEM) design.  Jason's numbers are certainly a place to start.  Like Jason, you need to match what you order and receive to the original parts found in your transmission.

 

     Note: I've not tested whether the use of an earlier 3rd gear and earlier 3rd/4th synchronizer assembly and synchro sleeve will work in these late Aisin gearboxes.  If the earlier parts do fit and work properly, that would be another solution—this has not yet been tested by any of us.  If Jason is correct, and if the parts in his transmission were only used in 1999, there is a problem brewing.  It will become increasingly more difficult to source replacement parts for the late AX15 3rd/4th gear synchronizers used in TJ Wranglers—and likely the XJ Cherokees from the same model year(s).

 

Look closely at GARYT's and Jason's photos.  Gary's photos are within the threads for this topic.  I added Jason's photos at the new post that I started yesterday on the 3rd/4th synchronizer differences: click here.  Open the photos by clicking on each one.  Note the features of Jason and Gary's 1999 transmission parts.  Compare these to the photos in my article on rebuilding the Aisin AX15, Part 1 (disassembly/inspection) and Part 2 (assembly).  In my article, the core transmission is from the early '90s era.

 

Moses

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  • 7 years later...

Was this issue ever resolved?  I just rebuilt an ax15 with the mismatched 4 gear synchro ring.  If it’s trouble some to go from 4-3 I think I’ll just double clutch it as I don’t want to tear this thing all apart.  Or maybe over time the synchro ring teeth will round over some.

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2001XJ...Matching the design and actual measurements of the synchronizer brass rings, hubs and other gear parts is essential.  AX15 aftermarket parts kits are "generic", leading to experiences like your situation. 

I compare the measurements and angles of new parts with the fit of the original gears and synchronizer parts.  Brass synchronizer rings must mate correctly (thickness, angles, contact points and depth, sleeve design, etc.) while matching steel hubs.  The contact between the inner brass ring and steel gear must create sufficient braking action—otherwise, there is no gear synchronization.

Which raises the question:  When you attempt to downshift 4th to 3rd, do you feel resistance early in the shift movement, as you lightly move the shifter past neutral and into 3rd?  You should feel resistance;  this is the actual braking action of the brass blocking ring against the gear hub.  We discussed mismatched synchro parts at length (above), and if the inner brass ring is not able to mate properly and grip the steel hub, there is no braking or gear synchronization.  This braking action brings both parts to the same speed—synchronization.

Many years ago (late eighties), when I was zealously promoting synthetic gear oil, I recommended that everyone try it.  A Toyota pickup owner shared that his truck's gears clashed after changing to synthetic oil.  The truck was nearly new.  He reverted back to conventional lube, and the synchronizers worked properly.

Here, the synthetic gear lube had too much lubricity, preventing the brass rings from providing the necessary braking action against the steel gear hubs.  That braking action is essential for normal synchronizer function.  Not sure what lube you're running, but viscosity and lubricity can play a role.  So can brass ring inner angles and opposing steel hub angles.  Or sleeve and brass ring teeth engagement points.

Moses

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  • 7 months later...

Moses Ludel I hope this post finds you.  I'm in the process of reassembling an NOS 1991 AX15 that was built as a 2wd unit and I'm converting it to 4wd using the output shaft and housing from a used AX15.  Here's my problem.  The 1/2 and 3/4 syncros that came in the new transmission sit ever so slightly higher on the gears than the ones out of my used transmission, which happen to sit just under flush.  So, when I go to press the new gears and syncros onto the used output shaft, it puts a slight bind on the syncros and keeps the gear from spinning freely.  The syncros on the used trans were replaced by someone before and that transmissions shifted great.  The height of the actual syncros are the same old vs. new, but the taper seems to be off on the new trans syncros causing the bind.  Is this normal when for an NOS unit to be this tight or should I seek out different syncros to replace these?  Thanks for any and all input, it's driving me nuts on what to do here.  My gut says just put all the new parts back in and let the trans break itself in, but my brain says not to listen to my gut lol.  Please help!

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Scott, your gut is right.  You need the proper clearances for the synchronizer rings to move freely on the gear hubs in the neutral position.  The ring is a "brake" that brings the gear to speed as you move the synchronizer sleeve.  In the neutral position, the rings (properly lubricated) should move freely and not bind or grip the gear hub(s).  This principle applies to any and all synchromesh assemblies. 

On new brass rings, be aware that lubricant on the ring is essential.  Otherwise, the ring will often bind on the hub whether the clearance is correct or not.  Make sure you're making this test with the rings lubricated, using a thinner oil or wide-range multi-vis gear lube.  Gently pry the ring(s) squarely (do not cock) away from the gear hub to see whether there is actually enough clearance.  If the rings can stand off from the gear hubs, the rings will slip, as necessary, when in neutral mode.  

There is another factor involved here:  gear endplay or thrust clearances.  You have control over the spacing between components on the mainshaft.  There are two housings/cases here and two sets of gears, shafts and thrusts.  The final assembly, when you mix the parts, needs to have the correct clearances. 

In my May 23, 2013 reply above, I linked to my AX15 transmission build, which has since been consolidated, edited and bookmarked/chaptered into a lengthy (hour and an half plus) how-to video rental at Vimeo On Demand.  (Link to the rental page:  https://vimeo.com/ondemand/ax15rebuild.)  Included in the popular rental video you will find detailed disassembly, inspection and reassembly how-to steps;  setting clearances is part of the assembly process.  This would be of value for your situation.  Here is the trailer for the lengthy video available at Vimeo On Demand:

You need the "stack height" along the gear train to add up properly and have the right endplay/thrust clearances.  This impacts the clearances at the synchro rings.  Compare the thrust and snap ring thicknesses and clearances between the two gear sets.  See whether this points to a solution. 

Moses

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Moses, thank you for the quick and informative response.  I decided to take your advice and rent your AX15 rebuild video.  To say it was a big help would be an understatement.  I would highly recommend it to anyone looking for an informative walk though of the process, clearances, torque specs, etc of an AX15.

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Thanks, Scott, I appreciate the feedback on the VOD AX15 rental video.  Let us know what you find and how your transmission build turns out...The AX15 is a busy transmission and has close-tolerance fit up.  The sequencing of the parts assembly is critical, and correct clearances are vital.  Done right, this is a great transmission with much to offer.  I trust you will find the end result gratifying!

Moses

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The feedback was well deserved.  I do have a curiosity question that you may have addressed in the video and I just missed it.  If (which thankfully mine didn't) there was excess clearance on any of the gears how would you go about fixing it?  Do they make thicker shims or do you just replace them with new ones that are factory spec?  I have another ax15 I'm going to tackle after this one and just trying to figure it out in case I do have that issue in the future.  Thanks again!

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Scott...Seldom are  the thrusts and gears worn to the point of needing a new thrust.  However, thrusts and spacers were available in single sizes from Mopar.  If the thrust or spacer is worn, you can compensate for minor wear with a new thrust or spacer.  If wear at the gear/thrust surface is too great, a new gear might be needed, though again, that's unlikely. 

So how does Aisin compensate for tolerances?  With the use of selective fit snap rings.  Here are the part numbers from Mopar for a 1999 version of the AX15.  Not sure whether you would have any success finding these snap rings through Mopar, though.  Toyota would be a possible source, as the A150-series transmissions are virtually the same.  You need to know which thickness snap rings you need.

Here is a 1999 TJ Wrangler AX15 gear train parts list that includes selective fit snap rings.  Zoom-in on the illustrations:

AX15 Transmission Selective Fit Snap Rings.pdf 

Moses

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  • Moses Ludel changed the title to Aisin AX 15 Shifting Problem After Rebuild
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Glad this worked out well, Scott.  The AX15 is a reliable transmission that will handle a good amount of torque.  Especially in a stock application, driven properly, these units hold up for hundreds of thousands of miles. 

A tip:  The synchronizer blocking rings act as a brake, grabbing the gear hub to match engagement speed.  Made of bronze, blocking rings wear.  The most significant damage to these transmissions is from forced downshifts at excess speeds, which increases the synchronizer friction.  This will wear out the brass blocking rings faster.  My synchromesh manual transmissions always last much longer by first using the vehicle's wheel brakes to slow down, then downshifting at the point where the synchronizer has an easy task of bringing gears to speed and meshing properly. 

I learned synchronizing gears and shafts from driving trucks with straight-cut or spur-gears with no synchronizers.  Engine speed and road speed determine the shift points.  Double-clutching allows matching engine speed to gear speed.  Double-clutching is not necessary with a synchromesh transmission.  However, reducing friction on synchronizers will protect parts and dramatically extend synchronizer life.

Inexperienced drivers have a fascination with forcing the 3rd to 2nd and 2nd to 1st gear downshifts, especially for Jeep 4x4s driven in low range where gear reduction exaggerates this braking effort...Now that you're "expert" at the inner workings of the AX15, you can better judge what kind of load you're placing on the synchronizers.  Use the recommended lubricant for the AX15 and make this a 200,000-plus mile rebuild!

Moses

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