W126 300SD revisiting/repair options re. frozen door lock cylinder issues

This common problem has postings all over the Internet by many but not discussed in a complete manner. A cost-savings exterior door handle swap has OEM alarm implications on an 1984. If you are beginning to have lock/key problems on your old 300SD please read on.

If you can’t get your driver’s door lock cylinder to budge you can try dousing w/ brake/parts cleaner/Liquid Wrench/WD40 and blow out w/ compressed air. If that doesn’t work, you have to get out the tools and remove the door panels and lock handles and mechanisms.

If you are experiencing difficulty with your door lock cylinders being balky don’t wait to remove/repair them until they freeze/refuse to rotate - or else the job will be much harder – you’ll be taking off the door panels and loosening the entire interior door lock mechanism to achieve the magic 30-60 degree key rotation to release the lock stem from the door/lock mechanism.

If your cylinders are balky but still rotate, removing the exterior door handles on the 300SD is easy, simply remove plug button in inside door jamb, remove the Phillips head - handle retaining screw, insert key and rotate 30-60 degrees and gently pull entire handle forward (towards front of vehicle) to release the protruding lock stem, and back portion outward to remove whole handle assembly – be careful not to rip out wiring/connector which you can disconnect from the inner door retaining clip and unplug..

If your cylinders have frozen you'll have to remove the exterior and interior lock retaining screws and rotate both the handle and the inner door lock mechanism to achieve the 30-60 degree release rotation - it can be done.

If you don’t want to spend some $300 on a new, correctly keyed replacement exterior driver door handle w/new cylinder, and your passenger lock cylinder is working well, you can swap the passenger to driver’s side with the *** CAVEAT that while the handles are physically interchangeable, the internal electrical switches in them are not the same and your 1984 OEM alarm system will NOT be happy. The OEM alarm door key responses will be reversed with door handles swapped. You’ll also note the wiring routing on the swapped handles isn’t optimal but there is enough slack so as not to produce a problem.

To counter the OEM alarm problem your handle swap has created, you can disconnect the alarm which is located on the interior passenger foot well/firewall by removing the carpet pad and then the plastic panel that covers the entire upper/firewall foot well area. On our 84 300SD it’s a gray plastic box with a multi prong connector on bottom (faces down/floor) – you can identify it as the box one w/ the clicking internal relays (as the alarm goes off). In the 84 300SD w/ air bags, there are also some boxes associated with the SRS/airbag so be SURE you’re disconnecting the correct (alarm) box – alarm not the SRS system.

From other posts, the handle swap on pre-84’s (e.g. 83) may not be a problem re. the OEM alarm issue but I can’t speak from personal experience.

Not sure how useful the OEM alarm system is these days and who'd steal an old Mercedes diesel (we love them but...) ???