On Sat, Oct 17, 2009 at 8:52 PM, David Beierl <dbeierl@attglobal.net> wrote:
> 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.

......

And of course in terms of this thread, since I was quoting from memory
(inferred by "similar to"), my quotes could confuse matters too. The
phrases used in the A3 Bentley are likely different than what I
quoted.

> 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.

Right. Makes sense. My knowledge of transistors is sketchy, but I do
recall that they are semi conductors. Upon Scotts direction, I figured
there was something like a diode in the path I was trying to check. As
you say the ECU uses transistors to switch loads. IIRC, they only
allow power to flow one direction.

Anyhow....

From an engine swap POV all this helps to make things clearer. As a
newb dealing with the Motronic wiring harness, finding all the grounds
and routing them properly was a bit of a mystery. With list help, I
sorted that out. And now I have a better understanding as to how an
ECU provides a path to the chassis.

Thanks guys!

Neil.

-- 
Neil Nicholson '81 VanaJetta 2.0 "Jaco"

http://tubaneil.googlepages.com/

http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines