Fixit Log

What skills to learn, what tools to get
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Sclass
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Joined: Tue Jul 10, 2012 5:15 pm
Location: Orange County, CA

Vacuum pump and DIY O-rings

Post by Sclass »

Fixing Vacuum pump for my air locks on Mercedes. My door locks on my S class failed again. I recall when I bought the car in 2010 the lock pump had failed. It needed some grease on an O-ring seal inside the pump.

I didn't want to buy a new pump. They're $140 online. A junkyard one will be old and worn out...like getting a heart transplant from a senior citizen. Taking it to a shop would cost a couple of hours of labor with is another $300. Yuck. Time to get to work.

The pump is mechanically complex. 80s German engineering. Build a mechanical control device instead of using some electronics. So there's a SPDT switch in the driver's side door that toggles 12v to the pump motor on two wires. The pump motor is DC so in one polarity it pumps air and on the other it sucks air. Each door, trunk and fuel door have an actuator that opens on pressure and closes on vacuum. Sooo then the motor has to know how long to turn on then stop, and latch the direction based on the last setting. They basically build an H bridge out of mechanical switches and use air flow through a pinhole to make a delay timer to give the pump time to build pressure before deactivating.

All easy with some digital circuitry. But MB engineers did it with pneumatics, mechanical switchover valves, switches and mechanical latching mechanisms. Alas after 35 years it leaks. In their defense, automotive grade digital systems were still pretty dodgy back then. They were shooting for reliability. And how did that work out? Pretty well admittedly. All the mechanical bits still worked. If you see a 1982 car on the road now chances are its a Mercedes...with rotten butyl rubber seals.

I tore the pump to bits and found the O-ring seal that I greased up to induce sealing upon 10 years ago had disintegrated. I needed a new o-ring. But the problem was this was an oddball metric O-ring with a 1mm cross section. Harder to find at Home Depot than P95s.

I ordered some I found on Aliexpress and the delivery dates were in April. Notes from sellers all said "we are temporarily away". Awww man. No power door locks till April. This is actually going to create havoc in global supply chains btw.

Anyhow, I'm just fixing my locks. I needed an O-ring. So I made some with Silicone glue and a 3D printed mold. Here is the photo story.

Here is the pump. So over built. Strong wires, bearings, metal components...but rubber O-ring seals that only last 20 years. Cannot complain, that's why I like 80s Mercedes. If you know what $0.25 seal to change you can keep them on the road cheap.
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Open it up and offending seal needs to go here between a pneumatic "flip flop".
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Make a mold on the 3d printer. Just a bunch of loops on a plate. I thought that I didn't even need draft if I used silicone rubber that will shrink a bit when you stretch it. Easy demolding in other words. Just pick out the seal and it peels out. It helps to use silicone oil on the mold as a release compound.

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The tools. Silicone oil lube for mold release. RTV silicone gasket maker from the auto parts store. I used high temp exhaust type since I had a lot of it. A putty knife to drive the glue into the mold and the mold itself. The real way to do this is use a two part anaerobic catalyzed silicone mold compound and use a two part mold with sprues and gates but heck, I think I can get away with this simple design and process.
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Filling the mold, demolding seals with a pin and fingernails. New seal shown with old disintegrated seal.
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Seal in place. The rubber is soft so it takes up a lot of the imperfections on compression. Seals nicely and my locks work again.
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A very cheap fix for a potentially pricey problem.

ZAFCorrection
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Re: Fixit Log

Post by ZAFCorrection »

Nice job with the o-ring mold. That's the first time I ever saw a 3D printer job that would be potentially useful for me. Much better than having to buy 1000 and wait a month just because I needed an oddball size. You think that o-ring would go to 10^-3 Torr in an otherwise decent joint?

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Sclass
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Re: Fixit Log

Post by Sclass »

ZAFCorrection wrote:
Sat Feb 01, 2020 4:47 pm
You think that o-ring would go to 10^-3 Torr in an otherwise decent joint?
No way. The limitation is the rubber which is soft form a gasket glue. Also, the surface finish on the mold is awful. It’s not like we cut it out on a mill or EDM machine like a real mold tool. The o-rings surface is too rough.

But, I measured the pressure on my door locks. It’s less than 10 psi under pressure and 20”Hg of vacuum. I can get away with murder. What I couldn’t do was use faucet packing, wax dental floss, pvc tape, Teflon tape and a bunch of other stuff I had lying around. I tried all that stuff and it leaked. I needed a 5.5id x 7.5od x 1mm cross section O-ring. It is oddball.

Now if you want to use better rubber like two part flexane or higher durometer silicone I think you have some hope of holding more pressure. But if it is anything critical I’d get real high quality seals made by a reputable manufacturer.

This was just a stop gap effort for the long delays from the Chinese suppliers.

5ts
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Re: Fixit Log

Post by 5ts »

Using vacuum for door locks seems like a typical German engineering exercise. Pointless when you can use the electrical system. I have never owned a Mercedes but with time travel I would go back to 1988 and drive a 190 D. Not many cars completely float my boat but that one does.

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Sclass
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Re: Fixit Log

Post by Sclass »

Yeah it is very German. It reminds me of the stories I hear about the V1 and V2 buzz bomb guidance systems that were supposedly clockworks.

5ts
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Re: Fixit Log

Post by 5ts »

Fascinating people. I study the language to try and get a glimpse into the Teutonic psyche but there is something impenetrable about them. Your repair is remarkable and this log is full of interesting examples.

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Sclass
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Re: Fixit Log

Post by Sclass »

My observation is that there is often a cultural bias for engineering solutions. Of course there is always some revolutionary mind that comes a long and changes the whole game now and then, but cultural influence in manufactured solutions is always there.

You can really feel it when you visit tech museums around the world. Japanese, German and British engineering has a particular flavor.

5ts
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Re: Fixit Log

Post by 5ts »

I agree. My predilection is towards Japanese automotive engineering. I am obsessed with reliability and they have a stellar record with that. Generally simple, robust engineering. Their tech UI is woeful, but that doesn't concern me. The Germans seem to have nailed the technology experience in new cars. Would you consider a newer Mercedes?

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Sclass
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Re: Fixit Log

Post by Sclass »

5ts wrote:
Sun Feb 02, 2020 2:47 pm
Would you consider a newer Mercedes?
Nope. I’m on some Mercedes forums and I don’t like what I read.

I worked a couple of years for NGK. They were very careful when it came to quality. It slowed things down but the products were robust. Some engineers had the single role of checking others work. They were big into Murphy’s law. I’m very happy with my Honda’s - I have a Civic, Accord and a motorcycle. They rarely break.

5ts
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Re: Fixit Log

Post by 5ts »

I have a similar view but have never owned anything German so wanted a real opinion. Seems like the 80s were peak engineering and reliability for Mercedes and possibly all German makes.

I am in the Toyota camp but there is absolutely nothing wrong with Honda and it's an option for me. I have had a Honda lawnmower, which is a notoriously finicky and high maintenance category of power equipment, and the Honda was absolutely perfect. Their engines are superb.

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Sclass
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Re: Fixit Log

Post by Sclass »

I love my Honda gcv160 mower and my Gx25 cord trimmer. They are the easiest starting small engines I’ve owned. I have a little Honda tiller too. I haven’t cranked it in ten years but I fog its cylinder with wd40 every few years and am confident it’ll fire when I need to convert my lawns into potato fields.

Honda Automatic transmissions are known to be poorly designed in some cars. I’ve owned two civics and two accords and not seen the issue. But I’ve heard when they’re bad they’re awful. My Civic has a Takata airbag and I never brought it in for the recall because I have it on PNO registration. No car is perfect. My advice on any vehicle is find a five year old model you like then go research long term user experiences before committing. It’ll help ferret out the terminal illnesses by design.

Recently Toyota has had some oil consumption issues on a few models. But again who hasn’t these days? Ok Tesla, but seriously it seems all makes has a line with oil burning.

By the way my locks stopped working today. I opened the pump and my silicone o-ring was shredded. I did a hillbilly fix of wrapping pvc electrical tape over the seal and jamming the halves together. After three attempts it worked. Maybe it’ll last till my Chinese o-rings get shipped.

Oh well. Not a perfect world. It’s easy to fall in love with our own solutions.

Riggerjack
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Re: Fixit Log

Post by Riggerjack »

It reminds me of the stories I hear about the V1 and V2 buzz bomb guidance systems that were supposedly clockworks.
Everyone used clockwork guidance systems back in the day. Post WWII, everyone was using engineers from the same source.

The SCUDs fired in early nineties were clockwork guided. Not terribly accurate, but when the target is the size of a city, and clockworks are what you know...

5ts
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Re: Fixit Log

Post by 5ts »

Ah, 4 stroke Honda trimmer, the stuff dreams are made of. I despise two stroke engines and the stupid fuel mixing. My Honda mower started up first pull every single time. Sitting over winter? Did not care.

I have heard about Honda transmissions. And you're right Toyota is not immune and I stick with around 5 year old models. At least you aren't stuck with some unheard of problem and spend a fortune getting a mechanic to tease out the issue.

I was thoroughly impressed with your fix but feared it might have an early demise. O-rings are persnickety creatures in my experience. One little imperfection and they bite the dust. It was an excellent write up regardless.

SavingWithBabies
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Re: Fixit Log

Post by SavingWithBabies »

Harbor Freight sells an assortment set of SAE and another set of metric nitrile rubber o-rings. I have the SAE set on hand. I took it out and measured and there is one that is ~7.76mm OD ~4.4mm ID. Box says section is 1/16" so about 1.6mm so 1.59mm and I'm using a cheap digital caliper to measure so the numbers are not quite right. So it might have an o-ring that just might fit? Although 1.59mm is quite a bit more than 1mm so... Hrm.

For the 3d printed part, I wonder if a flexible filament would work for that application? I recently experimented with TPE. I first printed a replacement extruder piece that allowed me to put some of the bowden tube material right next to the extruder wheel so the flexible filament would feed true when forced down to the hot end. I then printed an empty rocket and the feel was interesting -- I'd expected something more like lego tires but it was more like dry rubbery feel but it did hold together. I'd guess it would last longer than the molded part. It was surprising tough just lacking in grip.

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Sclass
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Re: Fixit Log

Post by Sclass »

Yeah I have the harbor freight red box of o-rings. None fit. They were too thick. It needs to be pretty close to fit in the recess and yet not have gaps.

Yeah RJ I guess if you didn’t have clockworks you had a bottle rocket back then.

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Sclass
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Re: Fixit Log

Post by Sclass »

I’m still a 3d print newbie. I am about 75% through my first spool of PLA. TPU looks interesting but I’m afraid of getting into new filaments for now. A big jump for me is changing PLA colors. I think I’ll wait for my o-rings. I drove out to dinner tonight. The electrical tape seal worked great.

basuragomi
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Re: Fixit Log

Post by basuragomi »

One thing you might want to try in the future is pushing some fibreglass strands into the mould after the silicone fill for additional reinforcement, then scraping more silicone over top. Might reduce the blow-by you were getting. The gasket silicone might also benefit from a heated post-cure, considering its intended application.

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Sclass
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Re: Fixit Log

Post by Sclass »

Hey those are some interesting thoughts. So you’re suggesting making a fiber reinforced gasket? The gasket maker was very fragile compared to other rubber o-rings. I’ll try a little heating (boiling water first) and see what happens.

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Ego
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Re: Fixit Log

Post by Ego »

The maintenance indicator wrench on the dashboard of our Honda Fit turned on last week. Over the weekend I completed the service. I thought it might be useful for others who have never done basic automobile maintenance to tackle this one. It is definitely doable for an absolute beginner.

The indicator (an orange wrench) shows up with a code, in this case the code was A12.

Just for fun I asked Mrs. Ego to call the dealer and a few local Honda shops to find out what they would charge for the service. The range was $240 at the lowest to a vague..."it probably won't be more than $500" at the highest.

I googled "2012 Honda Fit maintenance code A12" and found that it indicated the car was due for the following.

-oil change | https://youtu.be/d_KE5aK8ZuE
-oil filter change | https://youtu.be/d_KE5aK8ZuE
-cabin air filter change | https://youtu.be/LOog2qO5MJk
-engine air filter change | https://youtu.be/U2Zx-eAtR1g
-serpentine belt inspection** | https://www.youtube.com/watch?v=JpaAbn7jDkI
-tire rotation*
-reset the maintenance indicator light | https://www.youtube.com/watch?v=vunL-K-VGIw

I went to the automotive parts at Walmart.com and entered my vehicle make and year then bought the parts I would need for $54.27 including 5 quarts of synthetic oil with free delivery.

*we bought the tires at Discount Tire where they will rotate them for free.
** I just looked at our serpentine belt. I did not use the tool. It looked fine.

I poured the used oil into a cat litter container I got from the recycle bin and will drop it off at one of the recycle centers the next time I am passing one.

Total time was about an hour from start to finish with most of that being cleanup time.

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Sclass
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Re: Fixit Log

Post by Sclass »

Fun to save money. I kind of laughed at the clear maintenance light action. I always have to read our accord and civic book to remember the sequence of press the button and turn the key etc. Reading pays!

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