AX15 Transmission Popping Out of Fourth Gear

[cnyncrvr]

Hi Moses I purchased your AX-15 rebuild video on vimeo. I also downloaded the AX-15 service manual before I started this project.

When I started this project it was supposed to be a simple clutch install. How ever when I removed the transmission from the Jeep which is a 93 YJ Sahara. I noticed that the pilot bearing had grenaded and part of the inner race had welded its self to the input shaft on the transmission.

 

I figured since I will have to tear the transmission down to replace the input shaft I might as well do a full rebuild since I had noticed some grinding going into 3'd gear on occasion. I ordered a new input shaft, and full rebuild kit with bearings, seals, synchro rings and what not and began the process of doing the rebuild. I followed your video to the T and verified everything with the shop manual.

I got everything rebuilt and installed back in the jeep and everything worked fine albeit a bit on the tight side. The transmission shifted with no issues smoothly for about the first 5 miles. I then parked the jeep over night and when I went to drive it the next day it would grind horribly trying to get into 4th gear and would not go into 4th at all. Even if I put the jeep in 4th and then started the motor and let the clutch out it would just pop right out of 4th. There is also now a subdued but high pitched whining noise from the transmission.

Any idea on what could be wrong and why it worked fine until I parked it and then everything went to the birds?

The Jeep has a new friction disc, Pressure plate, Pilot bearing, Throwout bearing, New bearings, synchro's, seals, shifter bushing and seal, Transfer case was rebuilt with new bearings, new chain, new range fork, new mode fork pads, slip yoke eliminator kit, New heavy duty rear driveshaft, new Ujoints in the front drive shaft. Front and rear diff fluid changed and sealed.

Tons of work to get the driveline back into good shape and now the trans has started having problems it never had before.

[Moses Ludel]

Hi, cnyncrvr...If the transmission went together by the book and in the fashion described in the video, there are other possibilities.  Parts wise, synchronizer design has come up as an issue, we've discussed that at length in the forum topics on rebuilding the AX15.  Look over those comments, using  a quick search at the YJ/TJ Wrangler forum under keywords like "transmission" and "synchronizers".  The later AX15 uses a different 3rd/4th synchronizer brass ring style than the earlier design that is often furnished with rebuild kits.  You'll find that discussion...

There are a number of transmission issues that can cause this kind of problem, though the five-mile trip without incident does create some confusion.  There could be an issue of parts coming loose within the transmission, and my key focus here would be the mainshaft moving rearward under a variety of conditions, including but not limited to:  a loose transmission case or intermediate plate, a loose or shifting output shaft bearing, loose snap rings, or a situation that allows the main/output shaft or gear set to creep rearward.

4th gear on the AX15 is direct drive, a lockup between the input shaft and mainshaft.  A classic cause for a transmission jumping out of gear is when the input gear's back (pilot) bore bearing has too much clearance.  Visualize:  Flexing at the point between the input gear's  back end bore and the main shaft's nose.  This can also be caused by a misalignment of the transfer case with the transmission.  In this instance, the transmission's output (mainshaft) flexes enough to cause the front end of the mainshaft (at the pilot bore bearing in the input gear) to cock.  The synchronizer ring(s) get cocked, too, which would allow the transmission to jump out of fourth (direct) gear.  This trouble is always pronounced for the gear with direct or through-power flow from the input.

Another possibility is the input gear cocking from a worn or incorrectly sized crankshaft pilot bearing.  Here, the input gear rocks because there is no support at the front of the input gear.  The input shaft/gear, when working normally, aligns between the crankshaft pilot and the front case bearing on the input gear.  Again, a misalignment of the transmission, or the bellhousing with the engine block, can create this issue. Binding of any kind, including a driveshaft that is too long, the wrong angle or out of phase, can cause gear bind and jumping out of gear.

So, before condemning your transmission work, consider the fit of the transmission to the bellhousing, the bellhousing to the engine block, the transmission to the transfer case, the transfer case's input gear angle, loose hardware at any of these points, an out-of-center crankshaft pilot bearing, or the wrong crankshaft pilot bearing size.  Don't rule out the shifter mechanism itself, make certain parts are intact and feel "connected", with the shift lever moving the shift rails without slop and through their full range of motion—with no floor carpet or console interference.

Try to rule out the simpler and more accessible possibilities first.  Loose hardware prospects would be a place to start and shifter lever throw.  We can get into the transmission if/when you rule out these other possibilities.

Moses

 

[cnyncrvr]

Hi Moses, I did read the article on the differences between the late and early model synchro rings and sleeves previous to even attempting to rebuild the transmission and while it was a little confusing as to which was which I was able to get through it with what I believe to be the right parts.

You also mention the input shaft rear bore pilot bearing which made me remember something when I was putting the transmission together. The new bearing fit the bore of the input shaft fine, but there was what I would call "Considerable" play between the bearing and the mainshaft. I compared it with the old bearing which seemed like it was in Okay shape and noticed the same amount of play, Is this normal? Could I have gotten a faulty bearing? Could the main shaft be so worn that it will not fit the bearing tight and precisely? Or could be just be that I got a sub par quality rebuild kit?

I found the kit online and it was not a Crown kit that I so often hear referred to. It was a no name kit that cost a little over $200.00. The kit came with a new shifter ball, plastic bushing. I remember thinking when I changed the bushing on the shifter to the new bushing that came with the kit thinking, "Wow this feels really loose and doesnt seem to fit any where near like the original" which was a bear in its self to get off.

I will continue to check the basics but I am already more of the opinion that I will be pulling the transmission again and starting from scratch with a Crown rebuild kit.

[Moses Ludel]

cnyncrvr...I would be concerned about the mainshaft (output shaft) nose end play at the input gear's counter bore bearing.  If you recall this as radial play (essentially side play with the two shafts aligned), there's a big problem.  When fitted properly, there should be virtually no perceptible radial, lateral or side play at the counter bore bearing.  (End play is controlled by the two shafts being fixed in bearings with snap ring alignment.)  The input gear's caged counter bore bearing is a roller type designed to work with nothing more than adequate oil clearance.

A loose fit between the input gear counter bore bearing and either the input gear's counter bore or the output shaft's nose would cause gear whine and also the jumping out of fourth gear.  This is the cocking action that I mentioned in my previous comments.  The clutch hub of the 3rd/4th synchronizer assembly cannot stay centered or in a gear notch.  As a result, the synchronizer lockup in fourth gear cannot take place.  The gear whine is from constant mesh gears that are moving apart from each other, which changes their contact patterns.

Why the output shaft nose would wear to this degree is likely two-fold.  Heat and ineffective lubrication would be a factor.  The main issue, though, is that the worn crankshaft pilot bearing allowed the input shaft to cock at its counter bore end, and the binding caged bearing chewed up the output shaft's nose.  If the output shaft's nose wore evenly enough, you might not have noticed the wear; however, the loose shaft to caged bearing fit would be a clear clue, especially with a new caged bearing.  

These shafts are typically case hardened after machining, and if wear is severe/deep enough, the shaft nose could be worn to the softer, non-case hardened cross-section.  This would cause very rapid wear from that point onward.

Bummer that you'll be pulling the transmission down.  These are close tolerance gearboxes, and the alignment of the input/output shaft is critical to gear/tooth alignment.  You've described a trouble point that would readily cause whine and jumping out of gear.  Of course, there could be a wrong bearing roller diameter that might present the correct O.D. for the input counter bore and the wrong I.D. for the mainshaft/output nose diameter.  This would be unusual for any rebuild kit, I'd be targeting the mainshaft/output's nose diameter.  Take a diameter measurement and compare with a new or good used output shaft's nose.  It's a good sign that the bearing fits well in the new input gear's counter bore.

Your memories from the build have helped here.  The loose fit of the mainshaft/output's nose to its caged roller bearing will be an inspection target during your tear down.  In the meantime, don't drive the Jeep, the misalignment of the gears will damage the tooth contact patterns and lead to very costly gear and counter gear parts replacement.  Good you caught this trouble quickly.

Please share your findings and the solution.  Like you, I'm always looking at parts quality, parts that fit and "feel" right.  A loose shifter bushing mechanism would have me either reusing the O.E. pieces (if in good condition) or seeking better parts.  If the mainshaft/output is the culprit, you can use either a "good used" shaft (everything still measures on tolerance, including the built-in thrust and the bearing surfaces) or a new shaft.  New would be pricey, obviously.

Let's work through this and get that AX15 in shape!

Moses

[wardog73us]

Hi Moses. So cnyncrvr and I have been working on this transmission, as he said, for quite a while. And as he said, we ran into many re-assembly issues. So after basically losing 4th gear with barely 10 miles on the fresh rebuild, I finally removed it from the jeep again and have done a partial tear down on it, meaning I have split the case again but have left the assembly intact in the intermediate for the time being. I will be attaching some photos too to hopefully shed more light on this mess. The first thing I noticed was when I drained the case. The gear oil came out clear amber in color for about 1-2 seconds then became black and clumpy. I found a lot of metal shavings in the bucket I drained it into, some from the synchros and some from other sources, hopefully the new cage roller bearing in the input shaft. I didn't notice any excessive wear on any of the gears so I'm slightly hopeful there.

As far as the main shaft goes, there is a ton of play in the input shaft. It moves up and down, side to side and forward and backward a lot more than I would think it should. The 3rd gear synchro seems to be doing it's job correctly, but the 4th gear synchro will not engage at all and the entire assembly seems to bind up now in neutral, where it use to spin freely and fairly easily when it was first re-assembled. Hopefully the photos I'm including will explain things a little better than I have. I know I am going to have to remove the input shaft and likely have to replace the caged bearing again but I want to check for the wear you talked about on the nose, but don't have another shaft to compare it too and can only find the clearance for 1st, 2nd and 3rd gear bearing surface diameters. Nothing on the actual nose surface, but I'll keep digging. Well. As I said, hopefully the pictures will help.

 

[Moses Ludel]

wardog73us...The misalignment of gear sets in the photos could simply be that the snap rings on the bearings do not hold the shafts in their normal positions while you have the housing/case removed.  Align the bearing snap ring grooves as they would fit with the cases in place.  Then judge the movement of shafts, gears and synchronizer pieces.

One way to determine the nose diameter on the mainshaft is to look up the OEM bearing dimensions.  Get the Jeep AX15 or a Toyota A150 counter-bore bearing part number and cross reference it to Timken, NSK or another popular bearing.  Then look up the I.D. and O.D. dimensions. If you take the bearing number to a catalog listing for these kinds of bearings, you should find an I.D. and an O.D. dimension.  

We know that the clearance between the bearing rollers and the nose of the shaft should be minimal (essentially just enough for oil clearance with this type of bearing).  So the bearing I.D. dimension is very close to the normal nose diameter. 

If the nose turns out to have the correct diameter, check the current bearing number to see whether the input counter-bore bearing is the right one...If you cannot find either the bearing or mainshaft nose dimensions, let me know.  I'll research a bit further in my Jeep and Toyota service data.

Moses