[Diy_efi] Sending unit confusion + schematic suggestions

Fri Feb 21 07:44:32 GMT 2003

Definitely NOT JZA80.  I have that manual and the car, the tank he =
describes
is not JZA80 either.  My fuel tank is plastic with a metal undershield, =
it
would be impossible to split it into two halves; and the fuel sender is =
a
very simple circuit.  12v rail for all gauges, fuel level gauge and fuel
level warning light connected to the rail, variable resistor and level
"switch" in the tank, with a common ground for the meter and the other =
side
of the variable resistor.  Dead simple circuit, all passive devices.

Lance

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] =
On
Behalf Of Mos
Sent: Thursday, February 20, 2003 3:41 PM
To: List for general do-it-yourself EFI talk
Subject: Re: [Diy_efi] Sending unit confusion + schematic suggestions

J,

Guessing by the design of the OEM EWD you have a JZA80? (I just went =
through
all my MA70 notes to find them irrelevant ;p).

The problem here is that you don't really know where the wires connect, =
as
the EWD doesn't give you enough information. I'd speculate FE means Fuel
Earth, but that's not consistent with the Br-B (assuming that's Br-W on =
EWD)
having 15k to ground.

Chances are there's a resistor going to 12V on the FR pin, hence when =
the
SUB fuel sender is not hooked up you get 12V on the Y wire (no current =
draw
means no voltage drop across the fuel sender and possible resistor).

You could work out what the resistances are likely to be (if you can't
measure them - you can also pull apart the BODY ECU and trace the pcb) =
from
what you know, but in any case you don't really need to to simulate the
0-3.5V range. I'd do as someone else suggested and just use a pot (10k =
or
55k if you want, doesn't really matter) with each end connected to =
ground
and 8V, and then use the tap for your testing voltage; you'll get a 0-8V
range, but for testing that's fine.

The other issue you might run into is that your ciruit may draw some
non-negligible current from the fuel sender resistor ladder and cause =
your
fuel gauge to show an incorrect amount. The data sheets for the LM339 =
shows
less than 1uA so it wont affect it, but something to remember if it =
cause
weird problems.

Some notes on your schematic[7] (finally posting some to you after =
having
composed a few mails :)).

Typical NPN transistors are used in open collector applications pulling =
to
ground. So the typical way of hooking it up is as follows. +ve of the =
relay
goes to 8 or 12V, negative goes to the collector, emmitter goes to =
ground,
and the base goes to a +ve siwtch-on signal through a resistor. A diode
accross the relay is needed, Anode (arrow part) to negative, Cathode =
(bar
part) to positive.

In other words you should interchange the relay and transistor. The =
reason
for this is that you need to reliably develop over 0.65V across the BE
(Base-Emmitter) junction. If you hook up E to ground, and pin2 is at 8V =
you
will always have over 0.65V at B. The current going into B will be
(8-0.65)/1k=3D7.35mA which will turn it on without doubt.

If you hook it up the way you have, E will sit somewhere nearer 8V due =
to
the resistance of the relay and the transistor may not even turn on.

Another poster mentioned the LM339 having only open collector outputs,
meaning that it can only reliably pull something to ground - it wont =
pull
pin2 to 8V - you will need a "pull-up resistor" to pull pin2 to 8V. A 5k
pull up at pin 2 should turn on a transistor and is within the 339 =
limits
(but a better way is to use a PNP transistor and reverse the logic).

For PNP, E to 8V, C to relay +ve, relay -ve to ground, diode across =
relay
(cathose to +ve, anode to -ve), 10k pullup between 8V and B (to make =
sure it
turns off) and 1K from B to pin2 should be fine. In this way, when the =
-ve
339 i/p is over the +ver 339 i/p, the o/p will be low turning on the
transistor, and the relay.

In Schematic 5 the reference 3.2V (#2) is blocked by C3, so no DC =
component
will reach pin 4 (fixed now but..) Dave's suggestion was to hook it up =
in
parallel with the IC, ie positive of cap goes to pin4, neg goes to =
ground
(to help with possible noise problems).

Note: this has not been tested, just theoretical rambling.

Mos.

--=20
84 AE86, 90 ST185GrpA, 91 MX83Gr, Sydney, Oz.

On Thu, 20 Feb 2003, Toyota Supra wrote:

> hi all :)
>
> in my never ending quest to conquer this circuit, i have encountered=20
> something very strange. when i removed the fuel level sending unit to=20
> try and test my circuit, i found that i am not able to simulate its=20
> electrical function. the sending unit has 2 wires, a yellow and a=20
> brown w/ black tracer. when the sending unit is not plugged into the=20
> body harness, the yellow wire has
> +12vDC (or whatever battery voltage may be) on the body harness side=20
> +of the
> connector, and the brown w/ black tracer has approximately 15k of=20
> 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=20
> variable resistor will have resistance ranging from 0k to 55k.
>
> now, my problem is, i can't simulate the variable voltage outside of=20
> the vehicle, to test my circuit. if i put 12vDC into the yellow wire,=20
> the brown w/ black tracer will also have 12vDC, so what could that=20
> wire possibly be doing, or connected to, that would make the circuit=20
> and unit work properly?
>
> i have enclosed some scans of what the factory manual diagrams it out=20
> to be, and some pictures of the sending unit, with descriptions
>
> here is a description of the sending unit, and pics of its resistor=20
> http://68.7.7.84:8080/schematics/sendingunit1.jpg
>
> http://68.7.7.84:8080/schematics/sendingunit3.jpg
>
>
> here is the oem wiring diagram=20
> http://68.7.7.84:8080/schematics/oem-schematic1.jpg
>
> here is the way they describe the sending unit in the manual=20
> 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=20
> works, and implement it. thanks to all that have helped me through=20
> this!

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