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1997 TJ 4.0, AX-15, 130k miles. In reverse a loud knock occurs and I can feel it in the shifter. In 1st-5th gears there is no knock and the transmission acts healthy with no issues and the Redline MTL is about a 2 years old. The knock goes away when shifted to neutral and coasting backwards. The knock is loud enough to be heard outside of the vehicle. Which of course makes everybody scream bad u-joints. Sounds like a rotational noise.

 

Just to add; rear drive shaft is new only has about 100 miles on it (yoke at the pinion got smashed on previous one) and the knock was there before that. I noticed the knock after running off a small hill at about 25mph and the landing was pretty hard. Also, after that same launch my RPM guage pegged all the way (clockwise) back to zero and has not worked since. But, I don't really care about that just yet. 

I know it's been awhile since I've been on here, I hope to be a regular from now on.

 

Thanks Moses!

 

Wayman

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Hi, Wayman, always good to see  your posts!  Sounds like the leap was enough to jar the transfer case hard.  The speed sensor or its connections at the transfer case may be damaged.  Also, check the transmission/transfer case mount.  If loose, broken or separating to one side, this could cause U-joint bind when in reverse. 

 

I'm assuming that if your TJ Jeep Wrangler is lifted, you have a CV type rear driveline and the correct pinion shaft U-joint angle.  (1.5 to 2 degrees works well at the pinion U-joint if the driveline has a CV joint at the transfer case end.)  Any driveline bind or knocking, or the transmission/transfer case rocking and knocking on the skid plate, could be rear transmission/transfer case mount related.

 

Does the noise continue if you coast backward with the transmission in gear and the clutch disengaged?

 

Let me know if this turns up anything...We'll go from there. 

 

Moses

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CV driveshaft, 4.5" suspension, motor lifted 1", 1.25 body lift, and still has an uneeded t-case drop. So, the powertrain does sit at noticeable angle towards the rear. The rearend is a low pinion 9". In gear (clutch disengaged) going backwards is when it happens and the knock tempo speeds up the faster I go in reverse and of course is slower at normal 2 to 3ish MPH in reverse. With a magnetic angle finder the driveshaft sits at exactly 10 deg.

Besides the knock, it really seems to be fine. But the knock does seem to move the shifter a little, so maybe the rocking is a possibility. I do have an aftermarket new tranny mount that has been sitting the garage for a few years now. Maybe it's time to throw it on.

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WMCCALL...Could be transmission/transfer case misalignment.  Check the mating hardware when you deal with that rear mount...See if the transmission to transfer case attachment is secure. 

 

Of course, this could be damage at the back end of the AX15, causing the reverse gear to be partially engaged, but let's not borrow that trouble until you've ruled out the other, simpler possibilities.

 

Moses

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98 tj sport, 4.0 ax15 np231 150k same issue. Recently purchased it for my nephew so he can trail with us. Drives fine, shifts fine. I'm 40yrs old and have been an auto tech my entire life but have never encountered this exact issue before. The only time it makes any noise at all is in reverse during deceleration or coasting. (2wd/4wd makes no difference) Under power in reverse it is fine. Pushing in the clutch does not stop the noise while still moving in reverse, only shifting the trans to neutral will stop the noise. And yes, it's definitely rotational noise, loud enough to hear from a good 50yds away. It didn't have the noise when purchased. I had to rebuild the front cardan and a front axle joint anyways when we got it so I replaced them all since I had it at the shop. We noticed the noise one day when he was trying a slippery hill that he ended up backing down several times. We crawled under it and everything is fine with the ujoints and mounts. No excessive backlash. He's only 16 and drives it everywhere. Almost 10k on it since noticing the noise, still drives ok, just that noisy tick in reverse as soon as you let off the gas. Spring has basically started and that means it's tire changing season here in upstate ny so I've been too busy to get him back in the shop when he and I are both free(you know how 16yr old boys are lol) Personally I think he was beating on it and miss-shifted or something and chipped some teeth on the reverse gear. When I do get it in the shop I'll stick a stethoscope to the trans and tc to decide which to tear apart. I've asked all my wheeling buddies and coworkers at the shop and found no tsb's about it in mitchell. I just spent the last hour searching the web and there's one post in pirate 4x4 about the same coast reverse noise but no one there actually addressed the question lol. Typical. Sorry this is wordy and long. If anyone has input I'm all ears. I'll update as soon as I can get the kid out of it long enough for me to take it to work to diagnose. :-)

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DJDRAGON...I like your assessment, you do need to determine whether this is reverse gear in the transmission or a the transfer case issue.  If you need to rebuild the AX15, I'm getting excellent feedback on my step-by-step video how-to rental at Vimeo On Demand:  www.vimeo.com/ondemand/ax15rebuild.  Follow this link and watch the free trailer.

 

The reverse gear is hypersensitive and in close proximity to the rear of the case.  On the other hand, the NP231 transfer case could have a clearance issue on the chain, planetary damage, shift cog damage, or simply damage to the fork slippers

 

I'm pleased that you have the shop and resources to lift the vehicle and safely listen to the noise with a stethoscope.  Please share whether the noise is at the rear of the transmission (actually at the front end of the extension adapter) or at the transfer case.  It should be relatively easy to draw this distinction with a stethoscope.  If at the transfer case, I'm targeting the NP231's shift slippers as a first possibility, there's apparently no noise beyond this symptom.  Think in terms of backing off the throttle in reverse and the resulting TC shift hub movement caused by loose forks or shifter slippers.

 

Moses

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Hey I know this is an old thread but I have the same issue in my 91 yj but the other day it gave out and stopped the Jeep in reverse and I could not get it out of gear even with the Jeep turned off I did get it out of gear and I have gears 1-5 still but reverse bonds up now and you can feel the shifter twisting so long story short I was wondering if anyone found out what the issue is and could let me know 

I appreciate all the help I can get thankyou

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Mkymskar...First consider the shift tower parts, they're easier to access...Otherwise, it sounds like the reverse gear came loose or the shift fork for reverse is loose on its rail.  If the gear is loose, the reverse fork is being pushed around by the loose gear.  Unless there is bearing noise, the shafts are likely in alignment.

In any case beyond the shift tower or a loose transfer case-to-transmission situation, you will be pulling the transmission apart.  If so, consider my Vimeo On Demand streaming instructional, it has been helpful to many.  Watch the trailer for free to get some perspective and a parts orientation.  Consider the 94-minute rental if you do disassemble and repair or rebuild your AX15 transmission: http://www.vimeo.com/ondemand/ax15rebuild.

The relationship of the gear parts can seen in the gear layout/diagram below (courtesy of High Gear Transmission):

 AX-15-Gear-Diagram.jpg

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