Mystery dash smoke - '77 300D
 

Encountered following symptoms in 1977 300D with 180K:
1. Starting attempt number 1: smoke comes out driver side dash speaker grill and no crank. Inspect, wait 10 minutes, attempt start again - no smoke, starts fine.
2. Starting attempt number 2: after stop over at a store 30 minutes later, same smoke and no crank on first attempt. Wait again and try cranking = no smoke, but slow cranking this time, but does start.
3. No subsequent attempts to start.

Smoke had nondescript smell - not quite like electrical, but not oil (pressure gauge connection nearby) or coolant smell.

I'm suspecting the ignition switch, but, as I've learned over decades of trial and error, it's a good idea to do a little research first.

Any thoughts?

Smokey in here!!!
 

I have taken apart many ignitions to retrieve the vacuum shut off valve. And when I find the keys in the junk cars, I get the ignition and the locks too.
In some of the older Mercedes, as yours, oil does get sucked up into the vacuum lines of the car. Bad elements, or vacuum leaks. What ever. I can remember a couple of times when attempting to unplug the electrical from the back of the lock cylinder, that my hand was covered in motor oil.
I would suspect, that as the diaphram in the vacuum pump on the front of the engine failed, oil in the vacuum pump would be sucked into the system and make it's way to the shut off valve.
My first Mercedes was a 1980 300CD. This diaphram failure actually happened to me.
As the shutoff valve wore down over the years, oil made it's way into the lock cylinder. Oil is probably in other vacuum pods as well. Just open up a vac pod closest to the engine to find out. I started with the one at the driver's leg area.
So if the smoke is indeed coming from the ignition, the smell is a combination of electrical and burned oil.

Err. Seeing as how you live like, right next door to me, if you need another set of eyes, or a nose, I'm available, and I've spent hours and hours of my life dealing with the dark, mysterious abyss that is the backside of a W123 dash.

It just so happens I own a '77 300D, too!

Edit: Fished the sesame seed out from beneath the S key, and inserted a bunch of missing s's... Esses? Sesses? (Shakes head) It's early, guys. I need more coffee.

77 300D dash smoke update
 

I've been posting for my son who owns this 300D. He has removed the access cover and confirmed that there is oil seeping at the ignition switch.

Is this a sure indication of vacuum pump oil leaking through to the vacuum system? Does that require replacement/rebuild of pump AND somehow clean out vacuum lines?

Thanks - getting really helpful advice here!

I caught my vacuum pump just when it started to act up, so oil in the lines was minimal, and confined to areas under the hood.

As I understand it, a slightly failed shut-off valve can still work, but also introduce some oil in to the ignition switch vacuum lines. I haven't seen it personally, nor do I know how common it is. I've only read references to it here and there.

I think I'm a little concered at the direction this is taking... Smoke is one thing, but it won't happen just from oil in and around the ignition switch. Something has to get hot enough to make it smoke, and that would be an electrical fault. The switch gets a little warm in use, but certainly shouldn't be causing smoke.

The oil at switch could, in this case, just be leftover from an old issue that a previous owner had. Only now, maybe the ignition switch is failing, getting too hot, and cooking that old oil a little. Yes?

sounds like something behind the dash got fried like a wire,rely etc.

Oil can be leaking in the ignition or the burden tube but that wouldnt explain why there was smoke, For it to smoke there would have to be serious heat to change states.

Relay for the glow plugs is up under the driver's kick panel. My money is on some kind of short in the relay.
if it is the relay, don't bother with replacing with the loop style relay, just get a pencil plug upgrade kit which I think includes a new relay and change out the old system to a pencil plug system.

I would second this, since most folks don't have the MacGyver gene, or a box of old pinball machine parts to raid when they need replacement transistors to repair the original relay with. (You should see my Victorinox.) Oh, and buying a new one is more money than just upgrading to the better system.