2009 C300 Grinding gears!
As long as it shifts properly 90% of the time, and grinds twice a month, it sounds like operator error + a little syncronizer wear.
Going up thru the gears, the 1-2 shift is the most difficult for their syncroniser, vs other shifts, due to the large change in rpm.
A few simple things to try:
I've had cars, like that, and I found that either bliping the gas pedal before the actual lever pull, or a full double clutch eliminates the grind. Finally, if the trans oil has not been changed recently, do it, and consider a specialty gear oil like Redline, Neo, etc, especially if suggested by a member with a manual.
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Based on experience, I would suggest changing the MB MTX oil every 40K or so, more often if you track the car. Manual trans oil has different properties than plain rear end gear oil, as you know. The "friction additives" for the trans help in smoothly engaging the syncro/gear, and I found that smoothness was rejuvinated with a simple drain and fill.
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It had to get ridiculously bad before MB finally agreed to replace the trans. She has been shifting wonderfully ever since.
Make sure your clutch is adjusted properly and shift a bit more deliberately(slowly) especially when cold.
Are you still under warranty? If so, they may be open to replacing the trans as the synchros in some have proven to be less than robust.
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That alone will not prevent grinding the gears. With 30-50 K miles on the trans, especially if the shifting habbit put the max load (rpm differential) to the syncros. Wear at the slip/block ring will allow wear at the pre-gear (at the end of the integral main gear. This makes it difficult for the sycro to do it's job, and you will get a grind, with clutch fully depressed, as you are about 1/2 way through the actual shift knob travel.
If you have that crunch frequently:
change out the trans fluid
learn how to truely double clutch when the crunch occurs, just takes practice. Lots of info on web.
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The reason I say only use Benz approved oil is that the viscometrics have been optimised for evacuation from the cones & it contains no additives that are aggressive to the cone material composition. A standard sulphur phosphorus axle oil will attack the cones at certain temperatures.
Last edited by Glyn M Ruck; Jan 26, 2013 at 11:04 AM.
The new GM, Mopar, and aftermarket trans oils are blended in a way that they would not pass GL5&6 hypoid bevel gear tests in the rear differential, and as you said, optimised for syncro performance with MTX friction modifiers (not the rear end oil friction modifiers for LSD's)
The reason I say only use Benz approved oil is that the viscometrics have been optimised for evacuation from the cones & it contains no additives that are aggressive to the cone material composition. A standard sulphur phosphorus axle oil will attack the cones at certain temperatures.

I would have thought as the Germans probably use manuals more than elsewhere they would have perfected the shift.
Is there a reason they have'nt ?
Peugeots & VW have.
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On a side note, I remember a thread from long ago where it was pointed out that the stick vibrates noticeably more (hands off) while in second gear than any other. Almost like something was just off by a tad and it wasn't fully sync'd or engaged. Any comments on that Glyn/Kevin?

As I've said before I think their MT's spoil the cars. I would only buy auto for this reason. That's why I'm pleased they are now offering DCT transmissions for those that want to be more in control.
I suspect MB put some noise isolaters in the shift linkage. If so the shaking could be due to a resonance that is based on the typical rpm range in that gear.
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I do believe the SLK250 is the soul manual hold out in the US line up. Has anyone driven one of those?
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