[Diy_efi] Sending unit confusion

[Toyota Supra]

hi all :)

in my never ending quest to conquer this circuit, i have encountered 
something very strange. when i removed the fuel level sending unit to try 
and test my circuit, i found that i am not able to simulate its electrical 
function.
the sending unit has 2 wires, a yellow and a brown w/ black tracer. when the 
sending unit is not plugged into the body harness, the yellow wire has 
+12vDC (or whatever battery voltage may be) on the body harness side of the 
connector, and the brown w/ black tracer has approximately 15k of resistance 
to ground, or 35mV of positive voltage.

when the connectors are plugged in, the yellow wire will have anywhere from 
0 to 3.5vDC, dependent on the position of the "float". the variable resistor 
will have resistance ranging from 0k to 55k.

now, my problem is, i can't simulate the variable voltage outside of the 
vehicle, to test my circuit. if i put 12vDC into the yellow wire, the brown 
w/ black tracer will also have 12vDC, so what could that wire possibly be 
doing, or connected to, that would make the circuit and unit work properly?

i have enclosed some scans of what the factory manual diagrams it out to be, 
and some pictures of the sending unit, with descriptions

here is a description of the sending unit, and pics of its resistor
http://68.7.7.84:8080/schematics/sendingunit1.jpg

http://68.7.7.84:8080/schematics/sendingunit3.jpg


here is the oem wiring diagram
http://68.7.7.84:8080/schematics/oem-schematic1.jpg

here is the way they describe the sending unit in the manual
http://68.7.7.84:8080/schematics/sendingunitdescription1.jpg

and here is the latest evolution of the overall circuit

http://68.7.7.84:8080/schematics/schematic7.jpg


once i can simulate the sensor working, i can make sure the schematic works, 
and implement it. thanks to all that have helped me through this!

-j





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