1984 300D "harmonic vibration" at speed
 

Hi guys,

So I have noticed this since I bought the car, but haven't thought too much of it (and have been spending time 'debugging' the symptoms, so to speak).

In a nutshell, when I am at speed (typically over 90 or 100 km/h (over 55 mph)) I notice a sort of low-pitched harmonic vibration (which I can both hear and feel). I can't really localize where I feel it - I can feel it through my seat (though it's not a strong vibration) and through the armrest. Not through the gearshift at all. The vibration is not constant - it's more of a vibration that gets weak, and then strong over the course of about a second or a little longer.

If I shift into N while driving, and take my foot of the go pedal, the vibration is totally unchanged. I imagine that if I were really interested, I could shift into N, take my foot off and then even go so far as to turn off the motor and it would still be the same vibration.

If I let off the gas (while still in D, at speed) the vibration becomes a little stronger (but only slightly).

Turning the wheel has no effect.

I'm a bit stumped. Maybe I just need an alignment? The car doesn't pull perceptibly to either side. Tread wear is even so far as I can tell across all tires. Maybe I have a wheel that's out of balance? All my other tires are correct pressure.

My passenger-side rear tire is low on pressure (by about 6lbs) so I wonder if it could be this. I'll be repressurizing it tomorrow and we'll see if that makes any difference.

Any other ideas I should be exploring?

Crawl under and have a look see at the center bearing, try to rotate it. Sounds like maybe it could be on it's way out. Bearings usually give you some warning. Sound and/or vibration.
Otherwise, you may have a rear wheel bearing issue, harder to localize.
Mentioning "seat of the pants" usually refers to something in the rear of the car. Check trailing arm bushings also.
Good luck, and keep us posted.

You will find it at the front end of the rear driveshaft.

Tire balance?

hmm! I hadn't considered the centre bearing. How big is it? I am sure it will be easy to spot. Let's say it is going bad... how difficult should it be for me to replace?

Its about 4' across and takes a few horrs to replace. Check out and replace, if needed, your flex disks whole you are working on the driveshaft.

Great! Reading some other threads here, it looks like replacement basically consists of pulling apart the driveshaft at the spline, and marking it to reassemble the right way. With it apart, I can pull out the replace the bearing?

Time to check out my FSM...

Only if you have a heavy duty driveshaft!!!

Imagine how heavy that'd be... :D

4' sounds about right, becuase by "1984 300D" I really actually meant "my 1984 crude oil tanker" ;) hehehe

Need a hand?
 

Hey xdjio, let me know when you're doing this. Maybe I could swing by to help out (and learn a bit too:rolleyes:). Even better, if we could get Bens Lover around too, if he's available...cos then we'd have someone who really knows his way around the stuff :inquisiti.

Zu - I certainly will. It won't be until a week or two into the new year that I will have the time to even do a proper diagnosis of the intermediate shaft bearing. But if (when?) I replace it I will let you know. I was just reading over the FSM and the actual replacement of the part does not look too difficult, though so far I will suggest that it looks like I'll have to drop the exhaust.

Many hands makes light work, though :)

Ok - so I have one more data point on this issue. I'd thought "maybe I have a bad wheel bearing" or at least one that is on its way to becoming bad. But tonight after some extended driving at freeway speeds (about 50 km worth) I got home, parked, and felt all the wheel hubs to see if any were hot (or even warm in the least) and the verdict? all were pleasantly cool to the touch.

So, I think we can perhaps eliminate the possibility of a bearing that's going bad. Yes?

I suppose a poorly-balanced wheel could still be the case, maybe, or the intermediate shaft bearing. Really I should just meet up with Zu or Bens Lover and go for a drive to demonstrate my exact issue ;)

Cool bearings are a good sign, for sure.
To check your front bearings, doubtful if you can feel something in the seat of your pants tho, jack up the car, and gripping the tire at the top and bottom, pull and push to check for play. Side to side also. The rear wheels, you may be able to check those in the same manner.

Tire Balance
 

Have you had the wheels balanced since you got the car? I'd knock out the simple stuff before pulling the drive shaft to do a bearing replacement that may not be needed, though that procedure is not too tough.

If you do pull the drive shaft be sure to mark the two halves so you can get them back together the way they were before. Otherwise you'll throw off the balance of the drive shaft assembly and have yet another vibration!