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RawUmber
Samba Member
Joined: June 23, 2019
Posts: 303
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 Posted: Wed Jul 09, 2025 10:47 am Post subject: Re: AFM Advanced Test |
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The Bosch AFM potentiometer's construction has been a bit of a curiosity to me due to its relative complexity: not just a typical wiper over resistive film, but a resistive film with periodic fixed resistors in parallel to it.
US Patent 3750631 "Fuel Injection System Controlled by the Amount of Air Drawn in During the Suction Stroke" (Bosch) covers what appears to be the design concept of the AFM potentiometer:
https://patentimages.storage.googleapis.com/00/ea/a0/6b8fd590f7cbfb/US3750631.pdf
I applied the patent filing's reference numbers to a L-Jetronic AFM potentiometer resistive board photo from the gallery (thanks WhirledTraveller!) w/ the hope that it will make it a little easier to follow. While the patent illustrates a simplified example, the actual board has twice the number of taps/resistors, the remainder of which are denoted below with "...".
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RawUmber
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Joined: June 23, 2019
Posts: 303
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| Posted: Fri Jun 28, 2024 5:00 pm Post subject: Re: AFM Advanced Test |
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With regard to whether the AFM wiper board is laser trimmed while within the AFM or not, I came across a photo from Bosch that is said to depict 'Thick-film potentiometer for the L-Jetronic during laser adjustment in the production process' (original text: "Dickschichtpotentiometer für die L-Jetronic beim Laserabgleich im Produktionsprozess (1983)").
The board definitely appears to be inside the AFM for a number of reasons. And the wiper in the middle could suggest a trimming/calibration process is underway. However, laser trimming with all of the copper parts hovering above the board (see lower photo) present some obstacles. Most could be trimmed from directly above, but the fuel pump contacts on the right edge look like they might block a straight shot. Could they take an angle shot? sure. Could they hold/bend the right most fuel pump contact out of the way while lasering? Probably. Parallax effect? Possibly. Is any of this true? No idea. Other explanations?
FWIW.
from Bosch:
gallery photo had a good top view, for reference:
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telford dorr
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Joined: March 11, 2009
Posts: 3625
Location: San Diego (Encinitas)
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| Posted: Tue Jun 12, 2018 7:28 pm Post subject: Re: AFM Advanced Test |
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I was thinking of the pins other than 6 to 9 for resistance checks, as spec'ed in the FI manual, for a complete AFM checkout.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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furgo
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Joined: September 06, 2016
Posts: 944
Location: Southern Germany
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| Posted: Tue Jun 12, 2018 2:17 pm Post subject: Re: AFM Advanced Test |
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Oh, I see.
I wouldn't bother with the resistance measurements for the reasons already mentioned in the thread. Voltage is more meaningful and more accurate to measure. The curves are also on the thread, but we can put a snapshot of those in checklist form:
AFM in vehicle
With AFM in the bus, FI system all plugged in, engine off, ignition on
Equipment:
- Multimeter
Power supply: battery, voltage 12.6 V
Pin 6: 0 V
Pin 7: 1.48 V ~ flap fully closed
Pin 7: 6.65 V ~ flap fully open
Pin 8: 6.8 V
Pin 9: 10.6 V ~ ca. 2 V below battery
AFM on the bench
Alternatively, using Telford's method to test on the bench AND dividing the measured voltage at pin 7 by the voltage applied at pin 8
Equipment:
- Multimeter
- Means of applying voltage to the AFM connector (e.g. power supply and 7-pin AFM plug, or wires with crimped 2.8mm female spade terminals)
Power supply: fixed voltage applied at pin 8
Pin 6: 0 V
Pin 7: 0.21 V ~ flap fully closed
Pin 7: 0.98 V ~ flap fully open
Pin 8: 2 V (or up to a max of 8 V)
Pin 9: no voltage applied here with this test
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate |
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telford dorr
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Joined: March 11, 2009
Posts: 3625
Location: San Diego (Encinitas)
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| Posted: Tue Jun 12, 2018 12:07 pm Post subject: Re: AFM Advanced Test |
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I think he means a chart of expected resistances and voltages for each AFM type.
You could make up a test fixture using an AFM connector and wires from a dead FI harness, bringing all of the wires to terminals that you could clip a meter to. The fixture could apply a fixed voltage between pins 9 and 6. Testing would be as simple as plugging in the unknown AFM, then comparing the various resistances and voltages against the list.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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furgo
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Posts: 944
Location: Southern Germany
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| Posted: Tue Jun 12, 2018 11:42 am Post subject: Re: AFM Advanced Test |
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Apologies, I probably didn't express myself clearly there, overloading the term "reference". What I meant was that that diagram seemed to imply that the output was read as the voltage differential between pin 8 (+) and pin 7 (-).
In any case, as I said, I think too that the ratio theory makes sense.
| Tcash wrote: |
| Is it posible to create a bench test list for the AFM's used in the bays? |
What do you mean exactly by bench test list? A list of the equipment to perform a test/measurement?
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate |
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Tcash
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Joined: July 20, 2011
Posts: 12846
Location: San Jose, California, USA
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| Posted: Tue Jun 12, 2018 11:00 am Post subject: Re: AFM Advanced Test |
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Jag AFM specs
Quote:
Typical airflow meter terminal voltages are:-
Pin 6 0v
Pin 7 2.25v - flap fully closed
Pin 7 7.5v - flap fully open
Pin 8 7.6v
Pin 9 12v - nominally 2v below battery
Is it posible to create a bench test list for the AFM's used in the bays?
Thank you for the learning experience!
Tcash |
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telford dorr
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Joined: March 11, 2009
Posts: 3625
Location: San Diego (Encinitas)
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| Posted: Tue Jun 12, 2018 10:06 am Post subject: Re: AFM Advanced Test |
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| furgo wrote: |
| What still throws me off though, with regards to the purpose of Vpin8, is the way the output voltage is marked on the diagram, as if it were the reference for Vpin7. |
That's my whole point: it is a reference for pin 7. If you know the voltage at pin 8 and the voltage at pin 7, you can calculate the airflow. Because the voltage across the AFM resistor string apparently isn't regulated (and thus can/will vary), you need a "reference" voltage for the pin 7 voltage to have meaning. It's found on pin 8. Regardless of the voltage at pin 8, for any given airflow, the ratio between 7 and 8 will always be the same.
airflow = ratio = Vpin7 / Vpin 8
Same with any potentiometer. Suppose you have a pot set such that when 2 volts is applied across it, the output wiper puts out 1.5 volts. If the input is reduced to 1 volt, the wiper will drop to 0.75 volts.
1.5 / 2 = 0.75 / 1 = 0.75
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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furgo
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Posts: 944
Location: Southern Germany
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| Posted: Tue Jun 12, 2018 1:41 am Post subject: Re: AFM Advanced Test |
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| telford dorr wrote: |
I'll bet it's using the ratio of the voltage at pin 7 and pin 8, based on the idea that if the resistance values change, they all change proportionally, being all mounted on the same substrate.
ratio = Vpin7 / Vpin 8
As Vpin8 is always greater than Vpin7, the ratio can never be less than 0 or greater than 1. in my case, it's between 0.217 and 0.955.
The ratio values, for any given vane position, will always be the same regardless of the applied voltage at pin 9 (as long as it's greater than 0). |
Really interesting thought, thanks.
What still throws me off though, with regards to the purpose of Vpin8, is the way the output voltage is marked on the diagram, as if it were the reference for Vpin7. Well, actually the other way round, if the voltage needs to be positive.
| Tcash wrote: |
| Would this account for the regulated 7.5v on pin8? |
I can't answer the question myself, but after measuring the voltage there, it's neither regulated (varies with battery voltage) nor constant 7.5V. I know that's what that other diagram says, but I've found it not to be the case on my ECU. Perhaps it applies to other ECUs, though.
Digging deeper, it seems that the diagram that states "pin 8= compensation (7.5 V)" comes from a Jaguar website (a great read btw, I recommend it).
They mention:
| Quote: |
Typical airflow meter terminal voltages are:-
Pin 6 0v
Pin 7 2.25v - flap fully closed
Pin 7 7.5v - flap fully open
Pin 8 7.6v
Pin 9 12v - nominally 2v below battery |
Some things to notice:
• Their AFM circuit is the same (except probably for the resistor values)
• Their AFM voltage at rest (vane closed) is higher than ours (2.25 V vs. 1.5..1.7V)
• Pin 9 voltage is 2 V below battery. I mentioned this a few posts back. This also roughly applies to our AFMs
• Their pin 8 voltage with 12 V at pin 9 (14 V battery) is 7.6 V. In these same conditions ours is about 7.75 V.
Unlike most of the L-Jetronic documentation out there, which is mostly the reworded Bosch material, it seems that the docs and tests from the Jag website were original and carried out by themselves.
My guess is that they either created the diagram and added their measurements, or they extracted it from some Jaguar L-Jetronic documentation.
Those measured 7.6 V at pin 8 seemed to wander into 7.5V and be made an absolute figure.
In summary, I'm not sure what the best way to call pin 8 is, but compensation does not seem to quite apply. Neither 7.5V, as it's not a regulated voltage source as far as I can tell.
For the lack of a better explanation, Telford's hypothesis seems the most plausible so far.
As more food for thought, here's an schematic of an LE-Jetronic ECU I found a while ago, which is the next iteration of our L-Jetronic ECUs. It's more modern and does not apply to a bus. But it's still the same Bosch AFC system and some of the building blocks can be recognized.
Note again the three Bosch ICs as in our ECUs. AFM pin 7 and pin 8 are fed into one of them too. Note also on the top right hand side, below the diagrams, that what I believe is the signal we're discussing its called "REFERENS AIRFLOW", and divided by it, the "INPUT AIRFLOWMETER" voltage would result in a range of 0 to 1V.
I'm sure more ECU and sensos knowledge can be gained combining the information of these later LE-Jetronic schematics and the earlier D-Jetronic schematics Porsche folks reverse-engineered. Even if some guesses have to be made and considering the limiting factor of the Bosch ICs. Interesting as it might be, that's another project in its own 
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate |
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Tcash
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Joined: July 20, 2011
Posts: 12846
Location: San Jose, California, USA
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| Posted: Mon Jun 11, 2018 10:56 pm Post subject: Re: AFM Advanced Test |
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Would this account for the regulated 7.5v on pin8?
So the ECU only sees the ratio voltage no greater than 1.0v?
Thank you
Tcash |
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telford dorr
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Joined: March 11, 2009
Posts: 3625
Location: San Diego (Encinitas)
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| Posted: Mon Jun 11, 2018 7:40 pm Post subject: Re: AFM Advanced Test |
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| Quote: |
| In order that ageing and the temperature characteristic of the potentiometer have no effect upon the accuracy, only resistance values ratios are evaluated in the control unit |
I'll bet it's using the ratio of the voltage at pin 7 and pin 8, based on the idea that if the resistance values change, they all change proportionally, being all mounted on the same substrate.
ratio = Vpin7 / Vpin 8
As Vpin8 is always greater than Vpin7, the ratio can never be less than 0 or greater than 1. in my case, it's between 0.217 and 0.955.
The ratio values, for any given vane position, will always be the same regardless of the applied voltage at pin 9 (as long as it's greater than 0).
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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Tcash
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Joined: July 20, 2011
Posts: 12846
Location: San Jose, California, USA
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| Posted: Mon Jun 11, 2018 5:20 pm Post subject: Re: AFM Advanced Test |
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Resistance ratio, so is is it looking at the voltage applied to the current to calculate the resistance, to factor in for heat and degradation in the potentiometer?
Thank you
Tcash |
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Tcash
Samba Member
Joined: July 20, 2011
Posts: 12846
Location: San Jose, California, USA
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| Posted: Mon Jun 11, 2018 5:11 pm Post subject: Re: AFM Advanced Test |
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| That is strang. It shows up as page 19 on the PDF for me and of course in the book it is page 17. |
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furgo
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Joined: September 06, 2016
Posts: 944
Location: Southern Germany
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| Posted: Mon Jun 11, 2018 3:30 pm Post subject: Re: AFM Advanced Test |
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This picqued my curiosity enough to try to find it, as it didn't make sense to me. It's on page 17 of that linked PDF, not 19.
It's simply an inaccurate translation of the original German text (I've got the booklet on paper):
| Quote: |
| Damit Alterung und Temperaturgang des Potentiometers keinen Einfluß auf die Genauigkeit haben, werden im Steuergerät nur Widerstandsverhältnisse ausgewertet |
It should read:
| Quote: |
In order that ageing and the temperature characteristic of the potentiometer have no effect upon the accuracy, only resistance values ratios are evaluated in the control unit |
That makes sense now, but the ECU is still reading the voltage to the best of my knowledge (also as stated in several places on the booklet).
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate |
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Tcash
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Posts: 12846
Location: San Jose, California, USA
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| Posted: Mon Jun 11, 2018 2:17 pm Post subject: Re: AFM Advanced Test |
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furgo
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Posts: 944
Location: Southern Germany
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| Posted: Mon Jun 11, 2018 12:18 pm Post subject: Re: AFM Advanced Test |
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| telford dorr wrote: |
What Bosch did was use Piecewise Linear Approximation: take a resistance element and divide it up into seven zones along the travel path of the wiper. These zones are wired up to a series string of low value laser-trimmed resistors. The values of the resistors in this string have much lower values than does the main resistance element that the wiper travels over. This allows the voltage at the connection points of these seven zones to be precisely set, and the voltage between these points varies linearly. So when you try to measure the resistance from the wiper (pin 7), you're measuring through the high resistance midpoint sections of the resistance element, which drops radically as you approach one of the seven zone connections. The measured resistance will go up, then down then up, etc. Brain hurt! |
This is one of the simplest and most accurate explanations to describe the AFM workings that I've seen so far. Again, thanks for putting this post together and helping dispelling mysteries around this component which, while very precise, it is not that complex in function.
I thought I'd attempt to complement the explanation of why the output resistance seems to be all over the place when measuring it. Here's a visual illustration of what the resistance does when you measure it while moving the flap.
This is an approximation, but the magnitudes should be close to real world ones. The segments' resistance is non-linear, so in reality the peak is not in the center but rather towards the end of the segment. I didn't add this to my simulation to keep things simple, but I'm sure you all get the idea.
If you look at the graph, it becomes more evident why measuring the dynamic resistance is probably not the best idea:
• Within each segment and across the wiper's full travel distance, resistance changes unintuitively
• Within each segment, around the extremes, very small changes in degrees will produce relatively big changes in resistance
• Segments are small: with 15° of travel, it's difficult to move the vane accurately confined to that arc.
In comparison, for the output voltage:
• Within each segment (but not across the whole range, the voltage change is linear, with increasing value at increasing vane angle. More intuitive.
• The rate of voltage change per degree is much more manageable for making manual measurements.
• The ECU most probably measures voltage (or current), but not resistance.
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
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ve7kilohertz
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| Posted: Sun Jun 10, 2018 11:34 am Post subject: Re: AFM Advanced Test |
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Awesome! Good info.
Well given the extensive research we have conducted here, I feel pretty confident that I can use the 030 AFM I have which has TS1 in it, with my '77 or '78 ECU, but may need to do a little tweaking with the AFM spring and idle settings to get it just right as we are not building up a bus engine, but rather a T1 or T4. I also could just watch for a 018 or 020 AFM.
Great work everyone!
Cheers |
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furgo
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| Posted: Sun Jun 10, 2018 10:20 am Post subject: Re: AFM Advanced Test |
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| ve7kilohertz wrote: |
Based on all of this research and data,what can we conclude? Is it safe to assume that the only real difference between AFMs, besides TS1 or no TS1, is the tuning of the spring tension and initial setting of the wiper ? Unless the ECUs are sensitive enough to recognize the small differences in initial curve shape and the small end point differences. |
I don't think there is a difference in the initial wiper setting. Having seen these results, I'd say:
1) The voltage curve appears to be the same on all bus AFMs (and would seem to be the same for FI bugs). The fact that all PCBs are the same would seem to further confirm that.
2) One of the known differences between AFM types seems to whether they have (Late Bays) or don't have (-very?- Early Bays) an integral TS1. I believe, but I don't know for sure, that those busses without TS1 in the AFM had in fact no TS1 at all (see Preheating FI system).
3) Another difference between AFMs, which does not affect the AFR is the construction of the backfire valve (real valve vs. simple rubber buffer)
4) Whether there were differences in spring tension remains to be seen. I would think not, but without data, it's difficult to say.
The last one would be an interesting project. Thanks to Samba contributions along the years, we know much more about AFMs nowadays, but there still, as far as I can tell, not much info on the factory's initial spring tension. SGKent started a thread a while ago with a measurement of the weight to initially open the vane (45 g), which was the first and only one I've seen for a bus. That would be an initial torque of 0.98 N·cm to overcome the spring, if my math is right –and having measured the vane length to be roughly 45 mm.
There's another great AFM thread on the BMW K100 bike forum that also talks about preload measurements, and how to measure the torque across the whole vane range with weights.
To do a proper tuneup, you'd need to measure the AFR, but it would be nice for starters to have a baseline of spring tension to start with.
| ve7kilohertz wrote: |
| I was also thinking I could implement a TS2 sensor into the plenum as it is supposed to be the same value. |
That's good practical thinking, but if they are available in the classifieds, I'd rather recommend getting an AFM with TS1, with the right plenum and the right ECU. Much less work and trouble, and closer to stock: you'll get better help from others with the same setup if you need to troubleshoot stuff.
That said, if you really want to use the TS2, I'd suggest getting one of the later watercooled sensors, which had the nice L-Jetronic JPT connector for the harness instead of a spade terminal.
| ve7kilohertz wrote: |
Here are the numbers from mine...
280 200 006 = 1 288 310 021 Bourns date code 7520
280 200 012 = 1 288 310 021 Bourns date code 8426
280 200 030 = 1 288 310 026 SFERNICE date code 8714
unknown = 1 288 310 021 Bourns date code 8235 |
Nice. Mine:
0 280 200 020 = 1 288 310 021 Bourns date code 7736
0 280 202 007 = 1 288 310 026 SFERNICE date code 7903 <- Opel (GM) AFM, not VW
| ve7kilohertz wrote: |
| Interesting in that the 030 is supposed to be '80-'83 Vanagons but has a DC of 1987?? Unless I am wrong in my assumption that the 4 digits are date codes, they sure look like it and that is the usual format. |
Bosch discontinued the 0 280 200 030 AFM in 2001, so it's not all that unusual to see a date code of '87 there.
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
Decode your M-Plate
Last edited by furgo on Mon Jun 11, 2018 2:50 am; edited 2 times in total |
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ve7kilohertz
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| Posted: Sun Jun 10, 2018 9:21 am Post subject: Re: AFM Advanced Test |
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| raygreenwood wrote: |
| What are the part #'s on the ceramic circuit boards themselves inside the AFM? It would be great and simpler....if they were all the same part # ....which I doubt....that would be too easy.....Ray |
We think alike, I was just looking at the pics again and thought the same thing.
Here are the numbers from mine...
280 200 006 = 1 288 310 021 Bourns date code 7520
280 200 012 = 1 288 310 021 Bourns date code 8426
280 200 030 = 1 288 310 026 SFERNICE date code 8714
unknown = 1 288 310 021 Bourns date code 8235
Interesting in that the 030 is supposed to be '80-'83 Vanagons but has a DC of 1987?? Unless I am wrong in my assumption that the 4 digits are date codes, they sure look like it and that is the usual format.
Yes Ray, I forgot about the subtle shape changes made over the years, I think of the vane, and possibly the housing as well??
Mmmmmm and so it continues. |
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raygreenwood
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| Posted: Sun Jun 10, 2018 9:02 am Post subject: Re: AFM Advanced Test |
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What are the part #'s on the ceramic circuit boards themselves inside the AFM?
It would be great and simpler....if they were all the same part # ....which I doubt....that would be too easy.....and the only differences were mechanical adjustments, shapes and TS-1 or not. Ray |
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