My idle adjuster knob does not work on my '79 300SD and i have know idea how it repair or diagnose it. I have tried searching with many different phrases but to know avail. I've been lurking on this sit for about a year know and I have learned SOOO MUCH. Thanks

The cable passes thru the dash to the top of the valve cover. If your car has cruise control, you will see two cables attached to the metal pivot for the linkage just below the SHUT OFF lever. The cable is held in place by a ball at its end passing thru a nylon collar on the pivot plate. Usually the nylon collar falls off over the years so when the knob is turned and the cable shortens to pull back on the linkage the cable passes thru the hole rather than pulling on the linkage. I got a collar off another wrecked MB from the cruise cable. I pressed it out, split one side with wire cutters, slipped it over my throttle cable and glued it to the pivot with RTV black silicone! that was 8 years ago. Still works great!

my cable does not move though. Is the knob electric or vacume operated as far as moving that cable?

Neither, It's completly mechanical.

Get to a Pull-a-Part and get a new one, Mine didn't work, it was broken internally. Yours might be as well. I think it cost around 5 dollars.

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You are turning the knob counterclockwise to raise the idle are you not? It does not push/pull, the knob rotates. iNEON is possibly correct, the cable like any other, can break.

i have turned it clockwise, many many times, as well as the other way. with my hood up i can see the two cables, and they don't move. i'll pull it out tonight

My turbo 240's knob also with no movement from the cable. I need to order a new one form Phil.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

The cable for idle adj. is the closest one to the firewall. It will not move much. My knob rotates about 260*.

The location where the rotational movement of the knob is translated into the linear motion of the cable may need to be lubricated. Not sure if this is in the cabin or under the hood.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

When using a functioning idle adjust, help it along with the pedal when increasing idle speed. This eases the strain and wear on the mechanism pulling against throttle return spring. Steve

mine is definitly in need of replacement, it turns infinitly, but it's in the dash real tight. i'm going to try again after work. i fixed my tach amp/pod though.