Date: Sat, 17 Oct 2009 23:52:50 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Jetta swap: Check Engine Light Install Electrical Questions
In-Reply-To: <c4e7c5f90910171718i36b64fa1w5e8f2ac8dedc8f37@mail.gmail.co m>
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At 08:18 PM 10/17/2009, neil n wrote:
>Though not directly related, the Jetta diagrams do specify phrases
>similar to "Motronic Harness Ground" and "engine block ground". But in
>my mind, it seemed that they all ended up connecting to the frame.
A lot of this confusion is because of sloppy terminology by people
who mostly understand what's going on pretty well, so they use
shorthand. Strictly speaking nothing at all on a Vanagon is
grounded, because strictly speaking "ground" means literally an
electrical connection to the earth. In British English it's called
an earth, and grounding is called earthing. Because of the sloppy
usage problem we in the US often refer to it as an "earth
ground." Its symbol on a schematic is a horizontal line with three
short slightly slanted lines depending from it, to suggest the
connections driven into the earth.
The other kind, such as automobiles have, is better referred to as a
"common return" or in tech-speak a chassis ground or frame
ground. Its symbol is a set of horizontal lines that suggest the
shape of an equilateral triangle, point down. An open triangle is
quicker to draw neatly and is often used, and (at least in Germany)
sometimes just a short horizontal line.
In all cases this refers to the completion of a circuit (sine qua non
for anything electrical to work) by connection to a structure of some
sort (electrical chassis, "ground plane" in or on a circuit board,
automobile chassis etc., where one side of the power supply is also
connected to the structure. Sometimes this is done for shielding
purposes, but in vehicles it's usually because it's much cheaper to
use the large steel frame instead of running copper returns back to
the battery. Steel doesn't conduct very well compared to copper, but
there's a *lot* more of it so it evens out.
This all leads us to say ground when we mean common return, and
ground wire when we mean a wire leading to the common return, and it
muddies our thinking.
As Scott said, you can switch a circuit at any point in the circuit,
whether on the positive or negative side of the load. It's a matter
of convenience. In the case of the ECU it happens to be electrically
very convenient to switch loads with a transistor, and convenient
more from a manufacturing standpoint to wire the transistor so that
when switched on it provides a low-resistance path to power-supply
common, just as they did with the CEL, and just as the WBX (and
likely the Subaru) ECU does with the fuel injectors. Another common
example is our door-light switches -- it's very convenient to let the
button switch on the door simply provide a path to chassis ground,
and leave the positive side of the light circuit connected all the time.
Hope that helps some.
Yrs,
David
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