AX-15 Shifting hard

I'm having a little problems with my AX-15 transmission.

When I am idling in neutral at a stoplight, it is hard to get it to shift into 1st. I can usually put it in 3rd then go right into first no problem. But if I try to go directly into first, I have to force it in. There is no grinding, but I really have to strong arm it in. And if the weather is warm, and the car is warm, it gets hard to put it into 3rd as well. 2nd gear is also hard to go into. If I am rolling to a stop, it slips right into 1st.

When I am moving, everything shifts very well.

The previous owner replaced the clutch less than 30,000 miles ago.

I have already flushed all of the the old clutch fluid out. I have gravity bled the system, and also bled it with the pedal. I have a vacuum bleeder, but haven't tried it yet.

Do you think it is worth it to replace the master cylinder? I have an internal slave cylinder, so replacing it would require me to drop the transmission. Or is it the synchros going bad?

Any help would be appreciated.

Can you get it into reverse? If you can get it into reverse every time, it's unlikely to be a dragging clutch.

If you have any doubts about the synchros, you might try changing the oil. If the previous owner put GL5 gear oil in it it will be hard to shift and probably noisy as well, and it will eat up the synchros. Try Dino oil specifically for synchromesh, or sulphur free synthetic, or even just plain 10w-30 motor oil. Any one of these will shift better than GL5, which murders bronze synchros.

I just recently replaced the AX 15 in my 99, because after a nasty clutch explosion in city driving I had trashed the reverse idler trying to slam-shift my way through a busy intersection. I put in the one out of my 95, with 263K miles or so on it. It occurred to me to check the oil after I'd test driven it a while. Oil? What oil? I don't remember when I had last checked the oil in that old beater, but somewhere in the last ten years it had vanished. It shifted better with no oil than it did back ten years ago with GL5 in it.

I forgot to mention that I also changed out the transmission fluid first with synthetic 10-w-30, then with Pennzoil Synchromesh. I also have no problems going into reverse.

normal procedure for me,
into 3rd before slipping into 1st

1st gear is (believe) not synchronized. aka crawler gear
try shifting from 2nd down to 1st at any speead above 10 mph (wont go in)

Also, same procedure for shifting into reverse. (3rd to reverse)
120k on original clutch. tons of stop and goes. Shift are always clean (no clutch slipping by operator)
2nd gear chirps were the norm. I treat her nicer now.

Always shifted like this since I got her at 34k.
yeah.. i am bad.. never changed the fluid.
worried about using correct fluid.

I run Mobil1 10w30 in mine, first is a little stiff but I'm pretty sure it's because the synchros are a bit on the blunt side. If I get the clutch spinning a little in neutral and drop it in 1st gently it always goes in nicely.

Reverse is a mess sometimes but shifting it through 3rd or 4th lines things up most of the time. The AX15 has a reverse interlock IIRC (i.e. you need to move through the center neutral position before shifting into reverse) so that might explain it.

Mine, like Kastein's and MileHi's is a bit stiff going into gear, but generally speaking it goes into first all right at a standstill.

By the way, first is indeed synchronized, or you wouldn't be able to shift down to first in motion at any speed without crunching or needing to double-clutch. The final drive ratio is also (assuming stock) too tall to make second gear starts comfortable, so it's a functioning first, rather than a granny gear. My old F-150 had a four speed with an unsynchronized granny low, and believe me, you'd know it if you had it. But even with synchros you can get a little easier downshifting if you double-clutch it, especially between second and first.

In response to the original post, I think if you can shift into reverse without crashing, and can shift on the go all right, the clutch is probably all right, and I wouldn't worry much about it, even if the synchros are getting a bit tired. As long as you're not grinding the gears up in use, you can go just about forever this way. Even if the synchros wear down to a nub, you can drive it just about forever if you hone your clutch technique. Manual transmissions are pretty forgiving.

MileHiXJ said:

I'm having a little problems with my AX-15 transmission.

When I am idling in neutral at a stoplight, it is hard to get it to shift into 1st. I can usually put it in 3rd then go right into first no problem. But if I try to go directly into first, I have to force it in. There is no grinding, but I really have to strong arm it in. And if the weather is warm, and the car is warm, it gets hard to put it into 3rd as well. 2nd gear is also hard to go into. If I am rolling to a stop, it slips right into 1st.

When I am moving, everything shifts very well.

The previous owner replaced the clutch less than 30,000 miles ago.

I have already flushed all of the the old clutch fluid out. I have gravity bled the system, and also bled it with the pedal. I have a vacuum bleeder, but haven't tried it yet.

Do you think it is worth it to replace the master cylinder? I have an internal slave cylinder, so replacing it would require me to drop the transmission. Or is it the synchros going bad?

Any help would be appreciated.

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I had this problem as well. After I changed the clutch master cylinder this problem went away...

I'm having this problem with my '92 4.0 MJ now. It started about 2 months ago. I converted her to 4WD about 2.5 years ago using an external slave AX-15 from a late '94 XJ and a LUK clutch kit. The clutch master is a NOS Lucas I put in when she was 2WD with the internal slave AX-15. At the time I married this master to the donor slave and plastic hydraulic line. I replaced the slave yesterday with a LUK slave and, while she shifts better, the problem shifting into 1st from a stop remains. It seems that something is hanging up a bit or not releasing completely. Has anyone had issues with the LUK clutch kit?

If you can contact the previous owner, you might ask him if he had the flywheel turned.

I have a '91 XJ with the AX15. When I rebuilt the motor, I had the face of the flywheel resurfaced, which was a mistake. That increased the distance between the clutch pressure plate (which is mounted to the face of the flywheel) and the throwout bearing inside the transmission bell housing. So ... the throwout pushes the pressure plate a little less, and the clutch doesn't fully disengage.

When the trans oil is cold, it shifts into first just fine at idle. As it gets warmer, it's easier to spin the input shaft, so even though the clutch is only 5% or 10% engaged, it will spin the shaft and make it harder to go into first.

As my clutch plate has worn, it disengages a little more now, so it's not so bad. But when I pull the engine or tranny again, I'll either replace the flywheel with a new one, or shim the flywheel so it's closer to the throwout again (they make shims for that), or shim the bottom of the internal slave cylinder, so that it's closer to the flywheel.

As for fluids, I use Redline MT90 or MT75 synthetic oil, which is specifically made with the proper coefficient of friction, and has no corrosive effects on the brass synchros. I've also heard that synthetic 10w-40 motor oil is good.

I had the flywheel turned when the internal slave went out (back when she was 2WD) and I had to R&R the clutch. I converted her to external slave/4WD about a year later and the flywheel looked good (no hot spots, etc) so I didn't do anything other than hit it with some sandpaper to break the glaze for the new clutch.

I'm running synthetic Pennzoil 10W-30 + 1PT Lucas in the trans.

The distance from pressure plate and throwout bearing makes sense but I don't understand why the problem waited until now to appear. I've put about 30K on the resurfaced flywheel and 20K on the 4WD conversion.

1987Comanche said:

The distance from pressure plate and throwout bearing makes sense but I don't understand why the problem waited until now to appear.

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I honestly don't know the answer to that ... with mine, I noticed it immediately. I changed the clutch master cylinder, but that didn't do anything. Also, on mine, the flywheel face was scored up pretty badly, so they had to take quite a bit off.

Sorry to drift off topic, but I will probably convert my AX15 to external slave also. There is a '94 xj being sold on craigslist in my area for cheap. It has the AX15 external slave, AC, and a few other goodies mine doesn't have. I'm thinking of buying it as a parts vehicle. A personal junkyard for my '91.

MontanaMan, don't know if you're still reading forum messages, but I've been having same problem - difficulty esp. going into 1st and 2nd - since having a clutch job 3-4 months ago.

I've got a 90 XJ. We installed a new clutch disc, pressure plate and throwout bearing, new master and slave cylinders and a new hydraulic hose. You wrote,

"When the trans oil is cold, it shifts into first just fine at idle. As it gets warmer, it's easier to spin the input shaft, so even though the clutch is only 5% or 10% engaged, it will spin the shaft and make it harder to go into first. As my clutch plate has worn, it disengages a little more now, so it's not so bad."

I have also noticed harder shifting once warmed up which is the opposite of what I'd expect, but your explanation above could be what's happening. On the other hand, your notes about flywheel resurfacing possibly causing clutch drag don't make sense to me. Either flywheel resurfacing or clutch plate and pressure plate wear should make it easier for the clutch to be disengaged and harder for it to engage fully, I'd think.

My current best guess is that either there's a pesky air bubble somewhere in the line or that either the master or slave cylinder installed is bad (bought everything at Autozone). Only other possibility I can think of is that the pilot bearing on the input shaft is sometimes sticking.

I'll post a followup if-when I get it fixed...

AX-15 lubricants

I'm also curious if anyone has theories why it's so hard to find either dino or synthetic GL-4 in the parts stores. It's a strange situation given all the people out there looking for GL-4 due to having yellow metal synchros, esp. because you find tons of GL-1 in its stead.

When my 1990 XJ came out, GL-3 was originally recommended. Then, it all clocked right past GL-4 to GL-5. I have a Haynes manual for my 90 that recommends GL-5. Once the dust settled, GL-4 became the AX-15 recommendation - and for many other transmissions with bronze synchros - yet all I find in parts shops are GL-5 and GL-1.

Some GL-1 brands now say right on the label that they're on the shelf because they don't have EP additives that can harm yellow metals in trannies. I bought 4 quarts of Quaker State GL-1 "Green Oil" last week and put it in my tranny just to see what it'd shift like. I was looking for a GL-4 but nothing in stock at the three stores I visited and I didn't want to order Redline MT-90 then pay $70 for 4 quarts.

Back of the "Green Oil" bottle says its formula is suited for manual transmissions with bronze parts because of not having EP additives. So, as for debate about whether GL-5s general harm bronze synchros, there's a "formal confession" from a major firm.

Although it's evidently possible to have high EP protection without the sulphur compounds that cause yellow metals pitting, consensus seems to be that the GL-4 formulas maximize gear protection while not causing the bronze pitting associated with GL-5s.

So, my question is - given that gear pressure protection increased from GL-1 thru GL-5 over 60+ years - why have major oil firms reached all the way back to the GL-1 spec versus GL-4, esp. given so many trying to find GL-4 and paying Redline prices? My theory is that the goal is to ease reliable old vehicles off the road. Well, big Wall St. banks are major owners across the entire oil and auto sector. Neither they nor governments make as much off older vehicles and the preference is to keep the "consumer" treadmill running. Or, maybe there's some other completely valid reason.

I've also tried 10-40 synthetic motor oil in my AX-15 and it shifted real smooth, definitely smoother than now with GL-1. But, the smoothness wore off after 6-8 months. While tranny temps are lower than engine temps, evidently the higher pressure on gears causes breakdown, which is what the GLs formulas are meant to deal with.

10w30 syn can also damage your synchros, I know that first hand. I use nothing but redline MTL in mine. it may not be cheap, but how often do you change it?

Re: AX-15 lubricants

jdw-dd said:

I've also tried 10-40 synthetic motor oil in my AX-15 and it shifted real smooth, definitely smoother than now with GL-1. But, the smoothness wore off after 6-8 months. While tranny temps are lower than engine temps, evidently the higher pressure on gears causes breakdown, which is what the GLs formulas are meant to deal with.

Click to expand...

I'll second the recommendation to use MTL or MT-90. The MTL has friction modifiers to help the synchros work properly. I couldn't decide on which and actually mixed them 50/50 when I had the AX15. It was a night and day difference between that and the NAPA GL4 I put in after I rebuilt the tranny. I started with the cheap GL4, intending to run that for a week and then drain/refill with the good stuff. I changed it after 3 days because it was shifting so bad with the GL4 that I thought I buggered the rebuild. It shifted beautifully with the Redline after that.

https://www.redlineoil.com/gear-oil-for-manual-transmissions

Re: AX-15 lubricants

Thanks for the Redline MT-90 recommendations. I'll bite the bullet and go out and buy 4 quarts today and report any difference experienced.

I also suspect my shift issues may associate with a pesky bubble in my new clutch line so will make another effort to bleed the system later today.

As for regular motor oil damaging brass synchros, I've never heard any reports of that, because regular motor oil lacks the EP protection coating issues involved with GL-5 damage. However, everyone seems to agree it comes down to the "additives."

The problem also isn't "brass corrosion," per se. The GL-5 additives don't "corrode" brass. They simply form a harder and more durable EP coating than you want, if yellow metal is involved. Damage occurs because the GL-5 EP coatings are so hard and thick that micro-thick layers of brass start getting stripped off along with the EP coating itself. The way it's supposed to work is that the EP coating keeps rebuilding and stripping, but not the metal underneath it!

The GL-4s all also have EP and usually via the same sulphur compounds that create the GL-5 problem. However, by having an average of half or less those compounds, their EP coating is thin enough that it doesn't bond so strongly that it starts stripping off a weaker underlying metal.

I'd be curious to know if Redline MT-90 uses a different compound create EP protection but it shouldn't matter, in that consensus is that as long as the GL-5 spec isn't included, there shouldn't be yellow metal damage.

The great experience everyone reports with Redline must relate to its "friction modifiers." Plus, a synthetic formula that resists breakdown. Synchros absolutely require friction to do their job (a reason I don't plan to put Lucas in my tranny). At the same time, the whole point of lubricants is to "reduce friction." Thus, a "Goldilocks Porridge" formula involved and Redline has evidently nailed it.

Hey, did you ever figure out what was causing your ax15 to be hard to shift into first? I am having the exact same problem as you, hard to shift into first after a few minutes of driving. It will not go into first if I am at a complete stop but if I'm rolling it goes in no problem. I just read through your thread and was curious if you figured it out, this problem is driving me crazy. Thanks, Cameron

chook98 said:

Hey, did you ever figure out what was causing your ax15 to be hard to shift into first? I am having the exact same problem as you, hard to shift into first after a few minutes of driving. It will not go into first if I am at a complete stop but if I'm rolling it goes in no problem. I just read through your thread and was curious if you figured it out, this problem is driving me crazy. Thanks, Cameron

Click to expand...

If you haven't do so already, I'd recommend a drain and refill with Redline MTL or MTL90. Make sure you can remove the fill plug before you drain it.

I second the recommendation to change your tranny oil as the first step. In many cases, that will be all you need to do. Let's just say, having the right lube is CRITICAL for the AX-15 to shift properly. How much more sensitive the AX-15 is to lube quality than any other manual tranny, I don't know, but I can tell you it's super sensitive. As part of that, the temperature range in your area should also inform your mix.


I've posted here about the claimed dangers of EP (extreme pressure) additives to GL5s for trannys with yellow metals. Most manual trannys have bronze synchros, as does the AX-15. I have a bottle of Quaker State GL1 "Green Oil" here that says on its label it is "safe for yellow metals," contrasting that with GLs with EP additives.


So, there is at least some official acknowledgement of the EP vs. yellow metals issue even from major oil companies. This said, most tranny specialists I've spoken with say they've never seen convincing evidence of GL5 EP damage to bronze syncros. My conclusion is that: a) not all GL5 EPs are created the same, some pose a definite scaling risk to worn synchros, others only minimally so; b) even the worse EP formulas will not immediately eat up synchros, but they will wear faster.


This as backdrop, a year ago, I put a GL5 into my AX-15 with 280,000 miles on it. A month later, I began hearing a soft whine from the tranny at idle and began having rough shifting. I've been driving this Jeep 20 years and never had any such problems, thus immediately suspected the GL5 yellow metals issue.


I drained the GL5 then made the bigger mistake of using a GL1 (for the EP reasons mentioned). Only later did I learn that the lube qualities of GL1s are way inferior either to a GL4 or GL5 or to any popular motor oil (regular or synthetic). The GL1 hastened the hard shifting problem, becoming going into 1st and 2nd.


To make a long story shorter, I decided that, at 280,000 miles, probably time for new synchros and a rebuild so took it in. Turns out the synchros were still in excellent condition - even 1st and 2nd, though they showed minor wear - bearings and gears in such good shape my mechanic was reluctant to swap in the new parts. He has 20 years prior experience rebuilding transmissions at AAMCO.



He filled it up with 20-50w synthetic motor oil plus 20% his preferred GL5. He said he found this mix to provide an ideal combo of smooth shifting, gear protection and longevity. BTW, evidently all brand name synthetic motor oils have EP (pressure protection) equivalents to GL4 or better standards.


Meanwhile, I had a gallon of Redline MT90 and didn't want to waste it. So, I drained his mix and filled with MT90. For whatever reason, the shifting was not as smooth esp. when cold. After a few weeks, I drained the MT90 and put back in his secret sauce. Immediately, it shifted more smoothly so I donated the MT90 to a friend.


This is just my personal experience. My Jeep is a 1990 Cherokee Sport (2 dr), AX-15 tranny (and NP231 transfer case). Anyone might have a different experience for whatever reason - less wear, different climate, etc.


BTW, while the tranny was out, I took the opportunity to swap out the crappy stock T-case linkage with a link kit from Azzy's Design Works. Brian Spiegel there was a big help getting that working and just professional and a pleasure to deal with. I recommend them for link kits, custom gauges and anything else they sell.