[spuggy]

You could try undoing the 2 screws that hold the electrical portion to the back of the lock assembly and dropping the electrical portion off (easy to say, but will likely take you about 30-40 minutes of cursing, even with a flexy screwdriver or a mini ratchet handle and a screwdriver bit...

But then you can leave the electrical portion connected via the harness and just try turning the center of the switch with a screwdriver - eg via the part that the lock interfaces with.

It should then be pretty obvious if there's an issue with the electrical portion or there's some mechanical problem with the lock assembly/barrel/tumblers etc.

As far as interchanging the original electrical portion 911.613.017.01 with the later replacement that superseded it, 964.613.012.00, note that the 964 electrical part has an internal interlock; once you turn it to "Start", you have to turn it all the way back to "Off" before you can move it to "Start" again - eg, if you crank long enough you want to give the starter a rest for a few seconds, you cannot just leave it at "On" and then move it to "Start" again.

I don't know when this started, but I have an electrical portion I was assured was removed from and "probably" original to an '86 930 - it also has the interlock. And certainly the 964 parts do - I bought one when they were $60 and the 911 part was $160, and never fitted it...

Both my original '77 lock assembly and an (almost certainly) earlier one I picked up, have no interlock whatsoever with the electrical portion removed - the key and the lock barrel are completely free to move round and around and around. And the electrical portion that came with the other lock also has no interlock.

However, John Walker (and others) have reported that SCs have a mechanical interlock in the lock/key assembly itself. And if that's not enough proof, there are pictures of this in the thread that shamrock posted. At least this is easily removed, if you choose or it's causing the problem moving to the "Start" position...

I suppose it's possible that both of mine had the interlock removed by a PO at some point in the distant past, I don't really know - I just presumed that the older cars don't have it in the lock assembly itself.

I can't imagine what the reasoning was for any of this, much less why 2 different ways to accomplish the same thing (surely they didn't use both?) - I've never sat in a 911 that you couldn't readily tell it was running, so why would you move the key to "Start" if it was?

I need to re-fit my old switch (it was swapped out while chasing down a "solenoid clicks" problem 15 years ago), because the interlock behavior drives me nuts sometimes, LOL...