? Replacing Support Bearing
 

I have to replace the support bearing on the '84 300D Turbo. Should I replace the bearing dust cover and bearing support at the same time or just the bad bearing?

Is the u-joint at the front of the rear driveshaft replaceable or does it require replacement of the entire shaft?

Thanks.

Replace the support and dust cover. If the bearing is worn out, it is a safe bet that the support is too. The u-joint is staked in. Replacement is not a "routine" job like it is on most cars. Make sure you mark your driveshaft so you can reassemble it in its original relationship. You might also want to consider replacing the flex discs at the same time.

The PO replaced rear trans mount, and front/rear flex discs and there have been very few miles since. I observe some runout accompanied by noise in the driveshaft around the support bearing. It goes with the vibration felt around 35mph so I am starting with the support bearing. I would not be surprised that the parts were not put back together with markings prior to my purchase. Of course that could mean the vibration I am feeling during acceleration, coasting, deceleration around the 35 mph mark is due to improper reassembly . . .

Guess you can see where I'm at on this one???

Yeah, I'd replace that "dust cover". They do start to tear, especially when you work on it, like especially if you replace the bearing and as you are done putting it back in and rotating that stupid driveshaft nut (almost done with job) - then it tears :mad: Since you know the shaft has been out recently and you have vibration I would take it to a driveshaft shop. They can check it and balance it, then you'll be sure about it as long as you own the car. I would leave the u-joint alone. In my book, u-joints rank at the top of the list for most often,unnecessarily replaced parts on a car. I've been guilty of it myself. :o

You need to check the FSM to confirm but, I believe that in post-1983 production, the cast-in witness marks on the driveshaft halves were used during assembly at the factory. If that's the case, you should be able to inspect the driveshaft 'in situ' for proper phasing/orientation and, if the FSM supports my memory, you could reassemble it correctly according to the marks. Prior to some point in '84 the marks were on the shaft halves but weren't used at the inital assembly.

Other:
Generally, a support bearing makes a whirring noise, not vibration when failing. Medium speed vibrations under increasing or decreasing loads or speeds is more than likely a worn out u-joint. I've written up some stuff in the past on this; a search in here on driveline vibration should turn it up.

Also, you can visually inspect for bad support bearing isolation rubber w/o pulling the driveshaft; poke and pry on the assembly and look for tears/holes in the rubber.

The u-joint on a Mercedes is easy to check. Pull the entire shaft and then deflect the shaft at the u-joint. If there's excessive wear, you'll feel a distinct change in resistance/friction when the arc of the yoke moves beyond it's normal range of motion. The resistance you feel out side the normal range is the proper clearance, the 'looseness' close to the 'normal' arc is the play that causes driveshaft vibrations.

nice. Wish my SD had that.

I second that. Had a 300E with a bad U joint and there is a very distinct feel when deflecting it by hand. Two different shops told me it was not replaceable and I had to get a new driveshaft. Of course that was before this forum existed and I found that that it IS replaceable if you find the right person to do it.