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I am at my wits' (and budget's) end

My recently acquired Mercedes W126 has a vibration which stats at ±120km/h. It is dependent only on the driving speed, whether in-gear or coasting, under power or braking, 4th, 5th or neutral. I have had the mags, which are genuine Mercedes, refurbished to remove all warping and ovality; the tyres are brand new and the wheels have been balanced several times now; I have had the propshaft rebuilt and replaced the centre bearing and the guibos; I just had the entire front suspension and steering rebuilt along with new shocks, wheel bearings, brake discs and pads. All to no avail.

The vibrations are at much lower frequencies than the engine (±2800rpm, 46.6Hz) and propshaft (3500rpm, 58.3Hz) speeds, more like the wheel speeds (±1000rpm, 16.7Hz). The vibrations feel like they are phased: sometimes the car shakes transversely, sometimes the car shakes vertically, sometimes around a gentle turn the vibrations even almost disappear completely.

I don't believe in throwing money at a problem until it goes away. I want to know what to look for and how to look for it, so that a proper diagnosis can be made and the cause of the problem be resolved. I am open to advice and suggestions.

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  • Which country is this? – Solar Mike Nov 12 '20 at 7:24
  • I'm in South Africa – PlaneCrazy Nov 12 '20 at 7:25
  • One thing you could do is use a NVH meter to determine the frequency(ies) of the vibration. That might tell you whether it occurs at prop shaft speeds (e.g. transmission mounts) or half shaft / wheel speeds. – towe Nov 12 '20 at 9:45
  • I does not occur at engine or propshaft speeds, it occurs at lower speeds, likely wheel speeds, or about 16-20Hz. I'll update my OP to this effect. – PlaneCrazy Nov 12 '20 at 10:32
  • How did you measure this 16 to 20 Hz? – Solar Mike Nov 12 '20 at 13:06
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Although the explanation as to why the system vibrates at wheel frequency instead of propshaft frequency is a bit of a mystery, I have tracked the source of the problem to the transmission itself having very badly worn bearings. The only remedy is to overhaul or replce the transmission.

Thank you

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