[V8] O2 sensor

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Mon Oct 4 20:35:44 EDT 2004

... that is correct.  When you have a chance some time take an ohmmeter
and measure the resistance from the case of the OXS to the engine block
... you will most likely find that the resistance is very low.  That
said ... the fact that the 3-wire OXS depends on the electrical
characteristics of the exhaust system is why manufacturers went to the
4-wire setup ... 

... I'm sure that there are those out there thinking that a good solid
ground to a strut tower has to be better than depending on the rusty,
crusty exhaust system ... the thing is that the statement is not
necessarily true.  Consider Ohm's law ... voltage drop *is* dependent
upon resistance ... in conjunction with the current drawn.  An OXS
produces very little current, so the voltage drop caused by the "high"
resistance of the ground connection is small.  As I noted in an earlier
post, there can be a considerable amount of ground current flowing
between the engine block and the body of the car.  This current creates
a voltage drop between the two points that affects every signal using
this path as a return.  The auto manufacturers design the vehicle
electrical system so that only systems which are not as sensitive to
this very small voltage difference make use of it.  You can see the
level of detail by checking out the list of grounds on page 7 of the V8
wiring diagrams ... there are 3 grounding points called out, each one
detailing the use:

	137 - welded, in Motronic harness (final stage) - for 
	  outputs like ignition coils, etc. (referenced to 
	  engine block) OXS heater ground connects here.
	138 - welded, in Motronic harness (electronic) - shields 
	  for flywheel sensors, IAT & coolant temp senders 
	  (referenced to engine block)
	139 - welded, in Motronic harness (sender ground) - ground 
	  returns for coolant temp, IAT and knock sensors 
	  (referenced to pin 30 on ECU)

Steve B
San Jose, CA (USA)
> The rusty exhaust system, if my understanding is correct.
> -----Original Message-----
> So the V8 OEM sensor is a three-wire, isn't it?  Where does 
> the original
> wiring ground the O2 sensor?

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