Replacing L-Jetronic flap-type AFM with silicon pressure sensor

[Todd Knighton]

Fredrik Jeppsson wrote:
> 
> Hi,
> I need some advice. I am thinking of replacing the airflow meter of my
> Range Rover with a pressure sensor monitoring the manifold pressure
> ("vacuum"). I cannot make up my mind wheter to use an absolute or
> differential sensor. Can anyone advice me ?
> 
> My idea is to hook it up in parallell with the existing AFM and
> manipulate gain and offset of the pressure voltage until it matches the
> output of the AFM. Then I can substitute the AFM signal with the
> pressure signal (if I can get them to match).

	Well Fred, it's not quite that easy.  The thing reads backwards of the
AFM sometimes, and forwards others.  You'd have to rig up something that
took into account, not only the pressure, but rpm's as well.  Put the
whole mess together and you could come up with a good air flow meter
substitute.  there's a few of these floating around, HKS calls it their
vein pressure converter, and I've seen a few more, but they're not
cheap.
	Think about it for a second, if you sit in the parking lot at idle, you
might be able to get the thing to idle at the same voltage with some
sort of compensation, but now the thing starts to surge a little.  At
the lower rpm spot of the surge, the pressure will be the highest, caus
the engines trying to pick it back up, but the air flow would be the
lowest, and vice versa.
	Also, if you just rev it up with no load, at 1000 rpm's it might be ok,
but by 4-5000 rpm's or so the pressure sensor voltage has gone way down,
to follow the pressure in the intake, but the air flow has gone up, so
the voltage is expecting to go up as well.
	The time it does work is increasing the load at a particular rpm, the
pressure will increase and so may the air flow.
	Another easy example, is at WOT (wide open throttle) on a normally
aspirated vehicle, the pressure sensor will read approximately BP
(barometric pressure) less pressure losses through the intake, well
almost a constant across the rpm range, where the air flow meter will
read very low voltages, relating to airflow at low rpm, all the way to
max voltage somewhere around red line at higher rpm's.

	Good Luck!
	
-- 
Todd Knighton
Protomotive Engineering
knighton at net-quest.com