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  #1  
Old 01-07-2004, 10:04 PM
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M103 EHA/Lambda...Advanced question

He is an advanced question for all you folks pondering the mysteries of EHA/Lambda (for my 86 300E 5spd) I am wondering how the altitude correction takes place. I live at 6000ft and cannot find any official info on HOW the thing corrects, only that it takes place. Obviously, it has to correct for the lower o2 content of the air in the cylinder. Sooooo, 1) How is the correction made? a liner curve or just "steps" for certain altitude thresholds? 2) If I set my sytem up correctly, with altitude correction "in", how do I know it will work when I drive to a lower altitude? 3) Is there a way to bypass the altitude correction? Any thoughts, deep or otherwise, would be great! Thanks Scott

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  #2  
Old 01-08-2004, 12:16 AM
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I believe there is an "altitude correction module" (my name) near the ECU behind the battery, but I can only give you an overview of why it's there and how it works. Most devices like this have a sealed chamber and a diaphagm that communicates with outside air. Movement of the diaphragm due to ambient pressure change is converted to some electric analog via strain gage or other means, which provides an electrical signal to the ECU that is an analog for ambient air pressure.

As you are probably aware the KE is a basic continuous flow mechanical FI system, and the O2 sensor and ECU provide continuous fine fuel flow calibration to maintan a stoichiometric A/F ratio via the EHA at all throttle settings other than WOT.

Since the altitude correction (higher altitudes require less fuel flow to maintain stoichiometric mixture due to less air density) is implemented through the EHA, disconnecting the altitude correction module may not make much difference as long as the O2 sensor is functional. My suspicion is that it is their to work with the system in the even of a failure somewhere in the electronic part of the system.

Back then Mercedes made a big deal out of the fact that the engine would still continue to operate in the event of a failure in the "E" part of the system, but the electronics have proved, over the years, to be fairly reliable.

I don't think most modern fuel systems have any kind of specific altitude correction. They just rely on the O2 sensor readings, and as air density varies, fuel flow is modified in real time to maintain stoich. At WOT a rich 12.5:1 A/F ratio is usually programmed in and is maintained because mass flow meters actually measure mass flow, so they automatically compensate for lower air density.

There is a lot of redundancy built into the KE system, but it's debatable whether it improves reliability or detracts from it because there is more hardware to break.

Depending on your perspective, the KE system is either an engineering marvel of a giant kluge.

An '86 300E with a five-speed is a rare and fine ride. My '88 190E 2.6 is also a five speed. They were only available on special order. It took four months for it to be delivered after I ordered it.

Duke
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  #3  
Old 01-08-2004, 01:33 PM
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I might be able to add a bit to this, but I am not 100% sure it will be the same on a 300E, I have a 190E and as far as I know these cars are very similar.

On the 190E, the altitude capsule, which is what the manual refers to it, is directly fed into the connection of the MAF sensor and then to the computer. Meaning that a slight modification is added onto the reading of the MAF sensor by the altitude capsule, it more or less makes sense to me, just an easier way of implementing it. The manual states three readings for this capsule, I forget what they are, but since this is an analog device, and the way it is connected, I would say it works on a smooth curve, not steps. The only way it changes anything is to compensate for the air pressure, at higher altitudes, you need more air to burn the same amount of fuel, and this is reflected by the reading from the MAF. The final adjustment is made only by the EHA and amount of fuel mixed with the air, and idle control valve of course.

xp
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  #4  
Old 01-08-2004, 01:40 PM
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Eighties vintage KE systems do not have a conventional electric MAF, or a device so named.

My KE schematic shows electical leads between the "altitude correction indicator", the "air flow sensor position indicator", the ECU connector, and the fuel pump relay.

It appears the air flow sensor position indicator provides and electric analog for the position of the air flow plate, so it could roughly be described as a MAF, but is not called that by Mercedes or Bosch.

I have some KE documentation that says the signal from the air flow sensor position indicator only is used for transients when the engine is cold prior to going into open loop operation, however, others have refuted this and anecdotal evidence indicates that it is involved in normal closed loop operation.

Duke

Last edited by Duke2.6; 01-08-2004 at 01:49 PM.
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  #5  
Old 01-08-2004, 03:41 PM
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The air metering flap on the K-Jet (all versions) is a mechanical mass air flow sensor. No altitude compensation is required, since the actual mass of the air is mechanically measured as it goes into the engine. Proper fuel is added via the mechanical linkage in the fuel distributor, trimmed for correct O2 in the exhaust by the computer on the KE models.

No MAP sensor, no altitude compesator, etc.

Peter
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1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #6  
Old 01-08-2004, 10:29 PM
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Hmmnnnn...

After doing a bit more reading, a Bosch theory book stated the the EHA drops approx. 1 mA per 1000ft in altitude for certain KE systems that have it. Fair enough, how come MB never mentions this in thier factory info?? Also, the test proceedure in the generic Bosch book require the altitude compensation be disconnected in the basic Lambda test. If this guy can mention it, why can't MB?
BTW psfred, the CIS plate senses volume of air, not the density, which is the crux of the problem, the plate DOESN'T sense air in a way that accurately determines how much O2 per a given volume. Density drops roughly 2% per 1000ft, in my case, 12% total, and a corresponding drop in O2. I agree that the Lambda can correct for many situations that would ultimately result in the O2 sensors detecting an increase/decrease in exaust gas ratios. But that is only within a relatively small range. I am guessing that altitude correction was one parameter that could, under some circumstances, fall out of the range that the Lambda is designed to correct for. Why put it in otherwise, unless there was some EPA nerd trying to ensure the emissions reliabilty under just about any circumstances? Comments?
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  #7  
Old 01-09-2004, 01:48 AM
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The air flow plate reacts to the momentum of incoming air. Momemtum = mass time velocity, so less dense air will result is less plate deflection for a given CFM flow. It effectively compensates for variations in air density, but as a mechanical device, it has some inherent inaccuracies.

Mercedes tends to "overdesign" many things, at least back in the seventies and eighties. See my earlier remarks about KE built-in redundancy.

Duke
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  #8  
Old 01-10-2004, 08:57 PM
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Anyone else?

Anyone Anyone?
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  #9  
Old 01-10-2004, 09:30 PM
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I live and have worked my whole life at sea level so it this isn't part of my experience. My guess is basically lambda handles it when in closed loop. Those cars required technician centering of the control system and that would be done to suit whatever altitude one sets it at. Later cars centered their base setting through adaptation. KE cars with altitude correction would alter the base setting with altitude from the settings at adjustment (at least from sea level).

As to airflow meters. The one thing I know from training is that technology went from airflow meters to air mass meters to deal with air density differences which suggests to me that airflow meters wouldn't compensate directly for altitude.
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  #10  
Old 01-10-2004, 10:42 PM
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The K-Jet system (electronic or not) is a mass air flow sensing device. It isn't the most responsive, and required some help to give both low emissions and good drivability (the "pot" on the airflow meter for instance), and the electronic systems give better control. There is considerable lag between throttle opening and fuel delivery, and some rather troublesome restrictions on where the mass flow meter can be in relation to the throttle plate and the actual intake valves (serious resonance problems), and it isn't as accurate as the electronic systems. By the time you get the K-jet all strapped up and fiddled out, electric injectors are cheaper and easier.

Beats a carburetor six ways to sunday, though.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #11  
Old 01-10-2004, 11:21 PM
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Question Steve is correct, KE systems senses ....

and deflects the sensor plate, in relation to the VOLUME of air that passes it, not the MASS. The movement to MAF/Hot wire systems was primarily for acuracy because engines burn weight (or mass) of air. Colder air is more dense, has more O2 (for a given volume)and needs more gas to match the "ideal" 14.7 to 1 ratio, than warm air. Air Temp sensors are in CIS sytems (like alitude sensors) for this reason, MAF/Hot wire doesn't need them. Altitude increase, like warming, makes air less dense for a given volume, which means Less O2 and needs less fuel. The aircraft jet engines, that I fly and used to work on, measure fuel flow in lbs per hr to match the huge air temp swings on a normal flight because Jet fuel changes its BTU's per gallon ,but not BTU's per lbs. if temp changes (just like air/O2) it is the most accurate way to measure Fuel Flow. Geez, this makes my head hurt.....anyway....so should the EHA mA's in a car @ 6000Ft. look similar to a sea level mA's???
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  #12  
Old 01-11-2004, 10:25 AM
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The air flap sits in a venturi, it responds to the velocity plus the mass of the air -- this is explained by the Bosch manual, it's the reason they used it (it's been around since before WWII on gas turbines, BTW).

It's not completely accurate, and it reponds too slowly to meet newer emissions requirements.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #13  
Old 01-11-2004, 01:22 PM
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Re: Steve is correct, KE systems senses ....

Quote:
Originally posted by ChinaClipper
and deflects the sensor plate, in relation to the VOLUME of air that passes it, not the MASS.
No, as I said before the K air meter reacts to incoming air momentum (mass times velocity), so at a given CFM lower density air will deflect the plate less. This is explained in Bosch literature on the K and KE systems.

The K air flow meter does measure mass flow, but it is not as accurate or responsive as a modern hot wire MAF, which is why the electronics were added to fine tune the performance of the system for emission requirements.

The KE system is one of the most complex fuel systems ever devised, and that's why it was finally abandoned in favor of full electronic systems, which were both cheaper and more accurate.

Duke
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  #14  
Old 01-11-2004, 01:42 PM
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The actual reason MB went to electronic fuel systems was the US epa requirement that a misfiring cyclinder have its fuel shut off, thus requiring sequential fuel injection. (at least that is what we were trained by Bosch in 1992 with the introduction of LH)

Short of documentation I will stand by the standard definition of air flow meter versus air mass meter. I see no difference between the concept of electronic airflow meters versus electronic air mass meters which was to handle density issues and the airflow meters used in KE. If they were airmass then they would be called airmass.

Originally the two electronic systems were called speed/density or airflow: D versus L for the german druck versus luft.

When Porsche sold their Carrera motor to Mooney Aircraft for use in airplanes they got rid of the motronic fuel system for the more simple reliable K-jet. K-jet may be hard to produce but it sure ain't complicated.
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  #15  
Old 01-11-2004, 03:31 PM
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The K-Jet measures mass air flow, the combination of mass plus factored with rate, not velocity -- flap moves against a spring, it doesn't measure the pressure drop across a venturi (like a carburetor does). Not to split hairs, but it does account for the mass of the air as well as the velocity, egro a mass airflow measuring device.

It was orignially deleloped in the 50's, I think, for gas turbine engines -- very nicely, it allows one to measure a fraction of the air going into the engine and controls fuel for the whole thing, instead of trying to stick a mass airflow sensor in front of a 2000 kW generator! Doesn't allow for single part control, though, and the current EPA thinking (or the industries interpretation of if) is that every single cylinder cycle must be under complete mixture control.

Sounds like overkill to me, but I didn't get asked....

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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