[Diy_efi] Fuel circuit

[Bill Washington]

J, Matt,
    See my comments inserted below

>------------------------------
>
>Date: Wed, 19 Feb 2003 19:37:12 EST
From: fuelogic at ihug.com.au
>To: diy_efi at diy-efi.org
>Subject: [Diy_efi] re: circuit
>Message-ID: <E18lei8-0001fi-00 at grunt23.ihug.com.au>
>Precedence: list
>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>Message: 3
>
>Hey Circuit guy,
>
>Just to add my two cents worth...u are using the wrong symbol for a pnp 
>transistor in circuit 5...and the wrong pin configuration. I should be emitter 
>to 12V, base to R4 and collector to the relay coil positive. Otherwise I like 
>the circuit.
>
>Matt
>  
>
No! It should be an NPN as drawn!

>------------------------------
>
>Date: Thu, 20 Feb 2003 02:01:12 -0500
From: "Toyota Supra" <turbosupramk3 at hotmail.com>
>To: diy_efi at diy-efi.org
>Subject: [Diy_efi] Sending unit confusion
>Message-ID: <BAY2-F1482mFqF7PsgS00036b1e at hotmail.com>
>Content-Type: text/plain; format=flowed
>MIME-Version: 1.0
>Precedence: list
>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>Message: 5
>
>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
>
>  
>
My best guess is that the Yellow wire may be fed from a current source 
and the voltage sensed to indicate the fuel level.
The schematic by itself, is, in this case not very helpful in 
understanding how it is intended to function.

Now I assume that the tank is plastic, and that the top of the fuel 
sender unit is plastic(it appears so in the photo), is that correct?
If so there needs to be an earth connection, probably the other wire, 
unless there is a third connection which has not previously been mentioned.
Even if they are metal there should be an earth connection, because 
there will be a gasket between the sender and the tank which will 
insulate the sender from vehicle earth(chassis).
If there is a separate earth connection, the other wire may still be a 
return/reference wire for the sender because  it may be designed to 
operate isolated from earth.

To test:
1. connect up in vehicle (out of tank, sitting on rubber mat or 
cardboard to isolate from vehicle) and confirm normal operation, ie fuel 
guage goes up and down with float position.
2. If there is a third (earth) connection disconnect it only. does the 
guage still operate normally?
3. disconnect only the second wire (black?). The gauge should indicate 
empty (possibly full, but failsafe operation requires empty indication) 
regardless of float position, reconnect.
4. disconnect only the yellow wire. The gauge should indicate empty 
(possibly full, but failsafe operation requires empty indication) 
regardless of float position.

If behaviour follows this pattern, that combined with your previous 
evidence of full voltage on the mating spade to the yellow when 
disconnected suggests that my guess above may be correct.


Regards
Bill


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