Vibration While Accelerating

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ROBERT BONNER

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Since the vibration is gone for now, imbalance can be ruled out as a root cause. While manufacturing defects and damage are exceedingly rare possible root causes for driveshaft U-joint failure; the typical root cause is misalignment of the components at the opposing ends of the driveshaft (differential and transmission for 2wd/transfer case for 4wd). The two have to be parallel with very little tolerance for deviation. This is a very difficult thing to check in the vehicle, though not impossible. If the vibration returns, even a little bit, I would take it back and have them check the driveshaft alignment for parallelism.

The vibration should initially present as a very high frequency (for a vehicle) at road speed. The 10th gear is overdriven nearly 36%; so, at 80 mph/2000 rpm, the vibration will likely cycle at over 50 hz. It's like a bass frequency hum. By the time yours was replaced, it probably presented a lot more "music"; but, at the beginning of the failure, it will just hum at higher speeds.
 
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Since the vibration is gone for now, imbalance can be ruled out as a root cause. While manufacturing defects and damage are exceedingly rare possible root causes for driveshaft U-joint failure; the typical root cause is misalignment of the components at the opposing ends of the driveshaft (differential and transmission for 2wd/transfer case for 4wd). The two have to be parallel with very little tolerance for deviation. This is a very difficult thing to check in the vehicle, though not impossible. If the vibration returns, even a little bit, I would take it back and have them check the driveshaft alignment for parallelism.

The vibration should initially present as a very high frequency (for a vehicle) at road speed. The 10th gear is overdriven nearly 36%; so, at 80 mph/2000 rpm, the vibration will likely cycle at over 50 hz. It's like a bass frequency hum. By the time yours was replaced, it probably presented a lot more "music"; but, at the beginning of the failure, it will just hum at higher speeds.
This is excellent information, thank you!! I def have been very intent on listening/feeling for anything new since getting the vehicle back. If anything presents, I will follow your advice here. Thanks again!
 

duneslider

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Since the vibration is gone for now, imbalance can be ruled out as a root cause. While manufacturing defects and damage are exceedingly rare possible root causes for driveshaft U-joint failure; the typical root cause is misalignment of the components at the opposing ends of the driveshaft (differential and transmission for 2wd/transfer case for 4wd). The two have to be parallel with very little tolerance for deviation. This is a very difficult thing to check in the vehicle, though not impossible. If the vibration returns, even a little bit, I would take it back and have them check the driveshaft alignment for parallelism.

The vibration should initially present as a very high frequency (for a vehicle) at road speed. The 10th gear is overdriven nearly 36%; so, at 80 mph/2000 rpm, the vibration will likely cycle at over 50 hz. It's like a bass frequency hum. By the time yours was replaced, it probably presented a lot more "music"; but, at the beginning of the failure, it will just hum at higher speeds.
These aren't like solid axle vehicles, something would have to be way off at either end for there to be a misalignment issue since the transmission and rear diff are fixed in place on these. By way off, I mean more of a noticeable issue that would be seen before a cracked u-joint. Misalignment is way more common on solid axles (especially when you start lifting them) but I would think it would be pretty obvious on an expedition if misalignment were the issue.
 

ROBERT BONNER

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These aren't like solid axle vehicles, something would have to be way off at either end for there to be a misalignment issue since the transmission and rear diff are fixed in place on these. By way off, I mean more of a noticeable issue that would be seen before a cracked u-joint. Misalignment is way more common on solid axles (especially when you start lifting them) but I would think it would be pretty obvious on an expedition if misalignment were the issue.
Go ahead, you can say it...far more common on .....wait for it.... Jeeps!

Seriously, though, there is a lot of research on U-joint life. More than 1 degree parallelism misalignment will lead to greatly reduced universal joint life. (So, when you see the lifted jeeps that have 5-10 degrees misalignment visible...they're lucky to see more than a few thousand miles). 1 degree is less than my eyes could see. The components are fixture located at assembly in the plant to ensure less than 1 degree misalignment. If there is misalignment on this Expedition, it was likely caused by a botched past repair or accident related damage.
 

duneslider

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Go ahead, you can say it...far more common on .....wait for it.... Jeeps!

Seriously, though, there is a lot of research on U-joint life. More than 1 degree parallelism misalignment will lead to greatly reduced universal joint life. (So, when you see the lifted jeeps that have 5-10 degrees misalignment visible...they're lucky to see more than a few thousand miles). 1 degree is less than my eyes could see. The components are fixture located at assembly in the plant to ensure less than 1 degree misalignment. If there is misalignment on this Expedition, it was likely caused by a botched past repair or accident related damage.
Well, jeeps aren't the only things that have solid axles. But yes, very common on jeeps. However, most new stuff has CV joints or double cardans that are more flexible when it comes to misalignment.

So, I agree, if this expedition had been in a wreck or something it very well could have some issues with alignment but nothing was mentioned about it having been in a wreck and repaired. I am leaning more to it was probably a bad joint from the factory, or it cracked when it was installed. Whatever it was, he is lucky it didn't come apart while driving cause that is never fun.
 
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