Transmission click?

I have a 1978 300D, with an automatic trans and roughly 148K miles (odometer doesn't always work). The car was recently purchased and unfortunately I didn't get much in terms of its history.
Sometimes when downshifting I hear a soft metallic "click" going from 2nd to 1st. This is not accompanied with any sort of jerking. It's starting to make me a little uneasy. The trans also jumps a little from 1st to 2nd, though not all of the time. Otherwise shifting goes smoothly. Have any thoughts?
Thanks, erik
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First, some terms: 1st is engaged at a dead stop by shifting into L. You can start off in 1st, up shift to S and immediately BACK to L to hold 2nd gear. S is 3rd and D 4th.
As to the noise. The rear axles have "spider joints" inside their boots. These wear and make some noise. Rebuilt axles can be bought if it comes to that. Then there's the drive shaft which has "flex joints" and a center carrier bearing behind the transmission if I recall correctly. The motor has three mounts left, right and one under the transmission.
It could be that a motor mount is broken and the sound is from the motor and transmission moving from the torque. Or the running gear has some slop from wear. Suggest you first look at these and the transmission last.
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Nah, not likely. If the tranny is shifting properly even worn to the poit of near failing parts will still be quiet. MB goes to a lot of trouble and adds lots of parts for smoothness of he driveline. It's tranny vacuum i'm 99.9% sure.
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Thanks,
I'm glad to hear that it's probably just vacuum related; I was about to troubleshoot the vacuum system anyway, so addressing this can be my first order of business.
erik
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The harshness of shifts is controlled by vacuum. WHen the vacuum presented to the transmission is not to spec or the modulator is defective or outmoded (there is a new upgraded part that is better) then this is exactly what happens.
Calibration of the vacuum is a cumbersome precedure that does require a bit of time and patience but it is doable by the DIY mechanic.
If memory serves there's a cable on the 300D that isn't involced in (my) 300SD.
Now, the flipside is your tranny will last MUCH longer if it clucnks giong into low gear as there's less wear on the clutch packs. I live with is and so far has 3-4X the expected service life on the orignal tranny. (500K mi).
Here's the procedure:
    http://articles.mbz.org/transmission /
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