1991 SL300 Vibration under load.

[tecqboy]

My SL300 has developed a chassis vibration when the engine is under moderate to heavy load and at highway speeds above about 65-70mph. This is NOT a tire balance issue, as when the throttle is closed at speed (foot off of gas ) the vibration stops. The vibration is also noticable when accelerating, especially just before an up shift and particullarly when going from 3rd to 4th and 4th to 5th gears. If just cruising at 40 mph., there is no vibration. The two flex joints on the drive train were a bit hard so they were replaced and it made no difference. I've searched on "engine mounts" and there is an abundance of contardictory info about how to diagnose and when to change them. I can laterally push on my engine with my hand and there is a tiny bit of "play" on the passenger side mount. The mounts looks good, i.e., no cracks or leaks. They were replaced by the P.O. at about 60K miles / 8 years ago. So on to my question. Anyone else here experience a similar vibration? And what was the "fix". Should I be looking at replacing the engine mounts again at current 100K miles? Thanks for viewing. Tod

[Mike Miller]

Todd,
I have no experience regarding my R129 as far as vibration is concerned, BUT.... have you checked your rear suspension? By that I mean have an alignment shop do it. I missed some very badly worn rear suspension links on my SL even though I jacked it up myself and checked for play. You actually need to jack each wheel carrier, not the body. If you jack at the normal body points the suspension drops all the way down and when you try to "shake" the rear wheel, everything seems very tight. Well, it is, but it is masking the wear.

I am not sure this is your problem, but it beats replacing expensive parts.

Dont forget to check the center hanger bearing on the driveshaft mine is getting worn out and it causes a knocking noise when accelerating hard from a dead stop in my 91 300SL

[tecqboy]

Had this repaired last fall. I was indeed the center mount bearing. Also replaced the flex couplings for the 2nd time. I replaced them at about 100K miles as I thought they were the source of the vibration problem. But after only 4K miles, they showed signs of cracking. They were replaced under warrenty, but I had to pay labor again.

[iwrock]

Uhhh.. Engine/trannie mounts? My wagon had a little vibration at idle, and vibrated moreso when under a load.