There’s An Engineer In Germany I’d Like A Word With; Tale of a Crumbling Volkswagen Lock

In common with quite a few in the hardware hacking community, I have a fondness for older vehicles. My “modern” ride is an older vehicle by today’s standards, a Volkswagen Polo 6N made in the late 1990s. It’s by my estimation a Good Car, having transported me reliably back and forth across the UK and Europe for several years.

Last week though, it let me down. Outside the church in a neighbouring village the driver’s door lock failed, leaving me with my igniton key stuck in the door, and a mildly embarrassing phone call to my dad to bring the Torx driver required to remove the assembly and release it. I am evidently not 1337 enough, I don’t carry a full set of Torx bits with me everywhere I go. The passenger side lock has never worked properly while I’ve had the car, and this is evidently my cue to sort it all out.

Dodgy Die-Castings And Impossible Springs

The die-cast horrors of a VW door mechanism in all their glory.

Withdrawing the door handle and lock assembly, there’s a paddle on a stalk that’s turned by the key, and this locates in a slot on the door catch mechanism. At the base of the stalk assembly is a die-cast circlip, and it holds a spring in check that secures the stalk, a die-cast insert, and a die-cast outer shell. The insert in my lock had failed, it sports a couple of tabs which had fatigued and broken off, jamming the mechanism.

Taking it apart is easy enough but fraught with danger as the spring can make a bid for freedom, but in short order I had all the pieces on the bench. Having a quick look online it seems that this is a notorious problem with VW, SEAT, Škoda and other VAG cars from the early 1990s through into the 2000s, as the same mechanism found its way into many models. Mine seems to have been a good one, because most failures seem to have been on younger cars. An aftermarket kit of parts is available from multiple online sources, and mine cost me only around $5. In the pack, replacements for all four die-cast parts plus a new spring.

Looking at the assembly, it rapidly becomes obvious that this mechanism is hardly Volkswagen’s finest hour. Not only has its designer chosen to make the parts from a slightly dubious zinc-based die-casting, they’ve chosen to secure it with a spring that needs three hands and the dexterity of a contortionist to both compress length-wise and rotationally before it can be reassembled. A lot of swearing and maneuvering with a pair of snipe-nose pliers can get it into place, but I can’t honestly call it anything but an atrocious design in both materials and assembly. I’m sure a dealer would charge me £100 for the privilege of doing the same task, but it just shouldn’t be that way.

Stand Back, I’m An Engineer!

My OpenSCAD take on the part

In our community these things may be sent to try us, but they are opportunities as much as they are failures. I’m a hardware hacker, I’ve got this! So since I have the passenger side door to look at too it was off to OpenSCAD to create a CAD model for a replacement insert. All it takes is careful measurement of the die-cast part with my Vernier caliper.

It’s quite a complex shape, but in the OpenSCAD code it boils down to a simple case of a cylinder with a range of parts carved out of it to reach the required shape. An inner cylinder hollows it out, then some cubes to carve out and shape the various slots, and finally an additional protrusion on the rear. In about half an hour I had the part on the screen, and in a further ten minutes or so I had it printed. It fits into the assembly perfectly, I will never lose that air of wonderment at how we now have the means to make such complex high-precision shapes  on our desktops.

The part in the middle will do a better job than the part on the right of replacing the broken part on the left.

But before I replace my die-cast piece with a 3D printed one, have I just fallen into a trap? Because we have 3D printers there’s a temptation to use them to solve all problems, but am I just about to replace a part with a failure lifetime in years with one that’ll break after weeks? While the part is a minor thing of beauty I can’t even say in my wildest imagination that FDM-printed PLA is the strongest of materials Perhaps it’s time to back away from 3D printer evangelism and recognise that everything has its place. A nylon print would be stronger and lost-PLA casting would deliver a part equivalent to the original, but when I’m talking about a sub-five-dollar part what’s the point?

I’ll print a couple and keep them in my emergency stash in case I need to do a roadside fix to last me home, but I’ll order another kit for the passenger side. It’s a salutary warning, from an afternoon in the life of a Hackaday scribe, that sometimes when the only tool you have is a hammer, every problem is not a nail.