1984 300SD Acceleration Vibration
 

Looking for some ideas on a vibration with a new to me 84 300SD. Car has 118k on it, I've driven it about 3k now. Overall the car seems in very good condition. Almost everything works well and hasn't caused me any concern. I did recently replace the tires with new ones, motor mounts, engine shocks, and trans mount. I have done a valve adjustment, cleaned the banjo bolt to the ALDA, and plenty of other minor tuneups on the engine. New oil in the diff, the axle boots aren't torn or leaking and don't seem to have any issues I can tell so far.

On a recent trip, I noticed what felt like a vibration or hesitation from about 25mph to 30mph. After that, it smooths out and I don't think it comes back. I do get a vibration sometimes at about 70mph that comes and goes as well, not sure if they are related. One thing I did notice, the issue is much easier to notice if the rear of the car is weighted down, I had a full trunk on my last trip. This could have been occurring since I bought the car and I just noticed it after adding more weight to the rear.

I've searched the forum and most ideas tend towards the flex discs or driveshaft support bearing. Both flex discs look very good, and I haven't pulled the heat shield yet to look at the support bearing. But, I'd like to have some other suggestions to think about. I don't want to waste money trying new parts if I can help it.

Thanks

All of your motor mounts are New ?

You replaced them ?

If yes, did you check the FSM to see if you need to use a gauge to place them correctly ?

I could not find a reference on guaging the install for engine new mounts. I ended up loosening the transmission mount and making sure it was not binding anything up, then tightened it back. I had my wheels and tires balanced again just to be sure there weren't problems there.

I did a 500 mile trip, and the car seemed to drive better, very minor vibration under acceleration. However, on the way back I had the rear loaded down with about 300lbs again. I started to notice the vibration again when the rear end sat lower. I did remove the heat shield and look at the center support bearing and the flex discs. Both look ok, but there was some up and down movement on the driveshaft if I pushed near the center support.

Any other ideas before I start replacing parts? I think the shocks need to be replaced, and I'm going to look at the springs, pads, and subframe bushings, but what other things should I look at before I start buying parts?

I had issues like that when I first got my car. Turns out the PO had installed aftermarket rims that were actually off (bolt radius) something like .6mm. This caused them to be not concentric. Just putting this out there.

I have the same issue on my 1984 300SD. It doesn't really do anything when I am driving it unloaded, but when you get weight in the trunk to the extent that the back lowers, then there is this vibration as you start out.

I've had my issues with driveshafts before, and I think the problem is the driveshaft U-Joint. The single U-joint on these cars doesn't really flex very much, and over a long time tends to wear in one position. I have found that with the driveshaft out, you can feel where the U-joint develops something like a "center-detent" where it catches in place where it is worn.

So when you load the car down, you're exercising that U-Joint out of its normal worn operating area. And you get the vibration.

I can see how it might also be the center driveshaft support bearing.

In either case, the work involved is a high price to pay (either in parts money for a driveshaft, or labor effort to replace the center bearing) to resolve a minor vibration. I'm just living with mine until it gets much worse than it is now.

I jacked the Rear Wheels up off the ground on my Volvo Diesel and when I started the Engine and put it into gear the Drive Shaft Support and Drive Shaft started bouncing up and down.
Sure enough the Bearing was shot in side of it.
Do the above safely.

Below are some pics of a sticky U-joint and another showing that your U-joint is staked into the Drive Shaft.
This is my thread on that:
http://www.peachparts.com/shopforum/showthread.php?t=293976

Can you replace the universal joints????? - PeachParts Mercedes ShopForum

Tightening Drive Shaft Collar Nut 2011
http://www.peachparts.com/shopforum/showthread.php?p=2575372&highlight=U-joint#post2575372

... the u-joint is not affected by how low the vehicle is... you have independent rear suspension, and the diff is bolted to the frame of the vehicle... it could still easily be the u-joint, but more likely it's from the additional load from either weight in the car, or wind resistance at speed due to the rear being lowered, and losing the natural rake "spoilage" of the air.
either way, since it's more noticeable with a load, it could be,
1. Tires, just balancing does NOT determine if loaded they vibrate... try swapping front to rear to see if the vibration "moves"
2. Differential wear
3. worn axle joints
4. u-joint and or center bearing
5. flex discs allowing more travel than usual
6. broken frame parts allowing vibration
7. all of the above
8. None of the above
9. I've also noticed brake calipers being seized causing a vibration after a longish drive...

I noticed nobody has mentioned the diff mount itself. Replacement is pretty straightforward. Four thirteen mm bolts hold the top of the mount to the bod and two big ****ers bolt the diff to the mount. At twenty seven years old it probably requires replacement if you haven't already swapped it. (be sure to call Phil for your mount, 1.208.615.4005) it might even cure your vibration issue.

Thanks guys. I think in the next couple of weeks I'm going to try new rubber on the coil springs, the 19mm ones. They are pretty cheap and shouldn't be too hard for me to swap. And I'll replace the carrier support bearing and rubber around it. If that doesn't work, and my flex discs still look fine, I think I'm just going to live with the vibration until something more noticeable shows up. Unfortunately, I could replace most of the parts in the rear and still might not get rid of the vibration. Thanks again, I'll update everyone if I find a fix for my problem.

I bet the new diff mount will help. Look forward to hearing from you when you're vibration free mate, although lady friends who beg rides in your merc might not. ;)

If I remember correctly from doing a differential replacement, the differential is bolted to the subframe assembly in the back. The subframe is flexible and can move with respect to the frame of the car. It is the rubber differential mount that holds the differential and thus the subframe assembly in place with respect to the frame. I remember having the car up on jackstands in the back - and when the differential mount was removed the wheels sank downward and the differential moved upwards.

So I can agree that the differential should not move much with respect to the frame (thus changing the U-Joint angle) unless the differential mount is worn/loose.

Hmmmm.

Other than outright replacement, is there a way to test the differential mount to determine if it needs replacement? Sure beats replacing the driveshaft.

Ken300D

Check condition of the rear hub/drive shaft tensioning washers and that the correct bolt and spacers are fitted. If these become worn or loose they can cause a vibration that will eventually chew away the spline! Vibration usually is worst when loaded or cornering.

Correct parts for your car are:
1) Bolt - M8 x 85 (High Tens.)
2) Spacer - 67.5mm long.
3) Tensioning Washer - MA115-357 06 75 (Agent)

If the washers are dished or worn they must be replaced. FSM says they may only be used once.

Bad Half shaft !!!

Check for torn boots.

Well, replaced the rear shocks with Bilstein HD on the back, and replaced the rubber mounts on top of the rear springs yesterday. Overall, not that difficult to do. I was fortunate enough to be able to use a coil spring compressor I had for my 66 Mustang to help removal, took about 3 hours for both sides. I used the thicker 19mm coil spring pads, and it made a big difference in height. Overall, I think the rear came up about 1.5 inches on each side. I know it will settle, but not bad for now. There is still something more to do for the rear, the left side sits a half to an inch lower than the right.

As for the vibration and driving, major improvement. If the vibration is still present, it is barely noticable. We'll see how it is after driving for a while, but I'm happy so far. Next step is replace the driveshaft support, I think it is wise to do so, and probably the rear diff mount.

But, I'm going to drive it for a while with just the new coil spring pads and shocks to see if the vibration stays away.

I just wanted to update the post with some positive results, my opinion for now is the coil spring pads made the difference. Not bad for a few bucks and three hours of labor.

From your description, I would lean more towards a worn trailing arm bushing. Especially since you mentioned a mismatch in height on the rear. These can be a little decieving and difficult to notice the wear until you lower the trailing arm. Worn trailing arm bushings will show up in high speed curves as a "shifting of weight". Also, it will likely wear the rear tires, but not significantly at first.

The symptoms of a worn differential mount is hiking and lowering of the rear of the car when changing from reverse to forward. Usually noticed when backing out of a parking spot.

Driveshaft vibration seems to be pretty consistent and more noticable in lower gears, when you are applying more torque. It is also mostly felt in the tunnel area.

I've dealt with differential mount, subframe bushings, trailing arm bushings and driveshaft and they each have a their own symptoms when worn.