Driveline vibration in 4wd

[flick2614]

I started noticing some heavier vibrations coming from the driveline when in 4wd on the last few trails. I changed the front diff fluid to check, no shavings, oil came out pretty clean. Took it to a reputable shop, they went through everything and came up with the idea that the front driveshaft is likely the cause. I’ve got a 3.5” MC game changer kit, 37”s so I knew at some point the driveshafts would have to be changed out. We also came up with the idea that the FAD may also be having some issues. My question is what indicators would point to the Rzeppa joint going bad?

I’m not questioning the shop’s opinion, just making sure I’m not chasing down the issue past replacing the driveshaft. The drive shaft feels nice and tight and doesn’t have any real wear indicators.

Sponsored

 

[Apples491]

When I finally swapped to an Adam's front ds, I had no indication at all that the Rzeppa on my stock ds was shot. When I pulled it off, however, it was obvious that it had lost all of its grease.

The only thing I can think on the FAD being the cause would be it not disengaging. But, 1. that would just manifest driveshaft vibrations and 2. you'd feel the vibrations in 2wd drive as well.

IMO, go ahead and put on an aftermarket front DS. It's likely to be the culprit.

 

[AFD]

From the day I bought my JL brand-new I've always noticed additional vibration in 4WD. Nothing crazy, but it's definitely noticeable and mostly felt through the steering wheel or with your arm resting on the hollow center armrest while parked (while driving it's not noticeable at all).

Always assumed it was the FAD, since the same vibration doesn't always present itself in 4WD-auto mode until it actually engages the front driveshaft due to first loss of traction. In regular 4WD high, it's always present. Switching back into 2WD will always make the vibration disappear, as long as I drive a few feet (staying parked and switching from 4WD to 2WD the vibrations remain until the vehicle moves).

I asked about this here before and iirc, others considered the vibration normal since the FAD was coupling the driveshaft to the front of the vehicle and transferring vibrations from the engine further back into the vehicle. I honestly have no idea if that's correct or not, but it seemed like a reasonable explanation.

 

[flick2614]

When I finally swapped to an Adam's front ds, I had no indication at all that the Rzeppa on my stock ds was shot. When I pulled it off, however, it was obvious that it had lost all of its grease.

The only thing I can think on the FAD being the cause would be it not disengaging. But, 1. that would just manifest driveshaft vibrations and 2. you'd feel the vibrations in 2wd drive as well.

IMO, go ahead and put on an aftermarket front DS. It's likely to be the culprit.

Did you do the rear too?

 

[Apples491]

No. The rear driveshaft is much longer (on the JLU anyway) so a tall lift doesn't bother it nearly as much.

 

[Jtphoto]

Always assumed it was the FAD, since the same vibration doesn't always present itself in 4WD-auto mode until it actually engages the front driveshaft due to first loss of traction. In regular 4WD high, it's always present. Switching back into 2WD will always make the vibration disappear, as long as I drive a few feet (staying parked and switching from 4WD to 2WD the vibrations remain until the vehicle moves).

It’s not the FAD. When in 4Auto the FAD is fully engaged so it would act exactly the same as in 4Hi whether there is power to it or not. However it does sound like front driveshaft to me which would yield the results you observed in 4Auto when power is applied via slip and 4Hi.
Also be aware that caster/pinion angle are a lot more critical to minimize vibration with double cardon driveshafts.

 

[Reinen]

Caster & pinion angles matter. Keeping a high caster is good for steering but since the difference between caster & pinion angles are fixed on the axle, it's possible to have too much caster which results in too great of a pinion angle.
This video demonstrates how that can lead to vibrations.

 

[flick2614]

It’s not the FAD. When in 4Auto the FAD is fully engaged so it would act exactly the same as in 4Hi whether there is power to it or not. However it does sound like front driveshaft to me which would yield the results you observed in 4Auto when power is applied via slip and 4Hi.
Also be aware that caster/pinion angle are a lot more critical to minimize vibration with double cardon driveshafts.

For us non-392 pimps we don’t have 4-auto.

 

[flick2614]

Update to this: just got done with the Adam’s driveshaft install. Fairly easy install. A couple observations about the stock front driveshaft:

There were no obvious signs of wear, no play in the joints. However very oddly the torx hardware on the transfer case side flange looked like it had some rounded edges from contact which is baffling to me as there is nothing that it could contact (picture below).

Secondly I’m naive to the workings of the front stock driveshaft but I assumed the accordion looking boot and the fact that the Adams driveshaft has a slip joint to expand with travel meant that the stock driveshaft also has a slip joint to expand. This doesn’t appear to be the case or that joint is jammed up somehow.

 

[Jtphoto]

For us non-392 pimps we don’t have 4-auto.

4Auto is an option on other models as well. I have 4Auto on my 3.6 Rubicon.

 

[zouch]

but did it solve your initial problem?

Update to this: just got done with the Adam’s driveshaft install. Fairly easy install. A couple observations about the stock front driveshaft:

There were no obvious signs of wear, no play in the joints. <snip>

 

[flick2614]

but did it solve your initial problem?

The pinion to driveshaft angle after install was only off by a half of a degree so win there. I’ve only done 2 short trails in 4wd, the vibration wasn’t nearly as noticeable but at times I felt like it could still be there so more trial time is really needed to determine.

 

[zouch]

you could stick it in 4WD on the road to validate.

(yes, i know some people are whacky about not ever using 4WD on pavement, but running for a bit on a flat road can't put nearly as much stress on the driveline as when crawling over irregular slickrock, which has far more grip than asphalt.)
at least you'd find out if you solved your problem or not.

The pinion to driveshaft angle after install was only off by a half of a degree so win there. I’ve only done 2 short trails in 4wd, the vibration wasn’t nearly as noticeable but at times I felt like it could still be there so more trial time is really needed to determine.

 

[Jtphoto]

Driveline vibes are more of a highway issue. You’ll find that a double cardan driveshaft is a lot more susceptible to vibes and requires more critical DS/pinion angle as well as a lot more maintenance (removed and greased every 6 months or so).

 

[flick2614]

you could stick it in 4WD on the road to validate.

(yes, i know some people are whacky about not ever using 4WD on pavement, but running for a bit on a flat road can't put nearly as much stress on the driveline as when crawling over irregular slickrock, which has far more grip than asphalt.)
at least you'd find out if you solved your problem or not.

I'm of the school that 4wd on pavement isn't an issue and did test on pavement. Problem is I didn't do a lot of testing on pavement with the stock driveshaft so I don't have anything to compare to. But I didn't have any noticeable vibration on the pavement so called it mostly good.

Sponsored