[Diy_efi] air flow meters

[ScottyGrover at aol.com]

In a message dated 1/6/2007 4:54:13 P.M. Pacific Standard Time,  
dan at w3eax.umd.edu writes:

Hi Guys  (and/or Gals),

This isn't quite DIY_EFI but I'm sure someone here must  know the answer to 
my question.  I have an 85 MR2 (which I'm sure  will eventually get some 
DIY EFI) but for now it has Nippondenso injection  which is a licensed copy 
of Bosch L-Jetronic featuring the standard  vane/volumetric airflow meter.

Besides the fuel pump control and the  air intake temp thermistor, there 
are four pins: Vb, Vc, Vs, and E2  (ground).  The Toyota manual has a 
schematic that shows something  along the lines of:

Vb -\/\/\/- Vc -\/\/\/-Vs-/\/\/\/- E2 (where Vs is  a wiper of a pot)

Toyota claims the resistance between Vs and E2 will  "fluctuate" between 
20 and 3000 ohms as the flap is openned.   Bentley's BMW service manual 
claimed the resistance should increase  "steadily without any flat spots as 
the sensor flap is moved to the full  open position."  Bosch's "Automotive 
Electric/Electronic Systems"  claims that "The angular position of the 
sensor flap is transformed by a  potentiometer into a voltage."

However...  Toyota also claims the  resistance between Vc and E2 should be 
100-300 ohms, and the resistance  between Vb and E2 should be 200-400 
ohms.  My resistance is 180 and  280 ohms respectively, I don't see how 
this jives with the 20-3000 ohms  claimed earlier.  But it gets weirder.

I tried pushing the flap in  and measuring the resistance between Vs and E2 
and it seems to start at  about 120 ohms and then increases, but at about 
3-400 ohms it starts  dropping again, then increasing, then dropping, etc. 
The most I've ever  seen is about 900 ohms and often its only 600 ohms.

I opened the meter  up, and the inside looked clean and in excellent 
condition.  The  potentiometer was a dark material laminated onto a PCB 
which appeared to  be "toothed."  It was almost as if there were a bunch of  
potentiometers in parallel with the wiper moving from one to the next.  
Its hard to describe.

Can anyone explain whats going on with this  and what it SHOULD be doing? 
Is it right, is it wrong and fixable or do I  need a different AFM?

dan



Dan, You've got it exactly right, the voltage vs. CFM is a sawtooth; if you  
can find a repair manual for the Toyota EFI--in your public library maybe--you 
 can find a one-line diagram of the thing; oddly, if you supply 12VDC and 
ground  to the thing and use a ruler or some other graduated device to push back 
the  flap, the sawtooth goes away and you get something like a logarithmic 
voltage  curve rather like a RC voltage curve--which you may be able to 
compensate for.  I'm using this type of measuring device in my own DIY-EFI system; but 
I'm kinda  old-fashioned--no computer chip, I'm doing it with integrated 
circuits--and it  works on the breadboard but uses about 25 or 30 IC's to get the 
job done; and  doesn't use any ROM of RAM, just counter circuits to transform 
digital words to  elapsed-time to feed the injectors. Believe me, if I knew how 
to translate my  circuit diagram into a microcontroller program, I would; 
would use fewer  IC's.
 
Scotty