Engine runs on with key removed
 

My '85 500SEL engine continues to run after removing the key from the ignition switch. It is not dieseling but idles until I pull the coil wire from the distributor. I expect it is a ignition switch problem but would like any ideas on how to sort out and fix the problem.

I would endeavor to find out what relay(s) are involved and check them first, since a lock cylinder ain't cheap or easy to remove.

I doubt there are any relays controlling the ignition system, its probably a failure of the ignition switch, I believe it can be replaced by removing the cluster and unscrewing it from behind. Not too hard to replace.

This is the part:

http://img.eautopartscatalog.com/hir...300&height=300

I like how the part in the picture still has "W-Germany" in the plastic....I wonder if the new ones are still like that? They were probably too cheap to replace their plastics tooling/mold. :D Even 20 years later.

I had this very same problem with my 1995 W202 C-Class C200 Diesel.

But just this weekend it won't start, getting flashing red/green lights on rear view mirror. Have tried batteries in keys and car battery, no luck.

Could it all be linked to the ignition switch too?

Your car is much more advanced technologywise....it's probably an electronics/computer issue. The older cars just have a mechanical switch (pictured above)....

I have just replaced the ignition switch, that was fun! Didn't help, same problem. Seems to me, based on the basic wiring diagram that I have, if the ignition power supply, ie. #15 wire is not suppling 12 volts, the motor should stop. This being a petrol engine, it does not have the vacuum connections to the ignition switch. And CPU's aside, the ignition should power off the whole system. What else could cause this problem?? Help.