What the hell is that button?
My 81 240d 509,xxx miles
Is hard to start when cold
Just started this two days ago.
When I turn the spiral button on console
It seems to trun the plug on and start.

Rear windshield defroster. No connection to glow plugs. Do you have series or parallel plugs? Is the glow plug light indicating?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

If you mean the knob to the left of the steering this is the idle adjust for the injection pump. My daughter once set it to the lowest position when fooling around and the car would not start for her.

It is a knob designed both for finger grip and to indicate visually where it is set. Usually you may wish to lower the idle after the engine warms up or it is an option to do so. Rather than a switch it is a variable device making no sound like a switch does when turning. You may be refering to something else but your description of the symptom leads me to believe otherwise.

The light comes on and goes off like normal.
And i don't know I say series. I'll attach a pic

Those are the new updated plugs, not the series loop-type.
The good thing about your plugs is that if one goes out, you still have others that can warm up. With the series type, if one goes out, it breaks the circuit and none of them will glow. The series type do not ground individually on the engine, that's why they have ceramic insulators (which you do not have, sorry for the extra info.)

Your plugs each operate independently so just pull them, check the wires to each, measure their resistance and possibly ream out the holes. While the updated plugs are good for operation they can be bad in a way because they never tell you when you have a problem. If 3 out of your 4 plugs are bad, the indicator light will still come on and you may or may not get your motor started, so it kind of gives a false positive to the driver, saying that the glow circuit is all working.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

Just by looking at the condition if your plugs, I would say you are due for new plugs...plus ream the carbon out.....here's a pictorial on testing the plugs...

Mercedes Diesel Glow Plug Repair

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Can't tell for sure but something looks disconnected in our throttle linkage in that photo. The relay compares the draw on the first plug with the draw on the rest and if there's a fault the glow plug light wont work or will flash. But if all the plugs go out the relay detects no difference between the plugs. Also there's a strip fuse in the system which might have failed.
Check that you have 12 volts to the plugs with the switch in the glow position, If you do, then unplug the glow plug harness at the relay and test the resistance on each plug. Should be less than 1.5 ohms or so.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Somthing does look to be missing there. Perhaps you can tell me what is missing?

On later models, that "knob" is connected to a part that goes to the linkages on the cam cover.

Attached Thumbnails

 

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83 SD

84 CD

yes, what toomany_MBZ said.....Mine has the same empty knob.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT