Drivetrain Not So Good Vibrations

Ryan Hardy · May 28, 2015

Hey everyone. I'll cut to the chase:

1991 5.0 LX
AOD with 4.10 gears (installed by prev owner)

I have a speed (not RPM) related vibration. It's not really pronounced until about 65 mph. After that, it just gets louder an higher pitched.

It seems to be coming from the middle of the car. What's strange is that it doesn't happen under load, like accelerating or climbing a hill. It only happens when letting up on the throttle, or like if I'm on a level stretch with cruise control, it comes and goes as the system fluctuates just a little.

When I heard it, I just assumed it was u-joints, and replaced them. Apparently aluminum drive shafts have helped people with various vibrations, but that's out of the question right now. I've also been reading about the tail shaft bushing, and I may try that next.

What I'm worried most about, is the rear end. I suppose there would be considerable gear whine if a bearing were bad. Is there a way to check it without taking it apart?

rbohm · May 28, 2015

have you checked your tire balance? at higher speeds they come into their own, and if they are all not balanced it can be tough to figure out where the vibration is coming from.

the next thing is, did you remove the weight at the end of the tail shaft of the transmission? that weight was put there to absorb transmission vibrations.

the tail shaft bushing could be worn, and that will also cause a vibration.

one more thing, make sure your u-joints are properly centered in the drive shaft, or else you will get a vibration that will drive you nuts while you are looking for it, and kicking yourself when you find it.

7991LXnSHO · May 28, 2015

A vibration that gets higher in pitch??? I would be checking the u joints for dry bearings ASAP!

On my old tail shaft and worn bushing, it shook the shifter. Once put, we found one of the u joint caps was heated steel blue and dry. My LED trouble light hid the color.

I do not remember a high noise from bad or misbalanced tires. And I never drove one fast enough for the vibration to go away.

Ryan Hardy · May 28, 2015

Thanks guys.

The vibe is about 60Hz at 70mph, about the same pitch as the electric hum of a fluorescent light. Only it can be felt in the floorboard also, faintly.

I will check all that stuff, but like I said, I replaced both u-joints, and nothing changed. I'll post up what I find.

rbohm · May 28, 2015

Ryan Hardy said:
Thanks guys.

The vibe is about 60Hz at 70mph, about the same pitch as the electric hum of a fluorescent light. Only it can be felt in the floorboard also, faintly.

I will check all that stuff, but like I said, I replaced both u-joints, and nothing changed. I'll post up what I find.

like i say, if the u-joints are installed off center, they will cause a vibration.

Gearbanger 101 · May 28, 2015

Vibrates at speed under deceleration and cruise.....I say check the simple things first. Make sure rear wheels and drive shaft bolts at the pinion are tight.

...and no, I'm not joking.

Ryan Hardy · May 29, 2015

I agree, simple stuff that will bite you in the ass, first.

But I gather from these responses, that its probably not in the pumpkin?

Gearbanger 101 · May 29, 2015

Ryan Hardy said:
I agree, simple stuff that will bite you in the ass, first.

But I gather from these responses, that its probably not in the pumpkin?

Driveline vibrations can be tough to track down. It could be at either end of the car. worn engine/trans mounts, clutch, transmission, driveshaft, u-joints, rear end, wheels, Any one of them could be a culprit.

In my experience, vibration at cruise and under deceleration is often due to something being lose, than it is to any worn part in particular.

....but like I said, driveline vibration can be tough to track down, so check everything.

Ryan Hardy · May 29, 2015

Thank you. I may be able to get under the car this weekend. I'll make a checklist of everything here and anything else I can think of. I'll post what I find.

Thanks everyone for your input. I'm a capable technician, but it always helps to have insights from others.

Ryan Hardy · Jun 4, 2015

I checked with the trans in neutral, ass up, and nothing appears to be loose. All drive shaft flange bolts are nice and tight. There is no unintended movement in the u-joints. I don't know how to tell if its a tail shaft bushing, I may shotgun that.

As a note to myself: someone told me to check the pinion nut. Very good idea, since its kindof 'outta sight, outta mind' hiding behind that flange. Someone dealt with mechanics and dealerships for a year before a oil change/repair guy told him to check it. Figures.

Ryan Hardy · Jun 15, 2015

OK, I've eliminated (I think) everything but the output bushing. I'm gonna clean up the governor while I'm at it, since the tranny is shifting a little funky and the TV cable is adjusted correctly.

I have a question on the output shaft. Since my speedo reads 20% (of reading) high, I was gonna use the opportunity to change the output shaft to the earlier 7 tooth design, which would put my speedo at a much better 4% high. I can't find anything, even on alldata, about replacing the output shaft. Does the trans need to be disassembled for this, or can I just remove it from the rear?

Ryan

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