[V8] O2 ground

[Buchholz, Steven]

... I'm sure that we've discussed this on this list before ... let me
see if I can put something together based on what has been discussed
earlier and post it up ... might as well put it in the KB as well ... 

It turns out that even though the engine and body of the car are
connected by a piece of wire, for some purposes they are *not* the same
point.  It turns out that every piece of wire is a resistor ... hell,
even the engine block and car body have a finite resistance.  Ever had a
problem that created alternator buzzing in a car's stereo speakers?  In
many cases the reason behind the problem is one of improper grounding.  

... gone are the days of the "older VW" ... or even my old '78 Fox GTI
... with spark controlled by points in the distributor and fuel
controlled by a carburetor or an analog computer for a FI system.
Engines today are controlled with computers that base their decision on
what is going on inside the engine from low level signals from various
sensors.  It is very easy to create several tens of millivolts of
voltage drop between the engine and the body just due to the current
flowing between the alternator and the battery ... or when all the
headlights are on.  This very small voltage difference can change the
value the ECU is reading if there is a reference (read: ground)
difference between the sensor and the ECU.  This is why the vehicle
manufacturers deliberately design the electrical systems so that the
ground for sensitive signals are separate from that for the more mundane
circuits.  The various grounds must be connected together, but that
interconnection point is carefully chosen to minimize the chances that
they will impact the sensitive signals.  

Where you choose to ground the OXS can *and does* matter.  That is why I
made the statement earlier ... if you can't ground the ground wire from
a 4-wire OXS to the engine block it would be far better to stick with a
3-wire sensor and live with any issues with ground integrity from the
exhaust crossover to the engine block.  

Steve B
San Jose, CA (USA)
Systems Engineer with an EE background who has spent a lot of time
dealing with issues caused by poor grounding techniques ... 
> 
> So, here's the question. Why not ground to the body? Are the body and
engine
> not the same in reference to ground. All power sources lead to ground
> eventually. We ran into this quite often on aircraft components. The
only
> difference is when you are running a signal of a higher frequency.
Then, you
> usually use a resistor or capacitor to keep the signal from affecting
the
> power source (usually of a lower frequency). When you have a high
frequency
> riding over a lower frequency, it can cause surges that are heard
through
> audio components.
> 
> However, none of this is the case here. There is no resistor or cap
between
> the body and engine. So, if you have a good connection between the
body and
> engine, you can use either. If you don't, or suspect the connection or
> cabling is bad, replace it. If not, you are asking for trouble. Just
ask
> anyone with and older VW.