Altitude compensator shims

[jeffc280sl]

Took the altitude compensator off this afternoon and discovered there are 4 shims.  Two shims are 2mm in thick and two are .24mm thick.  The total is 4.48mm.  Is this consistent with your car? I know one member has 2.4mm total shim thickness.  If removing a a .10mm shim makes a big difference how can I be so far off the other members setting?

I've taken a few measurements.  With 4.48mm shims in place the length of the compensator plunger is 20.52mm.  It feels like it is spring loaded and with some force the plunger can be pushed into the compensator.  When force is removed the plunger returns to starting position. 

The bar inside the FIP can be pushed down with far less force that what is required to compress the plunger in the compensation device.  The plunger contact bar  is approximately 15mm from the surface of the pump.

With these measurements at 600 ft elevation my altitude compensator is adjusting the fuel injection pump rack by pushing it downward about 5.52mm.  Does this sound normal?

Thank you,

[Cees Klumper]

Took the altitude compensator off this afternoon and discovered there are 4 shims.  Two shims are 2mm in thick and two are .24mm thick.  The total is 4.48mm.  Is this consistent with your car? I know one member has 2.4mm total shim thickness.  If removing a a .10mm shim makes a big difference how can I be so far off the other members setting?

I just searched in the technical data manual - there is info on the barometric pressure adjustments for carburetor engines, but not (as far as I can see) for our fuel injection pumps. But I would venture to guess that each pump was individually 'shimmed' to specifications at the factory and may not be far off others' pumps. Then again, perhaps your car was originally sold in a higher-elevation area.

I've taken a few measurements.  With 4.48mm shims in place the length of the compensator plunger is 20.52mm.  It feels like it is spring loaded and with some force the plunger can be pushed into the compensator.  When force is removed the plunger returns to starting position.  

Guessing here, but I would think the compensator is designed to expand and contract with ambient air pressure. You pushing it in is simulating a probably extraordinary amount of air pressure, but the fact you can press it in at all and it returns to original position would indicate to me it appears to be working.

The bar inside the FIP can be pushed down with far less force that what is required to compress the plunger in the compensation device.  The plunger contact bar  is approximately 15mm from the surface of the pump.

As long as the pressure required to move the bar inside the FIP is less than the pressure exerted by the barometric compensator, I think you are ok.

With these measurements at 600 ft elevation my altitude compensator is adjusting the fuel injection pump rack by pushing it downward about 5.52mm.  Does this sound normal?

Short of getting reliable technical data, your best indication of whether there is a problem or not is whether your air/fuel mixture is within spec or not. Unfortunately, this is difficult to measure at engine RPM above idle, but an indication of a problem could be if you can't get the mixture within spec at idle, despite having all linkages etc set up properly. Joe Alexander has commented here that our pumps tend to wear rich across the whole RPM range, and that the pump can be leaned out across the entire RPM range by removing washer(s) from the barometric compensator. So if you have high fuel consumption, this could be worth a try. Don't want to run the engine too lean, though.

[Franz W]

Hallo,

first of all sorry for my bad technical english. In my Bosch service manual about the R 11 fuel injection pump there is written:

4.16 Checking the altitude compensator

100 mm HG changing (vacuum) = 0,58 mm stroke of plunger

First you have to mount the altitude compensator on the Bosch testing Equipement number EFEP 418 with gauge EFAW 7 and o ring seals.
Adjust 20 mm HG vacuum .
Adjust the scale of the gauge to 0 .
Increase the vacuum to 200 mm HG .
Dilation of the altitude compensator plunger now should be 1,00 - 1,25 mm. If not, it does not work correctly.

4.17 Mounting the altitude compensator

Calculate the difference between average atmospharic pressure of 737 mm HG and the real atmospharic pressure you have at your place.
For instance:
Average value for atmospharic pressure is 737 mm HG
Real pressure at your garage is 600 mm HG

Difference therefore 137 mm HG

The formula says :    0,58 x 137 / 100 = 0,79 mm stroke of plunger

Now mount the bosch testing equipement Nr. EFEP 417 with the gauge EFAW 7 on the injection pump altitude compensator area
Adjust 0 on the scale of the gauge, when you have about 3 mm pretension.
Alter the knurled screw on the injection pump for 0,79 mm. (you have calculated)
Now you have to notice how many mm is the movement of the control rack and notice the fuel delivery rate at full throttle.
(for this procedure the injection pump has to bee mounted in a bosch injection pump test stand)

Now you have to mount the altitude compensator instead of the testing equipement Nr. EFEP 417 on the injection pump.
Now add shims to get the same mm movement of the control rack and the same fuel delivery rate.

I am afraid, nobody of us has a bosch fuel injection pump test stand, so you can not really adjust an altitude compensator.
At best we can test the movement of the plunger when changing the vacuum.
Nevertheless it is interesting how it should be done.

Best regards Franz

[jeffc280sl]

Thank you Cees and Franz.

Interesting stuff.  This indicates to me that MB used a barometric compensator from the aircraft industry for our cars.  The test calls for taking a rod measurement at 9,000 ft or so.

I think we can look at the Bosch manual and use the data to extrapolate the thickness of the shims.  The manual states the rate of change for the plunger is .58 mm per 100 mmhg.  The manual also tells us the length of the rod at 220 mmhg.  Here is how the math works.

Pressure            Altitude                Rod length from sea level
220 mmhg        9,152 ft                    1 to 1.25 mm
 
Converting these numbers to US standards and if my math is correct the pin is supposed to extend .003 inches per 500 feet of altitude.

[Franz W]

Hallo,

mathematics is one thing, but Bosch and Mercedes also make the praxis test after having calculated the thickness of the shims. Because the point is, how much is the fuel consumption of the engine.
Therefore they adjust and afterwards take the injection pump to the test stand and make the measurement of the fuel consumption at full throttle.
I think the best you can do without this equipement is, to verify that the altitude compensator is working and to calculate and add shims and then watch the fuel consumption of your car when travelling around.

Good luck Franz

[JamesL]

i drove once in a Mk1 Golf from CapeTown to  Jo'burg and the car was running like a dog by the time we got up to the Highveld

So when I was planning on taking my 40 year old car over the Stelvio Pass I asked a mechanic if I needed to do anything to stop said dog like behaviour.

"Oh no", he said "it has a compensator for changes in altitude"

Silly me.

[jeffc280sl]

Thabks Franz,

The math was just an exercise to try and get me in the ball park so to speak.  Lacking the Bosch/Mercedes test setup I intend to buy a wide band air fuel sensor and read out to fine tune my FIP over various rpm ranges.  I'm looking at a portable version with data log capablity.  It is designed to also include OBDII inputs which we don't have in our cars.  I have contacted the mfg to determine if it will accept an rpm signal from the coil.  I could then plot rpm and a/f mixture together.

[awolff280sl]

Hey Jeff, please let us know which AF sensor you get. It's been on my list. Thanks.

[jeffc280sl]

Will do Andy.

[TheEngineer]

Today I put a .005 inch shim under the altitude compensator. Our air temperature is about 47 degrees F. and I noticed a few days ago the engine was running a little lean. This additional shim fixed the problem nicely. In colder weather the air is more dense and our cars don't measure air density. Last summer, when we had hot weather, the engine ran fine without the extra shim. How did I know the engine was running too lean? After it warms up, like driving it for a couple of miles, the cold enrichment device shuts off, but the engine is still not quite at hot operating temperature. At that time it is a little sluggish to respond to more throttle and will backfire when throttle is opened more. When engine warms up more, the symptoms disappear. With the 5 thousands inch shim installed, it runs fine

[awolff280sl]

Makes sense. I've seen the same phenomenon with my car. Here in central Florida during the winter, the temp can be in the 40's in the AM, but rise to the 70's by noon. I've considered simply using an adjustable bolt instead of the the BC, but I can't bring myself to alter the distinctive appearance that the BC gives to the pump.

[jeffc280sl]

Speaking of BC's I wonder how many of them are in working condition.  I have two and one has a frozen or stuck pin.  Another member suggested that I put the BC in a clear jar and cut a hole in the lid.  Place a vacuum cleaner over the hole and  the pin should move outward, simulating a higher elevation condition.  I'm told the movement is noticeable.  

[TheEngineer]

You can produce a vacuum using a hand pump. There is not much volume involved and the vacuum isn't that hard. Franzl gives very good instructions. The altitude compensator should work properly, even if you don't drive in the mountains, to compensate for changes in barometric pressure.

[jeffc280sl]

Curiousity caused me to take apart the spare baro that I have.  Here are some pics of what's inside.  The pin by the way has a maximum travel of 6.5mm.  The pin was not binding inside my baro so that wasn't the piece that was stuck.  The bellows at rest was 22.5mm thick.

[bpossel]

Jeff,

What was stuck?
bob

[jeffc280sl]

The bellows will not expand to push the pin down.  I thought maybe there was a mechanical binding of some sort with the pin.  A rusted shaft or something like that.  Turns out the pin was free to move.

I provided the length of the pin at max altitude.  In that way if you measure the current length you will have some idea of the max travel if you do the vacuum test talked about earlier.

[TheEngineer]

VERY INTERESTING ! It never occurred to me to cut the altitude adjuster open! So, why does the pin move down/out when the ambient pressure decreases? It must be a sealed bellows, like in a barometer. Maybe sealed in Stuttgart. When it gets up high, like on top of Mount Rainier, the ambient pressure is lower and the bellows expands? I'm just thinking out loud. When the bellows gets a hole, it doesn't expand any more? The instructions for testing supplied by Franzl from Wien are straight out of the Prüfanleitung, page 12. Of course, I don't have a fixture, but I can visualize a pickle jar with a nipple soldered into the top. I'd just watch the pin move when I connect to the intake manifold of my Toyota? Can I go back to bed now?

[Peter van Es]

Who is volunteering to put the info from this thread in the Technical Manual?

[bpossel]

Peter,

I will.  Just need Jeff & others to review when completed to ensure accuracy.
Bob

[jeffc280sl]

Engineer,

I think you are correct.  I believe it is called an aneroid barometer.  From reading these things are also temperature sensative.  To what degtree I'm not sure yet.  Plan to do some testing and more research.

[jeffc280sl]

Maybe one of my BC isn't dead.  I marked the pin as per the picture so that I could eyeball the pin extension under vacuum.   I reassembled the BC and put it in a mason jar with sealed top.  Took a while to get the vacuum fitting on the lid to hold a vacuum but I finally did it.

Here are the results:

mmHg vac               pin length          altitude

0                          1.8mm               600 ft
127                        2.4mm             5000 ft
254                        3.1mm             11,000 ft
381                        3.7mm             18,500 ft
508                       4.7mm              27,500 ft
635                       5.5mm              42,250 ft

 

[jeffc280sl]

If I compare the rate of pin extension as provided by Franz to my data I come very close, certainly within the margin of error.

My data indicates from 127mmHg to 635mmHg the pin extended 3.1mm.  The overall pressure change was 508mmHg.

Using Franz rate of 100mmHg equals .58mm pin extension the overall pin extension should be 2.95mm.  (5.08 x .58)

.15mm seems like a lot but my lines on the pin and my eye ball of the extension could easily be off this amount.

I guess I have a good BC after all.

Here is a pic of the bellows in the vacuum chamber.

[jeffc280sl]

Tested the BC that's installed in my car and it's dead.  The pin didn't budge.  I can push it in but it will not move ourward.  I'm not goint to mess with it.  It's stuck and I've adjusted the fip to work with it so I'm good unless I do some mountain driving.

[jeffc280sl]

I think these numbers are correct.  Based on the data provided by Franz and my own testing I calculate the BC pin should extend .003 inch for each 500 ft elevation increase.  This also means each .003 inch shim is equivatent to 500 feet of altitude. 

When the engineer added a .005 inch shim to his BC he made an 833 ft altitude adjustment to his fip via the BC.

Please remember that we are using the very low altitude portion of the BC and that a .04 inch (1mm) extension of the pin represents about a 6,800 foot elevation adjustment.  This is why we have been advised by JA17 and others to make very small if any adjustments to the BC.

I would be interested to receive the length of the pin on your baro and your approximate altitude.  I'd like to compare as many BC's as I can.