I tried my new multimeter w/ duty cycle today (KAL 3002 by Actron, I got it on eBay before I saw the post about the Sears model)
I started with the key-on/engine-off tests. Koeo: 70% (actually 68.81%) this is with (-) trigger.
Depressed airflow sensor plate; 10% (actually 8.78%)
Open throttle completely, 20% (actually 18.97%)

When I released the throttle, the meter returned to 10% (8.78%). It stayed there until I started the car and bumped the throttle a few times.
I repeated this three times with the same results. The third time, I bumped the throttle without starting the car, and on the fourth or fifth bump, the meter returned to 70%.
Upon starting the car, the meter hovers around 50% for a couple of minutes and then steadily rises until it fluctuated between 79.75 and 83.75.
On one instance it got as high as 94%, but over several tries, it seemed to predominately stay between 79.75 and 83.75.
While fishing for the O2 sensor wires to disconnect them, I noticed idle RPM rising. I stopped, and went around front, and saw the meter at 8.78% steady. With RPM around 2000, I bumped the throttle for about 2 minutes until the meter came off 8.78% and returned to the 80's.
After this, I did disconnect the O2 sensor, and got 50% (48.91%)

Since 10% indicates airflow sensor position indicator (I am guessing this is the potentiometer) or idle speed contact switch, that is where I am headed tomorrow.
I expect it to be the pot, else the PO would have had it fixed.

Question is, are airflow meters normal wear items, such that I need to get a new one, or is this a random occurance, such that I can get one off a salvage car?

On the optimistic side, can anyone elaborate about the 'eraser on the potentiometer' process?

Or has anyone seen this before and know I am barking up the wrong tree?

This is on an 88 300E with 112,000 miles, but the car is from Long Island, and may have another 50,000 unrecorded miles from sitting stuck in traffic on the Long Island Expressway.

Yeah, the airflow sensors have been known to cause some strange problems on the older MB's. It sounds like you did a nice job diagnosing the problem. FYI, the 70% reading with the key on indicates the vehicle has on-board diagnostic capability. 1988 was the first year for this, although California probably had it earlier. The 50% reading indicates the O2 sensor is not yet active (open loop). The oscillating readings @ 80% indicate a slightly lean mixture. In my experience the "sweet spot" for the 103 is 40-50%. As for your airflow sensor problem, I would exhaust all options/tricks of the trade before replacing it. If I remember correctly, it's not cheap. Good luck and keep us posted.

I was under the impression that if the lambda readout fluctuates at all, there is no fault in the system, and that only when it is static at 10% do you have a fault in the system ... in this case the air flow potentiometer.

Did you warm up the engine to operating temp before taking a reading? (80єc)

Someone please correct me if I'm wrong.

Good Luck

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

Once closed loop is established any fixed duty-cycle is considered a fault.

I would suggest getting a new replacement as the problem you are seeing is a result of normal wear. It shows up first at around idle as this is the single most common wear point. Idling on the expressway surely contributes.

Anorther reason to look for a new one at the moment is this: I just this last week replaced a 103 airflow meter. When I looked it up I found that the number had changed and the price had just about doubled to a MB list of over $800. I buy regularly from a couple of vendors that I have online access. The one that is the same source as Fastlane already knew of the price change and was the more expensive of the two. The other one still had the old list and was over a hundred dollars cheaper. These guys are really on top of these things and it is very unusual for them to be much different in price. It is an event that won't happen for long.

If you played your cards right you should be able to still get one for less than $500.

Just looked and fastlane still shows list at $462 and they are selling it at that list, which is a pretty good deal.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks Steve for setting me straight. I didn't realize he was testing with ignition on, engine off, and never knew you could get different readings this way. Learn something new every day.

I was at a dealer parts dept yesterday for a fuel line fitting to the distributor. He had to bring out a reman distributor to verify which fitting I needed. I asked him the price for the fuel distributor and he said they now cost $1,300 as opposed to about $500 three years ago! I suppose it's all supply and demand as the 103 engines age.

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

Actually I have never tried those tests with key on engine off. I am surprised by the results and will verify them at the next opportunity. I use the 70% (federal) KOEO (key on engine off) stat, only to verify my test hook-up.

It would have been my concept that the code duty-cycle only came with the engine running. Need to see about this.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

......just for information, I would like someone to please answer his question.

"can anyone elaborate about the 'eraser on the potentiometer' process?"

I have heard some people success stories (savings of $$$$).

Attached Thumbnails

 

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

The tests zhandax performed are outlined in the "Testing Electrical Components of the KE Injection System".
It is called the "Distinction Test", number 7.0 outlined at the bottom on page 35 of 173 of the CD service manual.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

I have taken apart a number of the potentiometers and have had instantaneous success with erasers, BUT the sweep arms are riding on a semiconductor base and the problem is almost always that the strata is worn through in the area it sweeps at idle and just above. There are only temporary fixes to this.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve, you nailed it.

The picture below shows the two semiconductor pads (arcs). The one on the right I took care of with an eraser. The one on the left is worn completely thru to the metal substrate; apparently the idle zone.

There is one thing I cannot visualize.
If I leave the original sweep arms in the air meter housing and just replace the cover containing the fixed circuit board (a new version of the part pictured below), the only calibration I can see is the 1-2mm of play when seating the cover.
And if that was enough to throw the calibration off, don't the odds favor miscalibration after using an eraser and reinstalling it?
If I understand this correctly, the resistance is a function of the distance along the arc from the sweep arm to the end of the arc.

What am I missing here?

Attached Thumbnails

 

Has any one had any luck just replacing the potentiometers?
m

__________________
Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)

I have wondered too why the pot would be hard to calibrate if you do it mechanically, taking careful note on the old one of arm position vs sensor plate position - but I haven't had the need to try yet. However, according to Bosch, the sensor element is non-linear, i.e. dR/dL is not constant. It is largest near idle, apparently. This would likely make adjustment quite tricky, and it is possible that production variations combined with this might make static geometric calibration useless.

Steve

Steve,

If there is that much variation in manufacture, I can see where the two arms (relative to their respective semiconductor arcs) perform some type of averaging, or some other mathematical function, such that the relative distance between the two arms along their respective arcs are the calibration points.

I don't know if that came across intact, but I am imagining the distance between the two arms themselves are the calibration...i.e bending them closer together or further apart might be the method of calibration.

The problem I have with this is that a professional body man can spray .875 mil of paint by feel/sight. Surely the machines Bosch uses to produce this part can lay down a strata +/- 10-20 microns. I cannot conceive that one of the founding ISO 9000 companies can produce a part of such varying consistancy that this can be an issue.

Of course, all this is academic at this point as the only source for this part I have found so far is this website
http://shop.classicmotors.ru/bosch/?page=44
and after checking tonight, the only person at work who speaks Russian left in January.

I would love to test this theory if anyone has any sources for the Bosch part 3-437-224-035 (the number on the original is 3 437 224 015), but it looks like I will be ordering a new air meter this week.

Part of the problem with only replacing the part in the picture is that the wiper arms are part of the rest of the air flow meter.

If the parts were readily available I'd fix it that way it a heart beat. Calibration would be no problem as far as I'm concerned. They even told us (hush-hush) in Bosch training how to pop the cap on the outside and adjust it with a little trim screw.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

If you need translations just do a search on BABEL- Altavista, there is a great tool that will translate russian to english and many other languages. I am now in the process of translating a lot of stuff from the link you posted. It is just a matter of copy and paste.
Example below:
Продукция BOSCH
Для квалифицированного подбора необходимого вам товара, вы можете позвонить по телефону (095) 932-70-22.
=
Production BOSCH for the qualified selection of the necessary for you goods, you can ring on telephone (095) 932-70-22
Потенциометр = Potentiometer

The price posted I assume is in Dollars, $51.00 is not bad compared to $500.00 for the whole unit. If I had your problem I would jump into purchasing the pot alone (that is just me, not a suggestion) and see if it solves it.
I read somewhere that XP190 (not sure of his screen name) was able to calibrate his, and I do not remember him posting anything about bending the arms.

Just out of curiosity, can you post some more pictures of the riding arms and the opening where the pot was?

Keep us updated.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified