Sbourg,

The parts number you listed must be a MB number. Both the Olympic and Bosch sites list the part number as a 0 261 210 066?

And Olympic states a $166 price???

Tinker

The part# I listed is lifted right from the old part. It is visible in the photos. If you go to the site, select the 'Bosch' parts link, and enter the number without spaces, it will return the correct part, and the $71 price. I ordered that, and that's what they charged. The part that came is as pictured, and you can see it has the same part# as the old.

If you need this part, I suggest you get the original part# right off the old part. They will probably refer you to an updated# if the old part has been superceded, but this is always more reliable than searching by function name.

Steve

Update - flaky behavior today and yesterday convinced me to swap out the pot for the new when we got home. Examination showed geometry and alignment of the new pot identical to old, so adequate results could be expected by just aligning the new one to the scribe mark on the airflow meter body. The low-end setpoint pot read 57.7 Ohms on the new pot and 108 Ohms on the old, so I set the new one to match. Everything reassembled, operation seems normal, so I'll see how it drives headed to work tomorrow.

Steve

Drive into work this morning was 16 mi. of uneventful commute on local roads, no traffic. Idle always returned immediately to about 700 rpm, no hunting, no surging on deceleration. This is the most 'normal' it has ever been, and much better than recently with the old pot. Too soon to say whether problems which occured intrmittently are gone - I'll have to wait about a week or so to be sure.

Steve

thanks for the info Steve, I am going to order one this week and
install it too.
thanks agian
Dave

A week has passed without a hitch in operation of the airflow meter - never any decel surge, idle always stays at 700 warm, briefly at 1100 when engine started cold, then 700. Initial tank of gas shows improved mileage, too, but a few tankfuls before final judgement on that.

Steve

Steve,

Glad to hear of your successful results. So your symptoms were a hunting idle?

You mentioned the three adjustments. But it sounds like you scribbed a mark on the meter housing and matched. You then stated; "The low-end setpoint pot read 57.7 Ohms on the new pot and 108 Ohms on the old, so I set the new one to match" So the low-end setpoint was checked by resistance through the pot? And what/where to adjust to achieve the desired 108Ohms?

Boy I wish the 16V`s were as easy to replace as the 8v`s.

Thanks for the posts.

Tinker

Hunting idle, with occasional swings up to 2000 rpm or more and back, and surging on decel. According to Bosch and Stevebfl, if I follow, the pot is used to supply enrichment demand input to the ECU, and not to track the actual airflow value. That is accomplished mechanically by the airflow meter coupling to the fuel distributor plunger. I don't know what the cause of the above symptoms is, but my guess is that the pot feedback is erratic at constant no throttle, and the ECU interprets this as demand for enrichment, and compensates for the resulting rich signal from the Lambda sensor by opening the idle control valve. This IS just a guess - I don't know the actual ECU logic.

The scribe mark is visible on the pic of the airflow meter body with the pot removed. The pot housing is designed to rotate precisely around the axis of the wiper arm - visible in that pic. The mark allows the new pot to be indexed in rotation depending on the alignment of the internal circuit board relative to the one in the old pot - so the same features line up in precisely the same way. In fact, the old and new units sported identical internal alignment, so the new one could simply be lined up to the scribe mark the same way as the old.

Another mechanical adjustment may exist on the end of the wiper axis. If so, it would be redundant, and in any event I did not need to investigate it.

The circuit board has three solder tabs visible on one edge. Two of these connect to pins 2 and 3 of the outside connector. The third, at the 'low' end of the circuit board, connects thru the trimpot to connector pin 1. This puts the trimpot in series with the pot, and the trimpot value can be measured between the board solder tab and pin 1.

Since the resistance value of my new pot was within close tolerance the same as the value of my old one, adjusting the new trimpot to the same value as the old puts the voltage monitored at the low end of the pot the same as before. If you are substituting a pot of a different value, my guess would be to keep the ratio of the trimpot resistance to pot resistance the same as the old, and maybe it will work. The trimpot adjusting screw is visible on the outside of the (new) pot housing. It is covered on the old one.

Bottom line, though, is that replacing and adjusting the pot alone is far easier than it must be to replace the whole airflow meter - which would require the mechanical mixture adjustment monitoring EHA current AND somehow a calibration of the pot.

Steve

Steve,

congratulation and thank you for the write up.

When we replaced the pots (I organized a group buy for 22 potentiometers ) we adjusted the new ones to 0.7V between pin 1 and 2 with the engine idleing. That 0.7V is the test value from the workshop manual/CD.

Can you check the voltage?

Problem with Ohms is that you can't get a correct value from an old and used potentiometer...

Tinker,

the potentiometer for the 2.5-16 is the same as "allways"...
Replacing that is not much more work than on a 300E I think.

bis denn,
Christian

1989 300TE

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Thanks for the spec, Chris - I don't have the CD yet, and my Haynes doesn't cover '89 on. Since part of my motivation for doing much of the work I am concentrating on at this time is an impending emission test requirement, I would prefer to have the adjustment 'dead-on' rather than 'very close'. Also glad to share my experiences - that is what this site is about for me, since I have learned so much from others sharing.

Steve

I have only a couple of things left to fix on my 89 W126 3.0. Inconsistent ( surging) idle is one of them.. The others are motor mounts, and both muflers..

Guys, I have a couple of questions I hope you can answer..

When I adjust the voltage to .7V between pins one and two, do I accomplish this by adjustment of the trim pot screw, or by swiveling the unit on its' axis????

Does anyone know if the part # on mine will be the same???

Steve, If I read correctly.. You simply removed the old unit, adjusted the trim pot screw to the same value, and re-installed in the exact same location.. With good results.. Correct????

THanks,
Jay

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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

Jay, since this pot has a complex and non-linear taper, precise mechanical alignment is necessary to put the wiper on the correct spot for idle. This will give the correct rate-of-change as the wiper moves off-idle. Based on this, I assume the mechanical alignment should be independent of measured voltage, and the voltage offset at idle set electrically, with the trimpot, after mechanical alignment.

The procedure I used is as you describe, as are the results. As long as the new pot (measured at the board connection pads) is the same resistance as the old, the trim pot adjusted to the same resistance value should give a very accurate result. Note that wear on the pot trace should have little or no effect on the total board resistance, since the actual resistance element is much wider than the area the wiper sweeps. Additionally - on mine, at least - the worst wear is on the 'tracking' element, which does not affect a measurement of the resistance element. I do intend now to check my claim with a measurement - probably this weekend.

The Bosch part# is embossed on the outside of the pot, and I would read it off yours to ensure you order the correct part.

Steve

Thanks Steve.. On mine, making any adjustment after it's installed will be quite difficult, due to its' location, and the fact that some things are in the way..

I took down the part # last night, and I hope I read it wrong..

I have 3437224015, which is one digit off from what you have, and is not found in the bosh parts at that website.. I'll double check the # tonight...

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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

Some searching answered my question.. the # ending in 35, is an updated part # an appears to be the correct one..

Jay,

3437224015 is the old number, this current part has a 035 at the end. That will do it

I really would adjust the pot to voltage, but: chacun a son goût!

Reason is, the resistor is weared, that is why you change the pot. You can messure the Ohms of the installed one - but how can you be shure it's the correct value? My pot showed chaotic values at or near idle position, every small movement of the AMS plate forced the needle of the Ohm meter to swing...

In the early KE instructions (for M102 in the W201) Mercedes advised to check the Ohms, in the later docs (current CD or manuals) they advice to check the voltage. It should be 0.7V +- 0.25V - so why not set the new pot to the requiered value?

If you do so, you will realize, that smallest movements of the pot will lead to relativly large changes in voltage - I don't think that an adjustment to a mark will be sufficient.

The adjustment should be done by rotating the pot. Carefully screw it down so that you can move it by small hits of a screw driver...

Installation is not that difficult but you have to disconnect the fuel pressure regulator to get access to the lower right screw.

Older pots are fixed by philips (spelling?) screws, newer ones have torx T15. You will need good tools.

hope that helps,
bis denn,
Christian

1989 300TE

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