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1973 450 SL
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Discussion Starter · #1 · (Edited)
Like Pandora's Box, I was compelled by the dark side to crack open a leaky MPS I have on hand. This MPS was salvaged recently by @joser85 from a 1973 W116; Bosch order # 0 280 100 100 (same as R107). It had lots of corrosion - crusty aluminum rust. I was about to pitch it but decided yesterday to see with my own eyes what is going on inside.

With a cutting wheel on a rotary tool, I cut the head off the 4 rivets holding the MPS halves together. As I pounded out the last rivet, the case sprang open. If I ever do this again, I will contain the assembly with a strap or in a vice with minimal clamping pressure. Here is what it looks like disassembled:

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Focus on the rubber flange seal at lower center. Inside that rubber seal is an aluminum button that rests in the armature shaft (center of the item above it). This little button has a innate desire to become lost. It is contained in that Dandy Sour tin with the other hardware as I write this.

Aside from the vacuum port on the outer half of an MPS, the only places I can see a leak possible are:
1. Flange seal
2. Harness receptacle seal
3. Threaded area for aneroid cells
4. A split in the aneroid cell closest to the adjustment point which is vented to ambient air by design

A smoke test would help me figure this out. While I'm getting my mind around that, I am considering replacement of that rubber flange seal so I took some measurements.

Inner half flange diameter is 66.26 mm

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Inner half flange depth is 3.29 mm. I had everything squared up - this pose is for the sake of a picture.

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Outer half flange inner diameter is 66.51 mm

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Outer half flange outer diameter is 72.29 mm

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Outer half flange depth is 2.09 mm

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I ordered some O-rings from the O-ring store; 3.0 x 66 mm, 2.5 x 66 mm, 2.0 x 66 mm and 2.0 x 66.5 mm. When they get here, I'll report back.
 

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2005 SRT Crossfire. 1973 Euro 450SL
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Is the lower center “O” ring around a disk? Or just a “O” ring with the little white button sitting inside it? If it is aluminum why is it white?
I got a 73.
Dan
 

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1973 450 SL
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Discussion Starter · #3 ·
The black O-ring, bottom center of first picture in post #1, fits the flange of the inner half of the MPS, upper right in same picture. That button just happens to be sitting there when I took the picture - it is aluminum, just looks white, not painted.

I ordered a sample set of the rubber panel grommets to replace the worn out originals for the MPS mounting plate. If they fit, I will provide details. I also bought some hardware to replace the M4 rivets destroyed when I opened this unit.
True Value hardware metric nuts and bolts section is a great place to spend time. Part numbers follow:

08236-22478 M4 x 20 x .7 screw (needs to be cut back to 15mm)
08236-18940 M4 lock washer
08236-17016 M4 nut
08236-92054 M4 Keps K lock nut (alternate to lock washer and nut above)

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That M4 x 20 screw works well rebuilding D-Jet fuel pumps too.
 

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1973 450 SL
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Discussion Starter · #4 · (Edited)
The O-Ring Store delivered today. After some experimentation with 4 sizes I ordered, the best MPS seal is obtained with this O-ring:

3mm x 66mm (NBR) Buna-N 70 Dura Metric O-Ring

I also had to repair a cracked electrical receptacle on this MPS that I suspected might be causing a leak

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I used Surebonder Clear 9001 to repair the crack and seal the interface between the plastic and the aluminum. I used a toothpick to build up a thick seal. This adhesive is somewhat pliable unlike super glue. I did not use the KEPS K lock nuts to reseal this MPS because they are only good for one tightening. I opted for standard lock washer and nut listed in post #3. The screws have been cut to 15mm.

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Following information on Oltimer.tips from Dr. D-Jet, I performed a vacuum test measuring the time it took to decay from -50kpa to -45kpa. After steps taken above, the MPS held vacuum and took 125" to decay. Before resealing this MPS, I could not get it to hold a vacuum at all.

Next, I adjusted the MPS to match an average of 3 unmolested units I have on hand. I measured inductance across primary coil (7-15), and secondary coil (8-10), and adjusted the 4mm Allen screw to meet the target values. This method is highly subjective but it is the best I can do with what I have on hand. Apparently, numbers will not translate if a different device is used . . . these target values are of no use to anyone not using my LCR meter in my garage at the atmospheric conditions of the time I made the adjustment. Technique only, I found it easiest to measure and adjust the target with the secondary coil first.

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This resulted in a satisfactory target result on the primary coil.

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You may notice the meter reading different values than the target numbers written on the index card. This is because I took these pictures and readings after successfully driving with this MPS installed in my car. That practical test heated it up quite a bit. Hours later, I took another reading and the MPS was still on target.

In summary, I was able to make a good spare MPS out of one that I thought was junk. Our M117 MPS, Bosch 0 280 100 100, is a version 3 unit which only has one adjustment screw. It is apparently much more durable than earlier versions installed in Porsche, VW and other makes. If you read Brad Anders very detailed D-Jetronic references, or any D-Jet stuff from The Samba.com, you will get the impression that these devices are prone to blow diaphragms. That discussion is for version 2 MPS; not the case with our version 3 MPS which does not have a full load diaphragm (full load interpreted at TPS). As Volker says on Oltimer.tips, the most common failure in our version 3 MPS is a leak. Leaks can be repaired.
 

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1985 380sl
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Like Pandora's Box, I was compelled by the dark side to crack open a leaky MPS I have on hand. This MPS was salvaged recently by @joser85 from a 1973 W116; Bosch order # 0 280 100 100 (same as R107). It had lots of corrosion - crusty aluminum rust. I was about to pitch it but decided yesterday to see with my own eyes what is going on inside.
I am glad I played some small part in this Jedi magic. I'm hoping that one day I will absorb a few stray Midi-Chlorians and learn how to do something awesome, like change my oil or something.
 

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1973 450 SL
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Discussion Starter · #7 · (Edited)
I have another spare MPS that has been tampered with. This one appears to have been professionally rebuilt because it has threaded M4 timeserts / helicoils in the outer half. This MPS held a vacuum but the bleed off was pretty quick; -50kpa to -45kpa in 28 seconds. I installed the 3mm x 66mm seal from The O-Ring Store and tightened everything up. Same vacuum test took 185" to bleed off. I adjusted the MPS to the target values as described above.

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I did not install this MPS in my car for a test because it has another problem. Notice the lack of nylon buffers on the aperture shaft highlighted with yellow circles. The one on the left is free standing, the one on the right is spring loaded into the shaft and has rubbed a clean spot along the side. I have no idea where to get these pieces, what they are made of or how to fix this.

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I'm wondering if these MPS adjustment covers can be 3-D printed. Missing from this broken cover is a domed cap that snaps into place into a groove around the inner circumference of the collar.

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Discussion Starter · #8 · (Edited)
I got a small sample of grommets to replace the worn ones on the mounting plate of these MPSs. Heyco G1090 fit well into the mounting plate but do not grip the metal standoff inserts quite as well as the originals. Another difference is a rounded upper and lower edge in lieu of the squared off edge of the OE grommet.

I like them well enough to order a batch to replace the old ones on all my spares.

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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I know I replaced those mounts somewhere along the way, at least on my spare MPS. Can't recall ordering them. Maybe found them at hardware store? Luckily not 'mission critical' :)
 

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2006 Z4 3.0Si
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Thanks @cushjbc! Just ordered some of these as I noticed when doing my valve adjustment that the originals on the car were decaying away.
 

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1973 450SL w/ 65,000 miles
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Saw Cush’s thread a couple of weeks ago and thought it would be interesting to try to emulate his success. Here is a recap of my experience/results:

This MPS (0280100122) does not pass vacuum test. My vacuum tester shows inHg rather than kpa so converting -50kpa = -14.765inHg; -45kpa=-13.2885inHg. This MPS would drop from -15inHg to -13inHg in about 8 seconds. Other tests passed though: Outer pins test at 93ohms. Inner pins at 332ohm. No continuity between any pin and MPS case. I do not have equipment to measure inductance.
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Knowing that I wanted to keep inside as clean as possible, I spent about an hour cleaning outside of MPS with nylon and steel brushes I had on hand.

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I had a very difficult time getting MPS open. I had a power drill and Dremel but no punches, drill press, or bench vise. I started with cutting heads off small side of rivet (small side is not much smaller).

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With small-side heads removed, I tried using nails as punches and drilling out rivets without success. Resolved to acquire some punches and find a better method of holding/supporting MPS when attempting to remove rivets.

Drilling a hole in a block of wood that was big enough for hollow-screw end of MPS to fit in hole and allow case to rest on wood made handling/supporting MPS during rivet removal much easier. With proper punches, 2 of 4 rivets came out easily and I was able to get MPS open. Other 2 rivets were stuck in one side of case and continued to give me trouble but a combination of Dremel, drill, and punches finally get them removed.

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I expected the inside of this MPS to look like Cush’s pictures (immaculately clean); instead it looked like:

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To determine if I wanted to mess with this MPS any further, I tested resistance, which was in spec, and I put MPS together with new o-ring, bolts, and lock-nuts to check if new o-ring fixed vacuum issue. Vacuum held at -15inHg in perpetuity with new o-ring. Decided to dissassemble and see what a vinegar soak would do to rust/corrosion. MPS spent the night in about an inch of vinegar.

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Thought vinegar did a pretty good job with cleanup. Lower-left of flat spring, which was in very bad shape pre-soak, did not make it through soak.

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I don’t understand exactly what purpose the flat spring serves (even after reading Brad Anders and Dr. D-Jet). My understanding is that the silver armature post has a spring on the opposite side, so the flat spring does not appear to be what pushes armature post back to the at-atmosphere position (maybe flat spring just helps to do so). With broken flat spring, armature still moves and springs back easily.

After allowing unit to thoroughly dry and a little more cleanup, I reassembled. I was curious if old o-ring would hold vacuum if I tightened bolts. MPS with old o-ring with bolts tightened down took 30 seconds for vacuum to drop from -15inhg to -13inhg. Here is pic comparing old to new o-ring.

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I installed new o-ring and MPS holds vacuum at -15inhg in perpetuity; outer pins measure at 97 ohms; inner pins measure at 340ohms.

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I used the following parts:

3mm X 66mm (NBR) Buna-N 70 Duro Metric O-Ring from The O-Ring Store LLC.
18-8 Stainless Steel Hex Head Screw, M4 x 0.7 mm Thread, 16 mm Long from McMaster-Carr
Steel Locknut with External-Tooth Lock Washer, Class 8, Zinc-Plated, M4 x 0.7 mm Thread McMaster-Carr
Heyco G1090 Grommets from Mouser Electronics

With 16mm length screw, no cutting was required.

I’ve ordered a cheap LCR meter of ebay that should allow me to measure inductance. I purchased a properly calibrated MPS from Cush a few weeks ago, so I’m curious to take measurements of that properly calibrated unit and compare/adjust this unit. With corrosion and broken flat spring this will unit will never be anything more than a temporary spare; nevertheless, found it be an interesting and worthwhile project to better understand this unit.

Many thanks to Cush for his experimenting to find the right size o-rings and grommets, which took all the guess work out of this.
 

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Discussion Starter · #12 ·
@Jfheflin has an intact adjustment screw cap with dome on that MPS (last picture of post #11). It could serve as a good model for 3-D printing if someone here is up to the challenge. I need 5 such replacements for broken caps on my spares. I would do this myself if I knew anything about 3-D printing.
 

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I have a 1973 R107 350sl and the MPS seems to be not keeping vacum. What actually needs to be replaced to make it work? Is there any sort of kit available? Many thanks
 

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Discussion Starter · #14 ·
Mario,

There is no kit. Recommend replacing the seal referenced in post #4 above. In order to do that, you have to open the MPS case. This requires drilling out the 4 rivets. Replacement of those is with screws, lock washers and nuts described above.
 

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Mario,

There is no kit. Recommend replacing the seal referenced in post #4 above. In order to do that, you have to open the MPS case. This requires drilling out the 4 rivets. Replacement of those is with screws, lock washers and nuts described above.
Hi thanks for your prompt help. So what actually needs replacing or fixing? Is there some sort of rubber umbrella that is perforated? sourcing one seems difficult
 

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1985 380sl
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@Jfheflin has an intact adjustment screw cap with dome on that MPS (last picture of post #11). It could serve as a good model for 3-D printing if someone here is up to the challenge. I need 5 such replacements for broken caps on my spares. I would do this myself if I knew anything about 3-D printing.
My cousin is a bit of a nerd and 3D printing enthusiast. Maybe worth a shot?
 

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1974 450SL (US), 2005 SLK200 (UK)
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My cousin is a bit of a nerd and 3D printing enthusiast. Maybe worth a shot?
Hmmm... I thought we had a thread on this but I guess it was all done by email. Cap was designed, tested and a batch hand delivered. If anyone wants one let me know.

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STL is attached (change extension from txt to stl). Has to be printed in ABS.

Andy
 

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