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AX15: Early and Late Model 3rd/4th Gear Synchronizer Differences

Jeep Wrangler Jeep YJ Jeep TJ Jeep 4.0L Jeep how-to AX15

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#1 Moses Ludel

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Posted 22 May 2013 - 02:13 PM

There has been a long thread of exchanges at Timmy960's topic on harsh shifting after rebuilding an AX15 transmission.  The problem around 3rd/4th gear shift issues has a basis in the redesign of the synchronizer sleeve, bronze blocking rings and the third gear cog teeth for the sleeve engagement.  Jason Logan and I had an exchange about this issue when he rebuilt his '99 TJ Wrangler unit.  That exchange was just prior to the launch of the forums.  For the benefit of all AX15 rebuilders, I am starting a new topic around this issue, beginning with the in-depth Q&A exchange that Jason Logan and I had...Since this exchange, Jason, Gary, Tim and I have been using the new forums to disseminate AX15 transmission rebuilding details...Here is the original exchange with Jason.  I have highlighted important facts:


Jason:  Hello Moses!  I have recently taken advantage of your detailed instruction on how to rebuild an AX15 manual transmission.  I am currently working on an AX15 manual transmission from a 1999 jeep tj 4.0L that has never been rebuilt.  I have a question regarding the stock 3rd and 4th synchro ring and the corresponding slider [sleeve]! The stock slider has an angle cut on one side and a straight cut on the other side. Also, the stock 3rd gear synchronizer is angle cut and the 4th synchronizer is straight cut. (I may have these backwards). 


Why is it that the dealer and aftermarket only show that both 3rd and 4th synchronizers are identical (either both are an angle cut or both are a straight cut).  I ordered a 3rd and 4th synchronizer from Chrysler and they are identical (straight cut), and when I ordered a set from the aftermarket, the 3rd and 4th were also identical but with an angle cut!  Should I use the Chrysler synchronizers (both straight cut), the aftermarket (both angle cut)  or should I use one of each to replicate the stock setup as described above?  I have put this rebuild on hold for weeks now trying to get some answers from many sources but nothing has been rock solid.  I was hoping you could help me and give me some clarification on what is the best set up and why?  Why does the slider [sleeve] have different cuts?  Why do the replacement synchros have different cuts, different part numbers and configuration of teeth? Thanks for any information you can give me. 


Jason Logan


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These photos courtesy of Jason Logan ("JayDLogan"), forum member...Note the 3rd/4th gear sleeve and cog design for his 1999 TJ Wrangler AX15 transmission. Click on each photo to enlarge image.



Moses: Jason, I would match synchronizers precisely to the originals and in their original locations.  Measure the diameters of the rings, their angles and, most importantly, their fit against the synchronizer hubs in their correct positions. Make sure you face the synchronizer sleeve in the correct direction.


Start with the original rings in correct relationship to the synchro hubs and sleeves.  Note the overall “width” of the synchronizer assemblies, and then compare this with the aftermarket (new) synchros from both generic and Chrysler sources. 


The goal is to have synchro rings that will behave and fit just like the originals.  Wear on the OEM rings is typically slight, perhaps a few thousandths, so you will know immediately if the parts are either wrong or in the wrong locations. 


AX15 synchro rings are not the same diameter, and the difference is slight, often confusing the assembly.  My article provides details on the kind of “fit” and end plays you should achieve during assembly. 


Take your time, Jason, you’re already ahead of the game by questioning the parts to make sure of their fit and placement.  The OEM layout and fit is your template.  Match this and you will “restore” that AX15!


Trust this helps…I’ll be at Moab through Thursday and out of communication.  Let me know your findings in an Email.  I will answer next Friday…Best of luck, I know you will do the right thing here…




Jason:  Thank you very much for all of your information and help - my dilemma is - when I have ordered the synchro rings from Chrysler (they sent me 2 identical 'straight cut' synchro rings) and when i ordered from the aftermarket (they have sent me 2 identical 'angle cut' rings) but my original synchro rings have 'one of each' cut (one ring is a straight cut and the other ring is an angle cut).  Although all synchro rings ordered are the same diameter, angles and fitment as my originals,  I'm not sure if I should be using what Chrysler sent me (the 2 straight cut rings) or what the Aftermarket sent me (2 angle cut rings) - or should I be using one of each cut to match identically up to the original set up?   I would have thought Chrysler would have sent me one of each (angle and straight cut synchro ring) based on the fact that my original rings are 'one of each' cut.  And my second dilemma is - the number of teeth and location of the teeth on each ring ordered are not set up identically to my original configuration of teeth on my original rings - does that matter?  I'm extremely grateful for your reply and value your expertise.  Jason Logan   


Jason added these parts details: I found some other information, Moses!  If you look at earlier years of the Jeep Wrangler 1997-1998 they have used part number 4897051AA (for both 3 & 4 synchro) or 4897052AA (for both 3&4) depending on the month the jeep was made.  Part number 4897051AA is a synchro ring that has teeth that are angle cut and part number 4897052AA has teeth that are straight cut (shaped like a house). The jeep I am working on is a 1999 that calls for part number 4897052AA for both 3 and 4.   It looks like at the factory, they have put part number 4897052AA for synchro 4 (near the input shaft) and 4897051AA for synchro 3 (near third gear).  Very confusing!  I also found, like I noted before in my second email, that the teeth of part number 4897052AA and 4897051AA are in a different configuration around the ring slightly than the originals as you can see in the pictures.  I have purchased many 3 and 4 synchro rings but none of the teeth patterns match up!  I had no problems matching up 1,2 and 5 synchros.  I hope this helps you understand what I am up against!  Thanks again!  Jason Logan  
P.S   I purchased part number 4897051AA today from Chrysler to clarify if it was angle cut and indeed it was! 


Moses: Hi, Jason, I’m just back from Moab, UT Jeep Safari…You sent great photos, this is all very interesting!  Since the gear/ring in question is really 3rd gear, my belief is that Chrysler/Aisin discovered downshifting to 3rd problems and implemented a remedy.  The angle cut would engage the shift sleeve differently, apparently allowing easier engagement on the downshift from 4th to 3rd.


It would seem like either ring would work, as they each engage the sleeve troughs accurately (taking a straight-on view).  The difference is that the angled ring would engage the sleeve with a different pressure and slight rotation of the ring.  Just a guess, but this seems the only identifiable issue.  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 design would work if all drivers shifted up and down smoothly and without taxing the synchros.  However, American drivers (and others with Jeep vehicles) want to affect a quick downshift to 3rd gear.  The concern here, though, is to match the bronze rings with the updated synchronizer sleeve design.


Does this make sense, considering the parts layout?  The tooth spacing and offset on the rings has more to do with keeping the ring as close to square as it moves onto the gear hub. 


If nothing else, this is a testimonial to the precision design of Aisin transmissions, more like Euro types than U.S. gear products.  The aim with a wedge ramp instead of arrow teeth is the shift “timing”, and that’s a precise consideration.  This reflects the rest of the AX15 design quality and fitment.  These are more complex units for a reason.


I’m sure your build will be stellar, Jason!  Let me know how this turns out.






Jason: Makes perfect sense Moses!  Would it make sense to you to use the angle cut synchros for both sides of the slider even though the slider on one side is straight cut and the other is angle cut?  I am still unsure if I should use both angle cut synchros, both straight cut synchros, or use one of each. The rebuild kit I purchased had two angle cut synchros, I am wondering if this is the upgrade? To me it is like rolling the dice on this decision!  Thanks for all your input.  Very much appreciated.


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Photos courtesy of forum member Jason Logan...Note the OEM synchronizer design for this late version of the AX15—Mopar parts.


Moses: I would follow the OEM approach if you’re sure the transmission is original, never before apart (which you believed from the start).  The model year is late enough in the game for the synchro "solutions" to be in place…I would match the bronze rings and synchro sleeve carefully to the OEM layout. If this transmission shifted well for all of those miles, I would take this approach!


As a final check, place the new synchro rings against the gear hubs to be sure the fit is aligned.  You can put a light film of grease on the ring contact surface to read the fit.  Wipe the grease away before assembly and coat the ring with a thin film of gear lube.


Let me know how this all turns out, Jason…






Jason: Thanks again Moses!  I hope you had a successful trip to Moab,  UT Jeep Safari.   I have wanted to go for many years.  Wish me luck on the AX15! 



Moses: Hi, Jason…Moab is breathtaking scenery, with contrast ranging from sandstone/slick rock to alpine peaks capped with snow this time of year, the Colorado River, arches and formations, all of it! I trust you’ll make it at some point, and if so, let me know.  I do make the Moab Jeep Safari each year…Like Canada, we live at a “winter zone”, 4400 feet elevation near Reno.  We look forward to winter’s end, and Moab Jeep Safari has become our annual end of winter/early spring gathering!


You’re going to do a great job on this AX15!  Follow the assembly steps faithfully, there are no shortcuts.  When completed, you’ll have a smooth-shifting unit, as new, and that will be very gratifying!  Let me know the outcome…


I launched a message board today with a large number of forums at www.4WDmechanix.com/forums.  It would be great to see your involvement if you have the time.  Tech forums need detail-oriented members!  It’s new, and I’d value your feedback about the forums you find interesting, the sign-up procedure, member validation and use.






Note...Jason and I moved this conversation to the new forums at this point...When rebuilding an AX15 that has never been rebuilt before, lay out the parts as you take the unit apart, identify the synchronizer design for 3rd/4th gear, and match parts to the original design. There is a distinct difference between "earlier" AX15 and "late" AX15.  The rebuild core in my magazine article and the A150 (Toyota version of the AX15) depicted at the Weber State University YouTube video (click for post topic threads containing the embedded video) are 1990-92 "early" AX15 design.  Later model Jeep vehicles with the AX15 use a redesigned 3rd/4th synchronizer assembly...Rebuild accordingly, matching and using the right parts! 


Jason has added charts from an Aisin direct dealer that show the synchronizer applications for 1998-99 AX15 transmissions.  Make sure you check your synchronizer rings and the synchro sleeve design for 3rd/4th gear.  Install matching rings for your transmission.  Be certain to install the synchro sleeve in the correct direction!  ("House" or arrow shaped points match the 4th gear ring with house or arrow point teeth.) Here is an approximate application list with Aisin part numbers (not Jeep/Mopar):


Attached File  Aisin AX15 3rd-4th Synchro Chart.jpg   60.87KB   6 downloads

Click on image to enlarge...Thanks to Jason Logan for the chart!

#2 ttippetts


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Posted 30 January 2014 - 06:55 PM

Great info.  I am in the process of rebuilding my AX15.  It has been a slow project as I don't have much time at any one moment to spend on it.  Before I ordered my rebuild kit I emailed the supplier and he informed me that all the kits he sells have the updated style synchros.  He stated they don't make the old style any longer.  My kit arrived and they are correct :)  Bummer part is I chipped away a few corners on my second gear while attempting to press it off.  I guess I'll be sourcing a new/used second gear. 

Tracy (ttippetts)

#3 Moses Ludel

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Posted 30 January 2014 - 09:00 PM

If you can find a very original (in premium shape) 2nd gear used, that could be helpful.  Good gears are out there, this is a constant mesh gear, so the wear points would be the endplay thrust face and the synchro hub engagement teeth on the gear.  Unless the synchronizer was severely worn or abused, the 2nd speed gear will be intact.  A new second speed gear could be costly, hunt for a used one first...


Keep us posted, TTippetts, we're here to help with your AX15 transmission!



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