[Diy_efi] trying to replace standard flap style afm with MAP sensor

[Marc Reviel]

You have to involve RPM in the equation. You must (at minimum) know 
the engine RPM AND the MAP value to be able to simulate the original 
AFM output. Plus most AFM outputs area not linear with air-flow. This 
non-linear transfer curve associated with air-flow must be accounted 
for in the calculation, too.  The upshot is you'll have to use a 
micro to generate what you need, ala one of the many commercial 
interceptor-style boxes out there designed to just this.

-Marc



At 4:39 PM +1000 7/20/05, Damian Long wrote:
>Hi,
>
>I am new to the efi digest. My first post! I eventually brought 
>myself here after many nights of trial and error with no solution. I 
>sense gurus live here
>
>What my problem is.....
>
>I have been modifying my vehicle and my efi system. I am fitting a 
>piggy back, interceptor style computer to my standard efi computer. 
>In doing this I also wished to do away with the flap style air flow 
>meter on the intake. I was replacing this with a 3 bar Denso map 
>sensor. I have discovered this is not a simple as first thought.
>
>The engine is a Mazda B6T 1.6l turbo with what I believe is a 
>Nippondenso efi system. The flap style afm fitted to the vehicle is 
>fairly standard and looks similar to those fitted to the Rx7 and 
>miata.
>I have a link at the bottom to a schematic of what I think the 
>inside of the flow meter is. Well its what I have measured in 
>resistances and from research on the net thats what I think is 
>inside. So how I think it works is, there is a 12v supply coming in. 
>A 100ohm resistor feeds a pin called Vc which is the 9v reference 
>signal to the ECU. Pin Vs is the flow signal. This is a varying 
>signal from 2 - 9v where 2v is no flow and 9v is 100% duty. the ecu 
>takes a ratio of Vs/Vc to select injector duty. Makes more sense 
>when you look at the picture.
>
>I measured the internal resistances across the pins and replaced 
>them with standard resistors in the configuration shown in the 
>picture. I used this in conjunction with the map sensor. The map 
>sensor gives a 0-5v signal 0v at absolute zero and 5v at 3bar (ppl 
>on here should know this anyhow). I then used the piggy back to trim 
>this to the normal 2v-9v etc.
>
>Oh I also fitted 550cc injectors instead of the 340cc units.
>
>With the map sensor configuration the ECU does not produce any error 
>codes. So I think as far as my knowledge goes that the ECU is 
>accepting my modification. However the vehicle runs rich, very rich. 
>With the piggy back computer I can force the cursor to stay on one 
>map point. I can fully vary the output voltage (going into the ECU) 
>from 0.6v through to 9v and the vehicle remains very rich at all 
>voltages. It will not lean out. The plugs are wet and lots of soot. 
>We have hooked up an oscilliscope to No. 1 injector and the duty 
>cycle is changing. You can tell by the sound of the car but whatever 
>it is doing it just doesn't seem right. It hardly varies duty. Some 
>other points
>
>* some time ago I intercepted the Vs signal with the piggy back, 
>with stock afm and injectors and successfully altered injector duty
>
>* I did not fully check out the original afms workings before I 
>butchered it for parts. Hence I can not go back to it to see it the 
>car will alter duty properly with the afm (ie. my dodgey map mods 
>are the cause)
>
>* my in depth knowledge of factory ECUs is fairly limited
>
>What I would like to know if anyone can is;
>
>* can you help at all?
>
>* can you enlighten me a little more on exactly how the flap afm 
>worked and if my circuit should also work?
>
>* Anyone know how the stock ECU would alter duty? my guess was a 
>look up table in which the ratio of Vs to the reference voltage Vc 
>is looked up and an injector duty is selected. If this is the case 
>then shouldn't my circuit work?
>
>any help at all is appreciated as I am at the point of giving up.
>
>thank you
>
><http://i7.photobucket.com/albums/y253/Jimbobtx3/ECU%20Code/bff1ae98.jpg>http://i7.photobucket.com/albums/y253/Jimbobtx3/ECU%20Code/bff1ae98.jpg
>
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>
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-- 
Marc Reviel

PowerLogix
http://www.powerlogix.com
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