Volvo/Saab Bosch MAF swap for VAF

bjanesi at juno.com bjanesi at juno.com
Sat Sep 11 01:05:38 GMT 1999


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)

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