Thanks Chris, and Steve.. I just ordered it, and I'll post my results in a couple of weeks..

In the mean time, I'm a little confused about what the trim pot adjustment screw does.. Is it factory adjusted and I don't need to mess with it???

I love this board If I had not found it, I never would have bought a Benz in the first place..

Chris, some questions and comments:

Wear on the pot should not affect the total resistance, since that is not affected by the wiper, and not noticeably by the wear marks either. I would expect more sample-to-sample variations than any effects due to wear. You are right - measuring the resistance of the wiper output on the old pot (pin 2 when installed) would be useless. This would be due both to wear, and due to the fact the ECU input is undoubtably very high impedance compared to the pot series resistance of the wiper - it is only concerned with the voltage, not source impedance variations.

I also feel that monitoring the output voltage in-situ is the most accurate way to set the mechanical alignment - but only if the trimpot is preset accurately. You don't mention the procedure for that - is there one? It was certainly set incorrectly on the new pot I received when compared to the old.

Rambling:

If I didn't preset the trimpot before installation, my monitored bottom-end voltage would be half the value it was previously. Still, we are only talking 1.4% vs 2.7% of full scale reading at a point lower than idle speed, so maybe MB doesn't think this adjustment matters. If the voltage at idle is supposed to be 0.7V, +/- 0.25V, this is an adjustment to an accuracy of 35% at a voltage level that is probably near 10% of full scale? It would seem that adjusting the trimpot would only have an effect on voltage that would be encountered during a backfire through the intake.

It will be interesting to see what voltage I measure - I will be surprised if my adjustment was wrong considering how carefully I measured. Of course, if the old pot had been moved in the vehicle's previous life, all bets would be off, but I don't think that was the case.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

Steve,

>Wear on the pot should not affect the total resistance,

you are right, the resistance between pin 1 and 3 is constant at 4 kOhm (at all 23 pots i measured).
Maybe I didn't understand you completely.

Which resistance did you measure and to what did you adjust the thing?

1-3 is fixed at 4kOhm
1-2 varies first on the position of the MAS plate and second on the wear
so what?

>I also feel that monitoring the output voltage in-situ is the most accurate way to set the mechanical alignment - but only if the trimpot is preset accurately. You don't mention the procedure for that - is there one?

That is one of the great miracles
AFAIK the trim pot is in serie to the pin 1-2 resistor. Adjusting the trim pot may help you to get the correct value, if not possible by mechanical adjustment only.

Since Mercedes / the dealer doesn't change the pot but only the AMS unit , AFAIK there is no official instruction for adjustment of the pot. The new and complete unit comes adjusted from a flow bench but that will not help us, I think.

In other words, I aligned my pot mechanically to 0.7 V and did not touch the trim potentiometer.

>Still, we are only talking 1.4% vs 2.7% of full scale reading at a point lower than idle speed, so maybe MB doesn't think this adjustment matters.

As said before, only BOSCH knows how to adjust that thing, in case they still know... To be serious, they will know, since they still (produce and) sell MAS units as replacements.

>I will be surprised if my adjustment was wrong considering how carefully I measured.

Your alignment will be good or at least very close to spec. If you turn the potentiometer, the idle will change dramatically. Since you like the new life of your engine, the adjustment will be fine...

best regards,
Christian

1989 300TE

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Chris,

"If you turn the potentiometer, the idle will change dramatically."

My understanding is that this pot does not affect idle speed nor baseline mixture. Are you saying that if you set the pot to a different position, the speed changes permanently, or just briefly?

"1-3 is fixed at 4kOhm
1-2 varies first on the position of the MAS plate and second on the wear"

The trimpot is actually in series with pin 1 and the board, i.e. pin 1 does not connect to the board as do 2 and 3. Therefore, 1-3 is actually not fixed - it varies as the trimpot is turned. Since the trimpot is at the end of the board closest to 'idle', it will affect the idle voltage more than a condition of high demand.

However, I measured the trimpot values directly, and in the case of my old board, it was adjusted to 108 Ohms, and in the new one to 57 Ohms. Since this affects the voltage bias at the 'bottom' end of the potentiometer, I adjusted the new the same as old. Note that if measuring 1-3 on the connector, this is a difference between 4057 and 4108 Ohms - not a lot.

I guess we still don't know the function of the trim pot, though it might be an alternative way to adjust idle voltage. In general, circuits provide two adjustments to track a range, and in this case that is provided via the rotation and the trimpot. It's only useful if there is a procedure for adjusting at two points, though, and it sounds as though Mercedes only provides one.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

I would think that it would be very hard for the manufacturer
to hold a perfect resistance tolorence with a carbon/wiper style pot with mechanical indexing/rotational positioning..
So, they add the pot to trim each unit ...which would translate
to me that one would want to use the trim pot to adjust the output of the unit in real voltage reading specs [in this case .7v]

Steve,

>My understanding is that this pot does not affect idle speed nor baseline mixture.

It does - at least at the M103. Maybe the M102 is different?

>Are you saying that if you set the pot to a different position, the speed changes permanently, or just briefly?

Permanently - at least until I stoped it. I got an idle at 2500/min by just de-aligning the housing for some millimeters.

>The trimpot is actually in series with pin 1 and the board, i.e. pin 1 does not connect to the board as do 2 and 3. Therefore, 1-3 is actually not fixed - it varies as the trimpot is turned.


Sorry, i forgot to add: "during normal operation" - ok?

>I guess we still don't know the function of the trim pot, though it might be an alternative way to adjust idle voltage.

No, we don't know yet.
Since the pot will be adjusted when mounted on the AMS housing on a flow bench, I think (guess) the procedure is as follows:

We are at the BOSCH factory that builds these AMS units. There are many mechanical parts inside the AMS, that are already assembled. Now the pot...

1st step - the pot is aligned mechanically at a given position and srewed down to theAMS housing. That is an assembling step during production.

2nd step - the AMS unit is mounted on the flow bench and the complete set is fine tuned so that at a given air flow a given resistance (or output voltage at a given input voltage) is reached. This will compensate all tolerances of the mechanical part (weigh of the sensor plate, spring inside the unit...)
Therefore the trim pot is used to reach the value of the test aparatus and afterwards the trim pot is sealed.
This is a calibration step during production.

Only guessing, but it sounds good to me. :p

What do you think?

bis denn,
Christian

1989 300TE

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Arthur,

why can you express this in only a few words - I needed a lot more
Maybe I'm better if we try this in german?

But we are assuming the same, right?

bis denn,
Christian

1989 300TE

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Absolutely..
Each unit is calibrated on an individual unit basis with the pot --bringing everything [ tensions,mechanics, posistions, etc ] to a spec. output before it goes in the box...that way, each unit should perform the same as a replacement without having to adjust its calibration..Trim pots are resistance circuit calibrators used to zero in on a required /design specification ..
But , when you are getting inside these things [ which I love to do] , you now have to get it back to spec... so, you play around
with angles and trims til you fiqure it out...FUN and rewarding...
and a cheap education.....in both money savings and mechanics

This scenario sounds good to me - but I doubt then if any 'pre' calibration of the trimpot is done when the pot is bought as a separate unit. I.e, the value I measured on the new part is probably purely arbitrary. Also, since I am convinced my mechanical alignment on the flow meter body is pretty precise, it seems the logical - and easiest - way to cal the pot would be to fine-tune the trimpot to get the 0.7 Volts (or leave it if the reading is 0.45 to 0.95 Volts).

Still puzzled about the change in idle speed you noticed, Chris, if it didn't stabilize back down in a few moments. My understanding is the idle speed is controlled by rpm sensor, not any mixture adjustment. Although I would expect any perturbing of the mixture to cause a temporary speed instability, the pot adjustment does not cause a mechanical mistuning of the mixture that requires a correcting offset by the EHA. It is simply another input signal to the ECU, which could be ignored if it is overridden by the lambda sensor. Maybe it just takes longer to stabilize than you allowed? Anyway, another puzzle.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

True
With any trim device , be it a mechanical [ an aircraft elevator trim comes to mind here] or electrical trim input,
one usually like to see the trim [ in this case . a pot] to be near that 1/2 travel point when the spec is obtained ..Reasoning here is that trims should have
a +/- adjustment where 1/2 way would be considered zero trim..
Guess we could call this Trim Center..This allows trimming in either direction and if a trim is exactly at zero to obtain the desired specs, all the better...

As it stands now.. I plan to place the new unit in the same position as the old.. Tighten it down.. Re-assemble everything. Start it up , and adjust to 0.7 volts or so with the trim pot. That just seems to make good sense.. That's my plan..

I'll post my results.. Thanks for all the great reading..

Jay

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89 300SEL Money Pit

92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week

88 Wagoneer Slightly modified (Not for soccer moms)

04 Kia Sedona with every option... NICE

I will be interested to see what results you guys have trying to adjust these.
I turned the setscrew at least 20 turns with the engine running and a meter attached, and did not change the voltage at all. The only difference I could see was with key on/eng off, where clockwise turns raised the voltage. The reading I saw under these conditions was a fraction of running voltage. I finally stopped after raising voltage from about .017 to .2v. Running, on the other hand, was below .7v at idle both before and after adjustment attempts, and simply installing the pot solved both stalling and hunting to 2k rpm sysmtoms.

I wonder if this relates to what SteveB mentioned about .7v being the magic number to get below to send a signal to the ECU.

I had mostly put this issue on the shelf until I addressed some other problems and will pick it up again after I get the head back on (hopefully) this weekend.

On the other hand.. I may adjust the resistance of the trim pot to match the old one..Just install it, and if it runs good, solves both my idle surging, and hot start problems.. I may not mess with it??

I feel fairly confident that this is the cause of both problems..

Hey Guys,

I just got my potentiometer today for the 16v, I installed it ,
and set it .7, started the car and it idled badly(but steady for the first time in in some while) and when I gave it some revvvs it would stall every time coming back to idle. I measured it agian it was .7 still,
so , I losened the pot screws and started tuning by ear ,lol
I found a sweet spot(1.13) where it was running very well,
but the idle is a firm 1100 or 900 w/A/C, I coundn't get it to idle at 900 and not stall after revvving it, maybe if I adjust the mixture screw? or is this what the little trim screw is for???

All and all I am very pleased with the new pot, no more screwie up and down idle, no more hesitation, cold start
was finally normal, it seems faster through the gears

I really have to thank Steve (my newest hero) and everyone , I have replaced ALOT for parts (almost everthing ) to find these problems.
I had no idea about this most important CIS feature, and the best part about this is it a cheap ,quick fix.
I will spread the word
THANKS AGAIN
Dave
190e 2.3 16v

My final research on the topic. This morning I popped the cap on the pot connector and tested voltages. Listed in sequence 1,2,3, measured relative to a ground point on the case of the fuel distributor, 0.3% accurate meter:

-Ignition on, engine off:

0.011V, 0.142V, 4.90V

-Engine on idle down and relatively stable at 700rpm:

0.010V, 0.62V to 0.67V, 4.90V

Because of these readings, I did not try to either adjust the trimpot or the rotation of the potentiometer - the results of which could have been interesting.

My conclusion in this whole adventure is that the pot is a wear item, and should be eventually replaced in every KE Bosch system. Checking for the wear is pretty easy on most cars, with some care in alignment. Since the airflow meter body is adjustable for wear - and a POA to calibrate, not to mention hideously expensive - replacement of the pot alone is a much better option. Too bad MB doesn't supply the part.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7