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Tcash
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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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furgo
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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 
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telford dorr
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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
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'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
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Tcash
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Joined: July 20, 2011
Posts: 12844
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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furgo
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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?
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telford dorr
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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
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furgo
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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
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telford dorr
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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.
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'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
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