AX15 4th Gear Vibration On Power

annihil8ted

annihil8ted

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Joined
Jun 13, 2023
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99
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Richardson, Texas
I'm getting a vibration in 4th gear when RPMs are between 1.5-2K and on power. Once I'm above 2K, the vibrations go away. If I'm coasting in this range, no vibrations at all.

None of the other gears exhibit this issue so I'm a little bit at a loss. The transmission seems to be pretty smooth throughout the range except for this.

Could this be driveshaft related? Or am I going down the wrong rabbit hole?
 
Mostly everything is stock, if anything the tires are slightly undersized at 215/70/15. I haven't cracked open the diff yet but I believe it should be 3.07

I am trying to accelerate; shifting from 3rd at almost 3K, when I'm in gear in 4th, it's somewhere right below the 2K mark.

I say 1.5K because I've experienced the vibration down that low before but typically I'm trying to keep things around 2K at a minimum
 
annihil8ted said:
Mostly everything is stock, if anything the tires are slightly undersized at 215/70/15. I haven't cracked open the diff yet but I believe it should be 3.07

I am trying to accelerate; shifting from 3rd at almost 3K, when I'm in gear in 4th, it's somewhere right below the 2K mark.

I say 1.5K because I've experienced the vibration down that low before but typically I'm trying to keep things around 2K at a minimum
Click to expand...

Have you tried to take 3rd to redline before shifting to 4th to make sure it’s gear related and not speed related?
 
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That's a good point, I haven't really taken any of the gears past 4K. I'll try giving that a go in the next couple of days and report back.

I want to say that early shifting to 5th doesn't cause the same vibrations at lower rpms but I can't be 100% certain
 
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annihil8ted said:
That's a good point, I haven't really taken any of the gears past 4K. I'll try giving that a go in the next couple of days and report back.

I want to say that early shifting to 5th doesn't cause the same vibrations at lower rpms but I can't be 100% certain
Click to expand...

Just trying to eliminate variables and isolate the issue.
 
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So did some tests in some super low traffic roads:

There is a vibration in 3rd above 3K but it's very very small.

4th has vibrations until 2K and then goes away but noise is only on power/acceleration

Any ideas?
 
Yes, they cover the same speed range which makes is kinda confusing. But this only seems to happen on power I should note. And at those RPMs with stock tires, I'm around 50-55 mph
 
I'd try changing the fluid to Redline MT. I'd see if I can replicate the noise with the transfer case in neutral running similar RPMs.

3-4 gear is on the same shift rail. You probably have a bearing going out on the AX-15.

Screenshot_2023-09-07-06-28-48-50_f541918c7893c52dbd1ee5d319333948.jpg


-Mac
 
annihil8ted said:
Closer investigation shows it closer to 55-60 MPH range, not sure if the 5 MPH makes a difference but it's definitely there
Click to expand...

So if I understand your troubleshooting correctly, the vibration is speed dependent and not gear dependent, correct? If that’s the case, it could be anything and not transmission related.

Have you ruled out the simple possibilities, like tire balance, driveshaft u-joints, etc?
 
Yeah that is my next plan of action; tbh everything probably needs to be replaced or rebuilt and the rear drive shaft was next on the list, already got the U joints on hand

Need to find a good weekend where weather is cooler to tackle the job
 
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