Volvo/Saab Bosch MAF swap for VAF

[Alex C. Peper]

I made some controllers for Bosch Hot Wire and Bosch Vane with PIC 73
Used built in 8 bit converter and 10 bit PWM output direct 
connection on hot wire.( used voltage divider and opamp on 10 V.
vane meter.)  Was used as O2 sensor adaptive feed back control,
with 12 point internal correction table.  Also had prototype to load
EEPROM values and rpm input. 
You could look into adjustable gain opamps and DAC's
Alex

> Some of you may recall that I've been investigating the possibility of
> swapping a Bosch MAF from a Saab or Volvo (they call it a AMM), for the
> stock  Bosch VAF meter in my Merkur XR4Ti (Ford Sierra in the rest of the
> world).  Here's what I've discovered about the two MAF on the bench:
> 
> '87 Volvo 740 2.3 Turbo MAF  Bosch p/n 0 280 212 007  (black plastic
> case)
> 
> Pin #1  Ground (MAF signal - local)
> Pin #2  Ground for filament burn-off and CO idle adjustment (at PCM)
> Pin #3  MAF output (~1 - 5v)
> Pin #4  Filament burn-off (4v for 1 second after shut-down, controlled by
> PCM)
> Pin #5  12v Power input
> Pin #6  CO Idle adjustment  
> 
> '87 Saab 900 2.0 N/A  MAF  Bosch p/n 0 280 212 005 (aluminum case)
> 
> Pin #1  Filament burn-off (4v fro 1 second after shut-down, controlled by
> PCM)
> Pin #2  12v Power input
> Pin #3  Ground for filament burn-off and CO idle adjustment (at PCM)
> Pin #4  Ground (MAF signal - local)
> Pin #5  MAF output (~1 - 5 v)
> Pin #6  CO idle adjustment
> 
> For bench testing purposes, I was only using three circuits:  The MAF
> output & ground and the 12v input (conveniently provided by a fresh
> battery).  Here's what I discovered:
> 	  	Powered, still air		Powered, maximum human breadth
> Volvo MAF	 	   1.32 volts			2.70 volts	
> Saab MAF		   1.55 volts			3.95 volts
> 
> Not surprisingly, the two MAFs are calibrated differently, with the N/A
> Saab MAF likely to max out (5v) with much less air flow than the Volvo
> turbo MAF.
> 
> Now, back to my replacement objective...  The stock Merkur VAF has a base
> starting output (powered but no air flow) of .5v, which is much less than
> either of these devices (relatively speaking).  So I'm thinking about
> sticking an in-line  POT on the output line to provide an adjustable
> base-line voltage and match it to the .5v that the PCM is looking for
> from the stock VAF.  I realize matching the base-line voltage does not
> guarantee a match across the full range, but it's a starting point.  What
> I'd really like to do is find a way to use the built-in CO idle
> adjustment screw, since it will otherwise go un-used since this function
> is not supported by the Ford PCM.   
> 
> If it turns out that adjusting the MAF output in a monolithic fashion can
> not closely reproduce the original VAF output, I'll have to look at a
> "black box" solution from vendors such as Split Second or Pro-M which
> provide the ability to have 2 or 3 adjustments to the MAF depending upon
> the flow.  I'm not smart enough to figure that one out myself.
> 
> Any comments, corrections, open discusions, etc are welcome.
> 
> Brad Anesi
> (bjanesi at juno.com)