Key won't turn past GP position, but steering unlocks
 

I've been pouring over dozens of threads about tumblers and ignition lock housings. No one seems to have my exact problem. So here goes:

1986 300sdl, 250,000 miles
I replaced the tumbler about a year ago with one from MB.
I can turn the key to the glow plug position, but it goes no further.
I can lock and unlock the steering wheel without any problem.

Is this a bad ignition lock housing? It seems like most people with a bad housing can't unlock their steering wheel.

Having just had mine apart, I'm inclined to think the problem lies inside the ignition assembly itself and not the tumbler. I can't imagine what would stop the tumbler from turning once it has already turned partway. You could remove the tumbler and try turning the locking assembly with a screwdriver to confirm the problem lies behind the tumbler.

I guess your problem is the ignition switch behind the tumbler. It is very demanding job to change this switch because the screws are almost not accessible. It took me a lot of nerves to change this part in my SDL. You need a special mini wrench with a screw driver head and a mirror to get this part off.
Good luck. Martin

I've had the same exact problem.
it's the collar around the tumbler.
take it out and try it again.

Is this cylinder the same as on an 84SD? If so, I don;t understand how it can be the collar. On mine, once you remove the paper clip and spin the collar, the dog locks into the collar and the collar can't move again until the dog is released again.

Well, I removed the key cylinder and used a screwdriver to turn the ignition. It moved over into the glow plug position fine, but it did not want to move over to position 3 without some extra force. I was able to make the starter motor turn, but when I pulled the screwdriver out, the starter motor kept running. I stuck it back in and shut it off. Then I lubed up the area with some lock lube and tried it again. It was just as hard to start it this time. But it spun fully past position 3 and now you can just spin it in place. :mad:

Looks like I have a ignition housing replacement in my future. Ugh.

Part is a little pricey but if you choose the right supplier you can save some money. Changing it out takes about an hour or so.

Hey, don't let the dog out without its collar on!

Instead of replacing the keyswitch, you could retrofit a gorilla knob from a W115 or earlier diesel. With that, you could also eliminate the pesky vacuum shut-off with a more reliable, mechanical system!