Dust and grit are the first suspects for making the door catches recalcitrant, followed by wear and tear. Fortunately the door catches are industrial strength and are quite easily fixed.
Dust and Grit
The door is held shut by a 2 position ratchet and pawl system. (See Pic 1 centre). Grit and dust can dry out the grease between moving surfaces increasing frictional forces. This makes closing and opening difficult since the plates won't slide smoothly and the pawl won't always drop into the ratchet, despite the strong spring. A full overhaul requires the removal of the door mechanism from inside the door, see the Power Door Lock blog post for details on how to remove the catches (about half way down).
|Pic 1. The Ratchet and Pawl System.|
Degreasing and cleaning out all the gritty sludge from the mechanism is the best solution followed by a full re-oiling of rotating components and re-greasing of the sliding plates. Also ensure that the ratchet to pawl spring is not stretched and is correctly fitted. It should be quite difficult to remove as it's quite strong.
Dust can enter via the rectangular cut out in the side of the door through which the catch protrudes, particularly the depressed rivet area, (see pic below), which allows free access for dust into the door cavity. The answer is a layer of thin closed cell foam strip stuck around the opening before the catch is refitted. The foam will compress during assembly and form a good dust barrier.
Wear and Tear
The next problem is wear and tear on the barrel which transfers the tension from the external latch to the internal ratchet and pawl system. Normally the ratchet plate is in line with the pawl, but when the barrel shaft or sliding surfaces are worn, under extreme shock (eg from a bump on a rough track or the door being slammed shut), the plate can move out of line. The pawl can then jump out of the ratchet and the door flies open, or won't close securely. (See Pic 3).
|Pic 3. A worn barrel allows the ratchet to jump free from the pawl.|
Wear on the barrel creates a gap between the door catch and the barrel spacer (see Pic 4). This is more difficult to fix since the plates at either end are sweated on and peened over to prevent movement.
|Pic 4. The gap caused by wear on the barrel.|
A simple solution is to wind one or 2 turns of fine galvanised fencing wire around the outside of the barrel to take up the wear gap. (See Pic 5) This reduces the inward movement of the internal ratchet plate and prevents the pawl from jumping out of the ratchet.
A large circlip would be a better solution but would be difficult to insert. By contrast, fencing wire is quick and easy to fit.
|Pic 5. Wires can fill the gap.|
You don't even have to remove the catch to make this fix. It can be done from outside. It might not last for ever but it's cheap, quick and effective, and easy to replace. It can also be done on the rear tailgate catches on the bus model.
The last(?) wear and tear problem is the latch plate which hits the striker. This is often worn and won't hold the striker rod tightly and securely.
The only easy solution to this problem is to lay some weld on the worn "C" shape (it should be a nice "U" shape) and file it down so it fits and turns on the striker rod smoothly. It's not a quick job but will make a lot of difference to door closing and retention.
If the striker rod itself is worn, as mine were, you could wrap a layer of steel shim around it to pack out the wear. This worked surprisingly well for several years. Alternatively, weld could be laid on the worn part of the rod and filed/ground round.
Finally, ensure that the striker plates on the door frames are adjusted properly (so that the door closes tightly without slamming and doesn't rattle) and the Allen headed screws done up tight so the plate can't move. BTW, never remove all the Allen screws at the same time. The threaded lock plate inside the door frame can drop down, never to be seen again.
And why not grease the door hinges while you're at it? I'll bet that doesn't get done very often because I've found it's necessary to remove the door stay strut on some doors to allow the door to open far enough to access the grease nipples.