From: Sky2high@aol.com
Message-ID: <17b.415539c.29a9652b@aol.com>
Date: Sat, 23 Feb 2002 16:35:39 EST
Subject: Re: VM Fuel probes
To: lancair.list@olsusa.com
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Reply-To: lancair.list@olsusa.com

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Adam,

Indeed, the Probes are a resistance/capacitance circuit that yields a 
frequency that varies with the fuel changing the capacitance.  When the probe 
is hooked to the computer the red is +5v, the black is ground and the white 
is the square wave frequency output back to the computer.  When you claibrate 
your tanks, you will supply frequencies to VM and they will program the ROM 
that converts frequencies to gallons. 

How do I know this?  VM insists that the probe must be at least 14 inches 
long and they are right.  I installed my LNC2 header tank probe vertically 
from the sump to the top - a distance of about 8 or 9 inches.  When I 
calibrated the tank, the frequencies increased until the fuel level was 
reduced to about 2 gallons and then the frequencies reversed themselves -- 
actually, they got so high that the computer could no longer recognize them.  
Using Radio Shack CMOS components (they run on 5 volts), I utilized a schmidt 
trigger to clean up the square wave, and another component to build a 
frequency divider.  Wired as a bulge in the cable that connects the probe to 
the computer, the new frequencies behaved quite nicely and the fuel level in 
my header is accurately reported from 0 to 9 gallons.

I hope this helps your understanding of the probe circuit. (And, I hope I 
remembered the 1996 wiring correctly).

Scott Krueger
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