I give up. I need some lateral thinking/help from the Brain Trust

[nemick]

I have been chasing an awful REAR end vibration. This only occurs between 15-19 mph. The severity of the vibration is determined solely by how hard I accelerate. This is not a gentle vibration when driving normally and feels like the ass end of the car is going to fall out.It feels like a heavy thumping on the underside of the car, or even a heavy shudder.

I have checked a great many things without finding any clue as to what is happening.

I can find no play in any of the rear suspension components. I checked this with the suspension at full droop and also loaded when on the lift.
I have replaced the forward diff mount with the ECS version
I have replaced the center driveshaft support with the 034 version
I have replaced the rearward diff mounts with the 034 versions
I have replaced both rear axles
Both rear wheel bearings are less than 18 months old
I have replaced the engine mounts
I have replaced the trans mounts
I have visually inspected the rear subframe bushings, in situ, but cannot detect any movement

What am I missing?

Car is a 2001 Tip with 215000 miles. I don't want to give up on her because she's a good car but this has been driving me nuts for over a year now

HELP

[Davdraco1]

Wow man. That's literally everything I could think of. It's such a weird time when it happens too. The only other thing I could think of is the driveshaft might be unbalanced? Or something weird like that. There's not much left to look into. good luck with it though.

[GrapeBandit]

driveshaft can be misaligned a tad too.

[pierreb]

I assume it couldn't be something simple like maybe swap the wheels with the fronts?

[bmos]

driveshaft can be misaligned a tad too.

Would it be as bad as he's saying?

[nemick]

Wheels have been rotated several times with no change. I'm stumped

[bmos]

Get a friend to sit in the back seat and find what side it's coming from.

No friends? Find a hitchhiker.

[bmos]

There's also a $30 borescope on Amazon with built in light that you could tape to the underside of your car and record some trips with to look for stuff that shouldn't be moving.

EDIT: sorry for the double-post!

[nemick]

There's also a $30 borescope on Amazon with built in light that you could tape to the underside of your car and record some trips with to look for stuff that shouldn't be moving.

EDIT: sorry for the double-post!

Funnily enough, this is my next step. It's become a mission now. It seems to be coming from dead center.

[LINDW4LL]

Could it be the differential itself?

[nemick]

Could it be the differential itself?

I guess anything is possible, but I'm really hoping it's not. The fact that the problem is only apparent when accelerating between 15-19 mph is what has me stumped. I would expect a diff failure to be there all the time

[Bordom]

Are you the same guy with the Avant that was doing the same thing?

Everything graciously mispelled by Android

[nemick]

Are you the same guy with the Avant that was doing the same thing?

Everything graciously mispelled by Android

No, different guy. I was the one following him closely though. I thought I might pick up on something

[13jfp]

Pinion could be thrusting or could have an issue with the diff itself. I would inspect the differential. Put it on a lift and remove driveshaft to rear diff and try rotating it back and forth to see how it feels before tearing into the diff.

[nemick]

Pinion could be thrusting or could have an issue with the diff itself. I would inspect the differential. Put it on a lift and remove driveshaft to rear diff and try rotating it back and forth to see how it feels before tearing into the diff.

Do you know how much backlash I should be able to feel?

[walky_talky20]

Check the universal joint in the middle of the driveshaft very closely (not the support bearing, the actual u-joint). Rotate it and check it with a flashlight close-up. If one axis of the u-joint is trying to freeze up it will cause some pretty bad vibrations.

[bmos]

Do you know how much backlash I should be able to feel?

The problem is that diff backlash is typically measured from the ring gear which means taking it apart. There's no spec for the lash of the pinion flange itself as far as I know.

[LINDW4LL]

Does it happen in both first and second gear?

I assume it doesn't happen when you're decelerating?

[nemick]

Does it happen in both first and second gear?

I assume it doesn't happen when you're decelerating?

yes, happens in first or second gear only. Nothing any faster than 19 mph and nothing on decel.

- - - Updated - - -

Check the universal joint in the middle of the driveshaft very closely (not the support bearing, the actual u-joint). Rotate it and check it with a flashlight close-up. If one axis of the u-joint is trying to freeze up it will cause some pretty bad vibrations.

if it does turn out to be a UJ, are they serviceable or will I need to find a replacement?

[13jfp]

Does it do it in reverse too?

[nemick]

Does it do it in reverse too?

I can't honestly say I've tried that

[13jfp]

I did some reading about quattro on your a4. It has torsen center diff and edl using an open diff in the rear. Only functions up to 25 mph. If noise goes away above 25 mph i would really think its an edl issue. U could have a wheel speed sensor issue making edl apply braking on and off to a wheel it thinks is slipping. Would explain symptoms and the fact that it is louder with heavier throttle. Not sure how to test system.

[MrSnickelsnizer]

^^Now that is an answer^^ But sounds like a full rear brake caliper check, rear upper and lower control arm bushing check, sway bar links check, exhaust hangers check, and rear diff fluid check are the only easy options left before jumping into center diff issues. You pretty much canceled everything else out.

[Davdraco1]

Is the EDL idiot light on? What about your abs?

[walky_talky20]

EDL theory is clever. I like it..

Easy way to check EDL - pop up the back seat and unplug either of the rear abs wheel speed sensors. This will completely disable all ABS intervention including EDL.

And for the u-joint - it is probably serviceable, although finding a replacement joint would be a trick. Easier to get a junkyard driveshaft for $60.

[nemick]

I did some reading about quattro on your a4. It has torsen center diff and edl using an open diff in the rear. Only functions up to 25 mph. If noise goes away above 25 mph i would really think its an edl issue. U could have a wheel speed sensor issue making edl apply braking on and off to a wheel it thinks is slipping. Would explain symptoms and the fact that it is louder with heavier throttle. Not sure how to test system.

I really appreciate the thought you put into that. I will investigate further and report back!

[nemick]

I'm going to check this EDL thing and if I can't find anything there, I'm going to start at the transmission output flange and work my way backwards...again. I am determined to find the cause of this problem if only to be able to help others in the future. Thanks so much for the brain storming, it's given me a lot to think about and work on.

[nemick]

Is the EDL idiot light on? What about your abs?

No idiot lights at all and ABS seems to be working in the rain earlier today.

[nemick]

Issue does not appear to be related to EDL. I disconnected a rear ABS sensor and it made zero difference to the vibration.Next up is disconnecting the driveshaft at the diff connection and checking the UJ closely and also feeling for any anomalies in the diff itself.

[walky_talky20]

Good work. I think you are on the right track.

[13jfp]

Any updates on this?