Door Lock failure

I have a 1986 D 150. Recently my driver side door won't lock. They are mechanical/electric locks. The key rotates from side to side but does not move the attached vertical rod on the back side of the tumbler. Is this a problem inside the lock tumbler or a problem inside the latch mechanism? Thanks for any help. Paul

Most likely a linkage problem. Does the cylinder have any resistance when rotating? If not most certainly linkage.

Good news is these are pretty simple systems. Might just be a broken clip on the linkage.

Or, something stuck in the latch itself. Pop the door panel off and have a look. If the latch is sticky, remove it and soak it in oil, then work all the levers individually. This usually gets them working for another 5 years or so.

If the key rotates but does not move the rod, it could be a key lock tumbler problem inside the tumbler itself, or it could be that the linkage that connects the tumbler to the rod has failed or simply fallen off the tumbler's arm. I have seen the problem caused by either problem, but probably the better odds are the rod has become disconnected from the key arm. Can you see if the tumbler arm itself is actually moving when you turn the key?

Another thing I've seen before was the arm did not move enough to move the level in the latch assembly into the lock, or unlock position on the latch. that can be caused by either the plastic rod holder has failed or come off, or it could be the rod or the latch lock lever has been bent. Usually with this, you can see the outside tumbler arm move with the key, and you can see it move the rod.

The picture shows a door latch assembly out of a 92ish Dakota door. The paper behind the latch is kind of hard to read, but it tells what each plastic clip and its attached arm does on the latch assembly. With each clip, the rod that pulls or pushes on the lever slides into the hole on the clip and the plastic arm locks onto the rod and keeps the rod in the proper position to move the lever as far as it has to move to chance what that part of the latch does. picture that plastic as the steel clip that holds the choke rod onto the carb choke linkage, only in plastic and on the door latches, handles, and lock rods, on both ends of the rods. If the plastic fails on either end of the rod, the rod can't do its job. Gene

The fact that the key is rotating but not moving the vertical rod suggests that the problem is with the lock tumbler itself, but it could also be caused by a problem with the linkage between the tumbler and the rod, or with the latch mechanism itself.

To determine the exact cause of the problem, you may need to remove the door panel to access the lock and latch mechanisms. From there, you can inspect the components for any visible damage or wear, and test the operation of the lock and latch mechanisms manually. If you are not comfortable performing these tasks yourself, it may be best to consult a professional mechanic or locksmiths Rotherhithe who has experience with these types of issues.

If it is the Door lock, Id get a new(different) lock assy and swap the tumblers from the original to the new one and call it good.