Thanks to many other forum posts I was well aware that the ignition cylinder in my 1984 300td someday might refuse to turn. Without any advance warning it finally happened. One day everything worked fine and the next day the dreaded 'my key won't turn.'

Miraculously, within twenty minutes of turning, shaking, lubing, and vibrating I got it to turn long enough to re-position my car first in line in the driveway and shut it off.

Yes, I violated the #1 rule-if you do get the key to turn by all means DO NOT turn it off.

Before pulling out the key i tried to turn it back and forth a few times and everything felt smooth. Made sure the wheels were straight with no tension on the steering lock, checked the key by turning it one last time (it was fine) and then confidently removed the key mentally prepared to repair it the following day.

The next day dawned, I made my way to the car, inserted the key and it would not turn. For the next several hours and well into the following day I tried everything (+ every variation thereof) I had ever read on this forum and others without success.

A quick list of the methods: new factory key with and without pliers, vibrated the key and cylinder with a hand-held massager and sander (massager seemed more effective), jacked up the front wheels and had a helper try to turn them as i tried the key, all kind of liquid including WD40, PB Blaster, Kroil, Brake Kleen, tried every possible combination of turning, jiggling, pulling/pushing the key in/out ever so slightly while turning.

So I resorted to the method of removing the entire lock assembly from the steering column by grinding off the tip off the pin that holds the assembly in the column and forcibly pulling the column to get the lock assembly free of the dashboard and out of the column. It involves pulling the cluster, loosening and moving a pinch clamp on the lock assembly to reveal the pin. I think Diesel Giant has a how-to with photos.

With the column pulled down and the lock assembly free of the dash I discovered why my key would not turn. When the ignition key is inserted the end of the key pushes a small chisel-shaped metal tab which allows the steering lock to retract from the column as the key is turned. Unless the tab is depressed the steering lock can't retract and the key won't turn.

But with the entire lock assembly in my hand I could push the steering lock into a retracted position and the key would turn. I spent about 15 minutes studying the way everything worked and was amazed how little the little tab needs to move in order to permit everything to work. On the flip side, if it doesn't move that little bit nothing else is going to turn.

One thing I noticed was if the steering lock was pushed in too far there was no noticeable click at each key position. The key just kinda flopped back and forth. So I decided to "fix" things by pinning the steering lock back in the shaft so that it could not engage but not so far back that I would lose the tactile sensation of each key position.

Figuring the steering lock would be hardened I wanted to avoid having to drill through it. Luckily, with the steering lock pushed into the optimal position, there was about an eighth of an inch (little over 3mm) open at the end of the shaft I could drill out without hitting the hardened lock.

The shaft drilled pretty easily and I used a small nail as the pin. Cut off the protruding ends, peened them, and filed everything smooth. The peening bent the nail slightly so it couldn't easily come out.

While everything was out and apart i degreased the ignition lock cylinder with lots of Brake Kleen so I could begin to again use graphite as the lube.

The reassembly went pretty smooth except it did seem harder for everything to clear the dashboard than it did taking it out. Be sure to connect the electrical plug to the back of the ignition BEFORE pushing everything in place. There may be a way to fiddle the connector in place afterward but I sure didn't see how. Be aware that key position is critical to disconnecting/connecting the plug. In 1984 (perhaps other years) there is a white plastic T that turns with the key and must be lined up with a slot in the plug to plug/unplug it. IIRC it is key position zero.

Also as the how-to describes, be sure to note which of the two vacuum lines is which on the ignition shutoff before removing them (tip- on my 84 one has a blue stripe). On the reassembly you can push the vacuum lines on after getting the ignition lock/steering column in place if you leave the rubber connectors attached to the shutoff valve. Just reach through the cluster opening.

The biggest surprise I encounter is the damage that can be done to a portion of the steering column when pulling it down to free the steering/cylinder lock assembly.

At some point in the W123 series Mercedes began to use a hollow corrugated tube in the steering column. The tube protects the driver from being speared in the case of a severe frontal collision. The piece runs from the firewall and attaches to the rigid portion of the steering column about 20" down stream of the steering wheel.

When pulling the steering column down to release the steering/ignition lock assembly that collapsible corrugated tube can get bent. Nowhere in any forum instructions did I read anything that gave me a heads-up on this potential damage. In looking at it (and diagrams of the column assembly) before pulling the column down it appeared to have a ujoint so I never considered the possibility of a problem. It was only as I reassembled everything that I noticed the tube had bent and a small rubber disk attaching it to the rigid portion had cracked. So be forewarned!!

One unintended consequence of pinning the steering lock is now my car thinks I've left the key in and buzzes even after it's pulled out. To prevent the buzzing simply disconnect the single wire from the bottom of the ignition switch but sure to remember to remove your key.

To summarize:
1) if your ignition key won't turn and you somehow do manage to get it to turn, DO NOT TURN IT BACK PAST THE NUMBER ONE POSITION!!! This may your last chance to fairly easily diagnose and repair this problem without having to stand on your head to accomplish it. if your car is running reach under the hood and manually stop the engine then carefully remove the negative cable from your battery.

2) just because your key won't turn doesn't automatically mean your ignition lock cylinder is bad. It could be the cylinder, it could be the steering lock, it could be both. if your key won't turn consider trying what I did instead of calling a locksmith and/or drilling out the lock cylinder.

For the ideal fix I would have spent $175 or so on a new steering lock assembly and another $100+ on a new factory lock cylinder. But this repair got me back up and running.

Thanks to everyone in these forums who has posted their tip(s) on this problem. Without their advice I couldn't have pulled this off!

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