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Discussion Starter #1
Hi,
I bought a used Porsche 944 turbo AFM for a 24v 3.0 Alfa 164 manual, Bosch number 0280 203 026, and have just taken it apart. I could feel the spring loading was incorrect so guessed someone had tried refurbishing it and failed.
I took the lid off my original too for reference and resetting the sping tension/arm position seems quite straight forward. The thing that seems odd is the arms are different. My original has 3 track contacts, the other just two. It is possible both are OK and one is simply a later version, or it could be the wrong innards with the correct black lid.
Anyone else looked inside their 24v AFM? Am curious as to whether there are any other two track ones out there.
 

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People have been known to open the top and reset the spring tension so it is lighter, there by giving a faster response to the incoming air request. As to the guts being different, I have no clue. What I do know is that the turbo 944 is supposed to be a replacement AFM as you said it has the same part number. I haven't tried one to really know if it works. You can share that with us. Supposedly, these can be rebuilt as I have seen some Bosch AFM rebuilt for sale. Fact or fiction, take your pick. :)
 

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Spring tension has nothing to do with throttle response. The flap measures air mass by converting flap position to a presumed volume of air. The spring tension would be specific to that measurement. Basically the flap position indicates air volume passing the flap which the ECU converts to mass. The K Jetronic used a pair of cones, male and female to measure the same thing. The J Jetronic metered fuel directly by converting that airflow directly to fuel volume. The clever bit in the lambda sond feedback adaptation to K Jetronic was to vary fuel pressure as well. L Jetronic of which that flap system forms a part used electronic inputs to correct fuel ratio for actual air density. An intake air temperature input was required. There were two main variants: LH which used a hot wire adjustment input and motronic which integrated a number of inputs. The best version was the LH utilizing an absolute barometric pressure sensor to adjust fuel mass to actual air mass. That used the hot wire (actually a grid) to measure air mass but the actual mass calculation was adjusted by the absolute barometric pressure sensor which adjusted for temperature and elevation. Worked great for supercharged engines, delivering correct air fuel ratio to elevations above 10,000 feet so any turbocharger would just keep boosting regardless of elevation.
 

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Michael, thank you for the great response! I learned something. My comment about changing spring pressure is what people perceived to help throttle response, but your explanation shot that idea down. :)
 

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Discussion Starter #5
People have been known to open the top and reset the spring tension so it is lighter, there by giving a faster response to the incoming air request. As to the guts being different, I have no clue. What I do know is that the turbo 944 is supposed to be a replacement AFM as you said it has the same part number. I haven't tried one to really know if it works. You can share that with us. Supposedly, these can be rebuilt as I have seen some Bosch AFM rebuilt for sale. Fact or fiction, take your pick. :)
Am not 100% convinced the internals for the ex-Porsche AFM are from the correct item, though there is no technical reason why a 2 track wiper can't have the same calibrations as a 3 track wiper. As my main air inlet flexible tube, brand new just 18 months ago, is perishing badly am reluctant to swap around AFMs more than necessary at the moment so won't be testing the Porsch unit just yet - unless it runs perfectly the results will be inconclusive anyay. Instead have now loosened the PCB on my original unit so going to slide it down a bit to give fresh track material and test the 'rebuild'. Might try adjusting the wiper position - depends how far this moves off position with the PCB move. Suspect not by much and the ECU might be able to relearn the 'adjustment'.

Anyone got any views on/experience of this?
 

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Yes, a lot. Theoretical first —the output (resistance which in turn is converted to a voltage drop) is individually trimmed, one each unit, with a laser that removes material(and thus changes the resistance, BOTH. ( importantly) baseline (or zero as sometimes called) and gain (“slope of resistance/ plate angle ratio). Note that the actual mass to plate angle ratio is not linear! That’s why the PCB has so much circuitry— not so simple as a “Wheatstone bridge” arrangement.

Practical — It’s difficult to work on AFM without the right test setup. The only “adjustment” I would make (as a last resort) would be to push the wiper down via shims or other— to move the contact points slightly outboard thus on a “fresh” track. Changing or adjusting the spring position is a surefire way to hose the unit. Even lubrication, if not done at factory, changes the opening resistance which changes the calibration.

Air mass as a function of flow is a third order relationship if I remember my undergrad physics correctly, even at constant temperature. Therefore the throttle angle doubling is not a doubling of mass.

Hot wire anaerometer is the way to go for sure, but even that has limitations (edge effects and the like).

I’d try Larry at APE on a replacement AFM.
 

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Ps - spring tension and response has much to do with throttle response but not in the way you envision. The flap is moved by force, being the product of density times velocity. It doesn’t move by volume. In this way, the flap has a semi-compensator for air density ( el cheap but still reasonable)! F=mv where m= volume x density.
 

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Discussion Starter #8 (Edited)
Thanks, Goats,

Moving the PCB is surely the same as trying to move the wiper position, only without the risk of possible contact pressure/angle change - things are only moving horizontally, on a flat plane. I was considering re-adjusting the wiper arm on the spindle too as right now it points to a marker on the PCB. If I slide the PCB down a bit and the marker is no longer pointed to, it seems to make sense to re-adjust it until it does. No electronic measurements required as only making physical position changes.
 

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Yes! I like that approach— it assumes equal carbon density on the track front to back but so does the “push wiper down” approach.

For such a simple appearing device, there is a lot going on. Measuring air mass is not easy especially when it’s accelerating!!
 

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That's the clever thing about the O2 sensor feedback loop invented by Bosch/Volvo. Transformed EFI. The feedback loop allows for a bit of error in the measurement of actual oxygen content of the incoming air. The main advantage turned out to be feeding the three way catalyst with slightly variable oxygen content but originally, in the 1970's, the lambda sond software compensated for variation in accuracy of oxygen content measurement. A similar idea was then applied by SAAB to their turbo engines using the O2 sensor for fuel metering and a knock sensor for ignition timing and boost pressure (both fuel mixture and boost pressure relate to ignition timing to achieve peak bmep for a given throttle position). SAAB christened their system Trionic for fuel/ignition/boost control. The knock sensor idea was also later applied to naturally aspirated engines for ignition timing.

The current engine management optimizes fuelling by use of very high pressure direct injection also. Mazda has developed a compression ignition gasoline fuelled engine using very precise and dwell controlled direct injection with the spark plug initiating the compression ignition.

Computer power and accurate sensor inputs have transformed the ICE.

Motronic was just one more step along the way, and powerful software it was too.
 

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I could drone on and on about these kinds of systems -- boring to anyone except those with sufficient math background and interest! I did some exploratory work on FCS using Kalman filters and adaptive feed-forward algorithms-- actually co-patented two approaches. In any case, its complex and needs lotsa computing power to solve PDE's using Powell's method and other numeric methods -- computationally intensive!

Anywho back to the dinosaur AFM --
 

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Discussion Starter #12
Job done.

After realising that moving the PCB down a bit would not have the new arm arcing in a totally concentric way to the original tracks, I decided to try swapping in the two contact arm from the Porsche unit. The idea being the two contact points would be fully concentric and on new track material, untouched by the original three contact point arm.
Electrically I tried measuring the resistance of the carbon track and while there seemed to be some level of electrical resistance, I couldn't measure it with any consistency or confidence of accuracy.

As the Porsche arm spindle and top connector was different to my original one, I swapped in both the Porsche arm and Porsche contacts to it and the external connector. This was quite a painless process and aligned the arm to where the original had been. Turning it manually it looked like the two contacts were indeed on fresh track so put it back in the car for a test drive.

Wow. The flat spots in performance, especially at low airflow so in the area of the most worn parts of the track, were noticeably erased. It could be argued this was just wishful thinking but for me the proof in the pudding was the curved exit of one of my local roundabouts, which I normally take full bore in second gear with front wheels just beginning to spin as it goes above 4,000rpm. Today, with the rebuilt AFM, I actually had to lift the throttle to stay on the road - front end powering wide as both tyres were being over powered. So, wow. No imagination. The fresh track now in contact really has revived the AFM.

As for the Porsche unit, the same can be done there - with the original Alfa 3 contact arm and connectors being fitted there. Am very happy and wish I had done this years ago.
 

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