bizard

Can anybody help diagnose problem with recently purchased 94 SL500 with 54,000 miles? Put a new set of Goodyear Assurance tires on it due to vibration from the old tires (i thought). Mildly better but the New tires didnt solve it. The wheels have been aligned and re-checked and the tires have been balanced twice. The annoying vibration starts around 60 and gets worse as speed increases. I want to take it on a road trip in a couple of weeks but don't think I could stand it for hours on the highway. Where should I look next to figure out what the problem is? Struts? Shocks?
Thanks!

J spaulding

If you have a floor jack then jack up the rear end (on the differential center section) off the ground about six inches. Block the front wheels so that if if accidently falls off the jack it won't go far. Start it and put it in gear with just the rear wheels off the ground. IRS rear ends don't particularly like this situation and get real clunky but don't worry about that since it will only take less than a minute for you to observe for the possibility of having something bent on the car such as an axle shaft or bent rim (the two most likely culprits). Have someone in the car slowly accelerate up to 70 MPH and try to figure out what is vibrating. Suspension shops do this all the time when the owners aren't around because of the nasty sound of the differential but it tells a lot in a very short time. Do it safely!

MB-Dude

On my SL600, I had a rear axle bearing that was making weird sounds and would randomly make a small vibration; especially around corners. In addition to J Spaulding's suggestion, while the car is up on the stands and with the engine off, check the condition of the rear flex joint. Since it's speed related, you may have one starting to tear or it's missing a bolt/loose nut.
Cheers,
Jeff

SamuelP

I would also suggest observing the drive shaft. A strobe light makes this much easier and vibration analysis is also a plus. I generally mount accelerometers and observe on a scope. This will pin-point the frequency and source the problem. Basically you are looking at once per rev or high frequency (sometimes detected by feel). You may want to set the car on blocks instead of relying on jacks and use a chain or cable to hold the car if it were to slip (forward only). If 'looking' or observing is your only means and since you're gone to this much trouble; tape a 'broom' straw (flexible) to the diff to observe its movement and characteristics (may or may not help, but how it shakes will let you know if it's fundamental or high frequency).