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

Fri Jul 22 00:29:07 UTC 2005

Keep in mind that the flapper style VAF on a 1.6 miata runs 5-0V (5V@ idle 
and 0 Volts @ WOT)
where most  map sensors runs the inverse (OV @ idle & 5V @ WOT)
You really have 2 issues to fix on the 1.6 ecu, invert the voltage from map 
to VAF, and corelating  a  voltage w/ a map sensor that will work w/ your 
ecu.  possible if you could invert the signal from the map somehow and used 
a mpx5100ap sensor, then you might get some where to start...

Best regards,
JEFF

1996 Miata --T25-
2003 P5 -  T3

From: Damian Long <bigcow_610 at yahoo.com.au> Reply-To: 
diy_efi at diy-efi.org To: diy_efi at diy-efi.org Subject: [Diy_efi] trying 
to replace standard flap style afm with MAP sensor Date: Wed, 20 Jul 2005 
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diy_efi-bounces at diy-efi.org X-OriginalArrivalTime: 20 Jul 2005 
06:41:33.0915 (UTC) FILETIME=[11D82AB0:01C58CF6] 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 Send 
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