Ignition switch or wiring harness?? Way Weird!!

[mr2dux]

Okay, so I come out to my 92 300sd after work yesterday, key in the ignition, wait for the glow plugs, and then turn the key to get my favorite car to rumble into life......but not today. Turning the key does not start the engine. It makes the horn honk!! Has been working perfectly until this moment. And the abuse I took at the office today was intolerable e.g. "Hey, if you need help just honk!" Things degraded from there....
I was able to jump it by connecting a wire across the terminals of the starter to get the car started, and now have a flashlight, and a dedicated piece of wire in the glove box, so I'm not stranded, but with that nice a car it is really bad form to have to jump start it every time.
Anyway, what could possibly be wrong? and more importantly, how do I fix it?? Is it a failed ignition switch? short in the wiring harness? failed ground that causes all sorts of weirdness?
Any thoughts and ideas are welcome!! Thanks so much!
Mark

[babymog]

Welcome Mark.

This forum is for gasoline-engine cars, there is a diesel forum for diesels.

However, the W140 is really uncommon in a diesel, so ...

Mercedes made some degrading wiring harnesses, perhaps that's why "Things degraded from there ..." My Some cars had the harnesses repaired, some are replaced, others replaced as needed, yours might fall in the 2nd category and possibly there's a new issue somewhere between your start switch and the horn relay, ...

Do you know if your car has had the harness replaced/repaired? You can look for crumbling insulation under the hood. I do not know for sure that the '92 has a suspect harness, somewhere on this and the diesel forum there is more information, search wire harness to find out.

I have a '93 SD (soon to be scrapped), harness looks good in mine, but I do not know the history of my car before 2009.