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Vibration on deceleration?

Cortez

Bronco Guru
Joined
Jan 29, 2009
Messages
1,193
Hey y’all - just did an AX15 conversion from an NP435. Test run…man this baby shifts so smooth and easy! When I get to highway speed, I noticed a vibration on deceleration…so when shifting between 4th and 5th that moment when it decelerates slightly I get vibration. The vibration goes away on steady state driving or acceleration. And I notice it less when down shifting and decelerating under engine drag. Don’t notice any vibration between shifts at slower speeds. Note: I thought the driveline length was unchanged between the AX15 and stock 3 spd or NP435…however I did note that while reconnecting my rear driveling I found that it was all the way compressed and would barely squeeze into the t-case yoke. So any situation that requires a compression of the driveline from unloaded static position may be stressing the drivetrain in some way?

let me know your thoughts here, and thanks in advance.

Dave
 

Okie69

Contributor
Sr. Member
Joined
Nov 28, 2011
Messages
774
Did you get new driveshafts after the swap? I think that swap changes your lengths right?
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
Guessing the transfer case moved back a little bit. Probably just enough to push the rear driveline angles into unhappy range. deceleration rolls the pinion down slightly, making the angle worse.

Something I really worry about when you describe the driveshaft fully collapsed in order to install it, what happens when the rear suspension compresses? Sounds like you will be having issues with bottoming out the driveshaft. The transfer case doesn't like thrust loads, neither does the rear end, nor the U-joints in the driveshaft. Hard to say which one will be unhappy first? My vote is the newest and most expensive parts.
 
OP
OP
Cortez

Cortez

Bronco Guru
Joined
Jan 29, 2009
Messages
1,193
Did you get new driveshafts after the swap? I think that swap changes your lengths right?
everything I’ve read says they should be the same, but or very close, but it must have been just enough to create a problem.
 
OP
OP
Cortez

Cortez

Bronco Guru
Joined
Jan 29, 2009
Messages
1,193
Guessing the transfer case moved back a little bit. Probably just enough to push the rear driveline angles into unhappy range. deceleration rolls the pinion down slightly, making the angle worse.

Something I really worry about when you describe the driveshaft fully collapsed in order to install it, what happens when the rear suspension compresses? Sounds like you will be having issues with bottoming out the driveshaft. The transfer case doesn't like thrust loads, neither does the rear end, nor the U-joints in the driveshaft. Hard to say which one will be unhappy first? My vote is the newest and most expensive parts.
Thx - I better get the driveshaft shortened and balanced the more I think about it any bump or compression could tear stuff up.
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
Messages
47,355
Also check the driveshaft angle now, while still in the current state.
You can then make any other corrections necessary. Such as rear pinion angle if needed.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
To add a little more to Paul's comment...
If you shim the rear axle to fix pinion angle, that will change the length requirements for the driveshaft.
First step, get the angles right. Second step, get the correct length.
 
OP
OP
Cortez

Cortez

Bronco Guru
Joined
Jan 29, 2009
Messages
1,193
Thanks Gents, I’ve done the 1” axle setback and shimmed the rear axle previously But will check things since the AX install.
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
Messages
47,355
Good to check.
Non-stock transmission length. Non-stock axle location. And a potentially non-stock spring perch angle.
All mean that off the shelf angle shims might not actually be correct for your setup.

Got pics?
 
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