This is going to be long for the sake of completeness.

Recently acquired 300CD has been a reliable daily driver for the last month since I have owned it. Today, while doing routine maintenance the switch suddenly refused to turn off electrical power and would not allow the car to restart. Some details:

The switch goes through all positions smoothly and locks/unlocks the steering wheel. It feels however that it has lost the "springloaded" start position, almost as though the car believes the engine is running although it is not and the starter will not run.

Vacuum-controlled engine stop worked normally last time I turned the key off.

Until now, the pre-glow worked well. Now, I do not get a preglow light in any switch position.

I now have to remove the negative battery cable to shut down electrical power to the car. When reconnecting electrical power comes back on but still no starting function or preglow light.

What I did today before the failure: Changed differential fluid, removed and replaced battery to look for rust under the battery box. The last time it started was when I drove it off the ramps after changing diff fluid. (Glad I was home and in my garage)

What I have done since: Removed cluster and driver side underpanel to examine the switch. Everything plugged in and looks o.k. mechanically.
All fuses are good. I have recently changed them all to copper.

I cannot believe this failure is unrelated to what I did today, probably R&R of the battery. Is there a relay in the starting circuit that could fail in such a way as to apply uncontrolled electrical power yet not allow starting? Can the switch fail in such a way as to cause this?

C'mon you Wizards.

Sounds like the ignition switch has broken internally to me.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
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1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Ignition Cylinder Tumbler replacement + Key issues; mixed diesel/gas
http://www.peachparts.com/shopforum/diesel-discussion/142232-ignition-cylinder-tumbler-replacement%3B-mixed-diesel-gas.html#post1065596




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Pull the tumbler, then try a screwdriver to turn the car on/off. That may help troubleshoot.

You could have one (or more) of maybe three things:

1) bad tumbler: recommended fix - order VIN-matched key+tumbler from dealer
2) bad ignition housing: remove, but new from parts link
3) bad electrical switch on back of housing: ditto from 2

There are a few threads from maybe 4-6 months ago that chronicle difficulties removing the ignition switch from the rear of the housing.

I'll be back when fixed.

Pulled the lock cylinder. It appears to be together and operates o.k.

Pulled the complete lock assembly (what a PITA).
Remove driver side underpanel
Remove cluster to unplug everything
Stand on head to depress locking pin and remove assembly

Disassembled electrical switch from lock assembly.

Examination showed the keys in the lock assembly that engage with the keyway in the electrical switch had sheared off thus allowing key to turn without doing anything. I have included a badly out-of-focus photo showing the disaasembled and failed pieces. The little particle in the lower part of the photo is a piece of the broken key.

New assembly on order.

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