air flow potentiometer switch - where is it?

Hi all,
well, I've finally tracked down at least one problem with my HC emission problem (it's too high at high rpm OK at idle). the computer responds to
the air plate position, but not to the gas pedal. when I push down the gas pedal the computer keeps sending the same signal as if the gas pedal wasn't touched. Ergo.... it must be my air flow potentiometer switch.... where the heck is it?? anyone know exactly?
any pointers hugely apreciated.
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Where is it on what????
I don't have a clue as to what model you have. My only guess is you have a Mercedes......

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all right, all right. It's a 300E, '89 in fact. Would have thought that this switch would have been about the same place in all engines considering the general similarity of all fuel injected Otto cycle engines :-) but yes, I should have been clearer. Sorry! Is this switch by any chance the same as the EHA switch?? I'm guessing they must be in the same similar location.
- best regards, guenter

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The air flow potentiometer is mounted to the air flow meter (obviously!). I'm fairly certain the you can't get them separately though. It's very easy to damage the air flap mechanism. A simple back fire is enough to bend it. This alters the mechanical ratio of air flap movement to fuel metering movement. Might be worth having your fuel metering unit checked out. It could be flowing too much fuel at upper limits.

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The EHA [electro-hydraulic actuator] is the 1"ish square black unit mounted on the rear of the fuel distributor head. That thing with all the steel fuel lines coming out the top.
Where the fuel head mounts, that aluminum foot long unit, is the mixture control unit. The potentiameter is mounted to the side of it. Look for the long black plug with 3 terminals. You cannot replace just the potentiameter, you have to replace the whole aluminum piece.

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Karl, many thanks! after I removed the aircleaner I saw it and, as well, noticed that it was leaking pretty bad.... I replaced it and now - at least- the gas smell is gone. I also noticed that the car runs/idles better. There is not much (none) of a dead spot when pressing the pedal anymore. I take it the EHA is essentially a electrically activated pressure control valve.
what still puzzles me, though, is that when I monitor the dwell times (pin 2/3 on the 9 pin plug on the fender) and push down the air flap, the dwell time goes to about 10% as it should (Voltage high around 11V). According to http://www.landiss.com/mixture.htm where he discusses fuel the Bosch fuel injection, he says that when the throttle is pushed all the way down, the dwell time should again go up but this time a bit less, say about 20% (sorry I meant dwell time should again go down). When I check this on my 300e = nothing! In fact, the air flow plate also stops having an effect on the dwell time. there is a little micro switch at the throttle linkages and once that 'clicks' pushing the air flow plate also has no effect on the dwell time which then remain around 50% identical to the static (motor not running) situation. Seems as if actuating this microswitch at the throttle linkeage has disabled the computer. The above web site revers to a 'air flow pressure switch' that needs to be replaced if this is so. Any ideas??
- your help is HUGELY appreciated.
cheers, guenter ..... lots of fun actually working on the car.

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The micro switch is one of two Idle position switches. The second is built into the Throttle body.

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thanks for your reply Lee! I must admit, I enjoy learing the ins and outs of the KE Jetronic CIS injection, there are still puzzling aspects. I did find 'a' microswitch on the throttle linkeage right behind the fuel distributor (air cleaner off) but where is the 'other' idle position switch? Are you saying it's right inside/below the air 'flap' that measures the air flow? In other words, I need to replace the whole fuel distribution setup?
- best regards, guenter

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The second "switch" is not actually a physical switch, like a micro switch. It's an electronic one that is part of the throttle body. If you look under the air flow meter you'll find the throttle body. There's a black cable that goes to a three pin connector on inlet manifold. One pin is the Common. I think it's Pin TWO. Pins ONE and THREE are for IDLE position and FULL THROTTLE. Not sure which pin is which.You can test the switch with a multi
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many thanks, Lee, for the great help. I will have a look tomorrow morning at the throttle body. Much appreciated!
- best regards, guenter
ps haven driven a 240D for the longest of times, and then switchting to the 300E, I had forgotten just how much more sophisticated engines had become since my old VW days :-) what does suprise me is how well all of this technical sophistication works, moreove, how robust it is in everyday use.

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Lee, again many thanks. found the 'switch' in front of the fuel distributor and couldn't find anything wrong with it. When I checked operation of the computer again it worked just fine. Must have had a bit of corrosion or lose contact and pulling the plug and reinserting it did the trick... now I hope that I pass emissions with the new fuel pressure valve installed and the CIS apparently working OK otherwise.
- cheers, guenter

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