Neutral safety switch plastic plug retaining clip

[ykobayashi]

I designed a replacement retaining clip for the plug lock on my 85 300D turbo diesel neutral safety switch. I posted it up for free download. The little white plastic clip that holds the plug on the switch snapped in my fingers as I tried to remove it. It was crazed with cracks and very brittle from age.

This happened when I was trying to make room to get some new bushings on the shifter linkage.

I realized this clip holds the plug in place and if it gets unplugged the car won’t start. So I decided to temporarily safety wire the plug on and go to work at designing a replacement plastic clip.

Here it is. This is for the late w123/6 with the 722.4 transmissions. When I was down there I noticed the shift lever doesn’t unbolt from the trans and the switch is a very differently type from the one on my 300sd. It’s also very expensive. $150. I wasn’t going to just buy one to get a new plastic retaining clip.

https://www.thingiverse.com/thing:6972825

[Shern]

I’d love to know more about your process.

I imagine simple geometry is trial and error, but there are contours on this piece I couldn’t imagine you’d eyeball.

[ykobayashi]

Hey Shern,

I start by reconstructing the broken part with super glue. The intent is to restore as much of the original geometry as possible.

Next I use a pair of calipers and measure off what I believe are the critical dimensions. Some dimensions can be just picked out arbitrarily. Others not. On this clip the holes where the clip slips over the switch were critical. Also the thickness of the same hoops had to be spot on because they slip into a slot on the switch. The little ears on the sides are actually what locks the switch retainer in place so their relative position to the holes is critical. Also the location of the slots in the middle need to match some tabs on the switch.

That was about it. I just measure and start drawing in Tinkercad…a free online SW made for small kids.

alternatively I use Freecad. They have a photo import which is kind of a poor man’s scanner. You take a picture and import it. Then you measure some feature in the photo with your calipers and tell the software how big it is in real life. The CAD then scales the entire photo. Now you just trace over the image to get your locations.

The contours are relatively simple. These parts were designed in the 1980s with old CAD tools. Most curves are just circular primitives because people drew curves with compasses and circles back then. I simply guesstimate a radius if I cannot measure it by holding a coin or other round object against the surface and just measure the coin with the caliper. Then I iterate with the printer to get closer to reality. Alternatively you can print your own radius templates with the 3D printer. On the switch retainer you can see I just cheated - it had an organic shaped thumb tab but I just made a simple round thumb tab to save time.

Once you get 3D models you print them. There will be issues with shrinkage, fits and such but you basically edit and print interactively. I printed this switch cover four times to get it just right. My prints for this design were $0.11 each and they just take a few minutes to edit and send to the printer. You can refine a fit really well in a few tries.

That’s about it. For the most part it’s using a bunch of kids’ toys. Tinkercad, Cura Slicer, a harbor freight digital caliper, and a $200 printer. Money is no longer the excuse not to do this. It mostly costs bench space.

I’m afraid we are going to be doing more and more of this. The good news is the materials are getting better. JLC PCB is offering metal printing now. Just send in your 3D models (after testing them in plastic for fit) and you get back metal parts.

I just spent the last day working on HVAC diaphragms on my new w202 c280. Behr has simply discontinued manufacture of the flap actuators for 5/6 of the vacuum pods. Gone are the days when you could just get what you needed from pelican. It is getting bad very fast. I was thinking my 1997 C280 would have better availability than a w123. Nope. I think the market for diy on that car is so small that manufacturers simply won’t support it. I cannot even get an antenna grommet.

I’ll be modelling one for printing in TPU (rubber) soon.

3D scanning cameras have recently dropped in price. Creality has just released one. I think these will democratize the reverse engineering business.

[Shern]

Great run-down Carl. Now I can understand the prototyping.

The potential was obvious, but the last time I played with one of these printers (10 years ago?)the feedstock was a crappy plastic and the printers were loud and comically slow. Wonder how that will evolve over time.

Originally Posted by ykobayashi

Hey Shern,

3D scanning cameras have recently dropped in price. Creality has just released one. I think these will democratize the reverse engineering business.

Funny you mention this. That was actually my first thought, but hadn't read about this in so long I wasn't sure they were mass market. Wild how far they've come along. Took a look at prices and seems they've become much more accessible. Wow, this will be a boon across the board.

[BillGrissom]

I have ~4 new Neutral Safety Switches like that for the later transmissions (my 1985 CA 300D). I bought a lot of 10 from a distributor ~10 yrs ago, to sell the extras off on ebay. Sold a few, but got tiring and not sure how well they work (look OE, but made in India I think) so didn't want to get bad feedback (despite very low price). Anyway, if someone is desperate for one, can PM me. They don't fit earlier transmissions (1984- and even 1985 federal?) which have a round connector.