Hi everbody I am new the MB world but am a long time wanter.
I just purchased a 1979 300sd 120 series with a 617 series motor and trying to get him back to top shape. My question is what is the black knob on the driver side of the dash for. this car did not come with a owners manual and I will be buying one soon. I have many more question to come.

Typical little black knobs are for the rheostat that controls the instrument panel brightness (or lack of), resetting the trip odometer, changing the clock, and, as on my '82 240D, a manual idle speed control.

What's it do? Can you push it in (or does it just rotate)?

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting

it makes a full rotation but I have not noticed any change in the dash lights. I have found it to be kinda hard to turn. it is located on the dash itself beneth p brake light and the bright light indicators

thanks for your help

The way mine are set up on the '87, is the knob to the left of the steering wheel does the instrumentation brightness by rotating, and if you push it in, will reset the trip odometer. The right knob does the clock (you have to push it in for it to change the time).

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting

Sounds like the manual idle adjustment. Did the SD's come with those?

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

^ I don't think so, as those idle control knobs weren't on turbos.
That knob is the trip odometer reset button, push it in while stopped. It also controls the dash light brightness. A common failure item, if spinning it back and forth a few times doesn't make any difference, you'll need to make an adjustment or replace it.
The lights at their brightest, aren't very, so check when it's dark outside.

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83 SD

84 CD

Yes, they did come with a manual idle adjustment knob. That's the knob you are describing. Press down your accelerator pedal gently and turn the knob. A wonderful little gadget in cold weather!

That's what that is!

You're right, I've seen those on 240's and NA 300's.
That knob will be flat vs a round button higher up on the dash for the dimmer/odo reset.

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83 SD

84 CD

THANKS!!! You might have just solved the mystery I had with "Next" --- my 1979 300SD.

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

Okay, welcome to the forum --- now can you do us a favor? Click on User CP and edit your details\profile and add a location. And you might want to add your car in your signature.

Also, we like to see photos of the new cars coming to the forum --- so post some pics. The search function works great on the forum and there is a whole DIY (do it yourself) section.

Oh.....and welcome to the w116 family --- I just bought a 1979 300SD a few weeks ago myself. Here's the thread for it:
http://www.peachparts.com/shopforum/showthread.php?t=261750

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more