MPS- delving into it's secrets?, Update January 3, 2011 Options

[jk76.914]

I collected a number of MPS over a couple of years. Most held vacuum, some did not. I measured inductance vs. vacuum on those that I could, but then took all of them apart (except one brand new one).

The is a lot of variety in the diaphrams, probably because they get replaced when virtually every MPS is rebuilt. The photo below shows four diaphrams. I'll follow with descriptions and observations...



#1- copper, 2 pleats. This came from a rebuilt MPS that was in a pond or river for some time. There was actually sand in it, along with tiny aquatic snail shells of some sort, and the copper was pretty much green. This diaphram was ruptured. The workmanship is very good, and the alloy is idential to stock (see chart below). The threaded bushing looks like stock in design and attachment.

#2- copper, 3 pleats. This is a stock diaphram from a riveted MPS. It is ruptured.

#3- brass, 2 pleats. This is from a rebuilt MPS. I have three of these diaphrams, all similar. This MPS was freshly rebuilt in a rebuilder's box. Looked like new with fresh paint and plastic cap on vacuum port, but it leaked slightly. You can see why when you look at the ripples in that flange. The O.D. looks like it was cut out by hand with tin snips, and the threaded bushing was taken from a stock diaphram and soldered by hand into place. At the bottom in the picture, you can see that the flange is pretty smooth- that's where I tapped it out with a machinist's hammer on an anvil. I have no doubt that the leakage was from around the O.D. of the diaphram, and that tapping it out would probably fix it. This material (brass, see chart below) is stiffer and thicker than any of the rest. It would add quite a bit of spring tension to the mass-spring-damper system. In general, I rate the workmanship on this as "crappy", though maybe you could get a running car out of it.

#4- stainless steel, 3 pleats. This one is a bit of an enigma. It is identical to stock except that it is made from stainless (see chart below). The workmanship is perfect. It is from a rebuilt MPS. Interestingly, it is the MOST COMPLIANT of all of them, while most steel ones are reported to be stiffer. The other part of the enigma- the aneroid cells from this MPS are also stainless- same alloy. My suspicion is that this is a late Bosch rebuilt. Who else would make stainless cells, because the cells don't fail very often, so there are lots of spares available out there, and the cells are pretty complicated to make...


*** EDITED *** Materials. I measured alloy composition at work using an X-Ray Fluorescence analyzer. This machine is very precise, but it has limits to the range of elements that it can detect. Unfortunately, it cannot detect Beryllium (Be), and it is likely that the stock (at least) diaphram contains Be to harden and strengthen it. You can see a couple of things here though- the stainless diaphram and stainless cells are of exactly the same composition- nickel-chromium stainless steel. The nickel explains why they are slightly magnetic, as many stainless alloys are not. ***



Conclusions (really opinions) - The diaphram was put in there (early VW D-jets did not have one) to provide altitude compensation and to soften transition from leaner (high vacuum) to richer (low vacuum) regions. Early D-Jets had a separate unit with a diaphram and a switch to inform the ECU to richen the mixture at low vacuum. Cost reduction may have been a third reason- eliminating the separate unit, wiring, vacuum hose, etc. Anyway, this switch was either on or off, no soft transition. The MPS had only 2 aneroid cells and no diaphram, and its inductance curve was essentially a straight line from 0" to 25". There was no mixture compensation for altitude with this arrangement.

I'm thinking that the lower the stiffness of the diaphram, the more consistency in the setup and responsiveness, while both of these objectives are met. By maximizing compliance of the diaphram, the springs acting in the system are mainly the coil spring and the leaf springs that act to locate the armature.

My stainless diaphram is the most compliant, but there is another feature of steel (if I remember correctly) that adds to the argument that this is a late Bosch design- steel has a much higher Youngs modulus than copper. The higher the Youngs modulus, the greater the fatigue resistance, and the vastly most common failure mode of the MPS is fatigue failure. Could this have been a Bosch attempt to solve a reliability problem, even as the technology was being superceded by more modern ones? Since the cells are also subject to fatigue, would they have switched them over at the same time?

I am planning on assembling my own personal MPS using the stainless parts, and seeing how close I can tune it to my engine.

There are lots of other parts in the MPS that I've formed opinions about, but I'll hold off for now.

These are my own opinions, which may not be popular, so BLAST AWAY!!

(eye candy below)

[pbanders]

It would be great if we could get Bosch to supply a run of replacement diaprhagms.

I might point out that even if a replacement diaphragm were available, that rebuilding and calibrating the MPS isn't a trival thing. Here are the steps as I see it:

1. Remove the epoxy from the full load stop - don't attempt to remove the full load stop yet.
2. Drill out the case rivets to remove the top.
3. Using a 3/8" tip flat screwdriver (filed to fit the slot tightly), drive the full-load stop through and out of the top half of the case. Don't try to back it out, as there is epoxy holding the threads. Heat and WD-40 will help this operation. You may want to break it loose before drilling out the case rivets first, as it may make it easier to secure the MPS.
4. Remove the old diaphragm and remove the adjuster assembly from it, reinstall in the new diaphragm to approximately the same depth. Use a vernier to set it exactly.
5. Clean out the interior of the MPS with contact cleaner.
6. Install the diaphragm. Assuming we don't have a supply of new gaskets, install the old gasket, and use a thin layer of heavy grease to seal it.
7. Tap the case and bore the top to accept screws to secure the top.
8. Install the top, leave the full-load stop out for now.
9. Using the tables on my MPS page (see my sig for links) and a Wavetek LCR55 meter, set the inner and outer adjusters as described on the page. Install the full-load stop as described on the page.
10. At this point, the MPS is approximately adjusted and should work. To properly optimize the adjustment, you'll need to set the part load and full load mixtures using a dynamometer and a gas analyzer. Suggested settings for stock 2.0L motors are part-load (i.e. 2500 rpm, low throttle angle) at a CO of 2.5% to 3.0%, and a full-load (i.e. 4000 rpm, wide-open throttle angle) at a CO of 4.5% to 5.0%.


To those out there who have done this before, please feel free to edit and correct this procedure as required.