[V8] O2 sensor

Jeremy Ward jward at mti-interactive.com
Fri Oct 1 11:19:13 EDT 2004

Oh sure, now you tell me Steve!  ;-)  I did this 2 weekends ago and had
good intentions of running my extra ground into the cabin, and grounding
it to the ECU.  After hrs of trying to fish it through several
firewalls, I gave up and grounded it to the chassis.  Next time I am in
there, I will re-ground it to the spot engine that you suggest.  BTW,
how good is the ground on an aluminum block engine?  Isn't aluminum
oxide a pretty good insulator?

Someone once mentioned that they didn't want to use a non-Audi part here
just in case the future owner (FO?) couldn't figure out what was done...
I realized when I pulled the Audi part off the car, that when you
purchase it from the stealership, that it comes with the 2 plug
connectors (one for the signal, and one with both heater wires) and a
long length of harness.  If someone pulled off my Ford part (by removing
the 2 factory Audi connectors) they would only have a mysterious ground
wire to remove.  This would be true weather or not you bought the
matching Ford connector and spliced it in between the Audi connectors
and the O2 sensor...

As I side note, I went to the U-Pull-It a weekend prior to doing the O2
sensor, and the newest vehicle they had there was late 80s.  Most cars
were missing motors, trannies, and exhaust systems.  I hadn't bought the
Ford O2 yet, so I didn't know exactly what I was looking for; not being
able to find a '93 pickup didn't help things.  I snipped a wire out of a
harness, and took it up to pay.  When he saw that I was going to write a
check, he said I could have it for free, so it only cost me the $1
admission. :-)  Turns out to be the wrong plug, oh well.

Here are a few pics I took to help anyone thinking about tackling this
job: side by side comparison of 4
wire (left) and 3 wire (right) with the dust cover removed snipped the harness off of the
old Audi sensor, and spliced on the new Ford sensor note Bosch part number, short
Ford harness (white plug) on the right push button tester I made (with
an LED in the switch) for blinking out the codes. :-)

-----Original Message-----
From: v8-bounces at audifans.com [mailto:v8-bounces at audifans.com] On Behalf
Of Buchholz, Steven
Sent: Thursday, September 30, 2004 6:30 PM
To: v8 at audifans.com
Subject: RE: [V8] O2 sensor

... IMO you've got to be a bit more careful than this with grounds ... 

The ground wire provides the reference for the OXS signal ... you have
to be careful not to bias the signal because it will cause the ECU to
get an improper evaluation of the state of the car.  There are actually
two ground points on the back of the engine block below the left
distributor.  I heartily recommend that if you are doing the 4-wire
conversion that you connect as close as possible to one of these to
ensure the signal the ECU signal sees is the one the OXS is generating.

To be a bit more detailed about my concerns with the attachment to the
body of the car ... there can easily be a voltage drop between the
engine block and the frame of the car in the hundreds of millivolts.
Furthermore this voltage drop will vary depending upon what loads are
being placed on the electrical system ... coolant fan, head lights ...
even a discharged battery ... and if you attach the ground of the OXS to
the body of the car you will automatically apply this voltage offset to
the signals the ECU is reading for the OXS input.  In addition to this
offset, the long ground path can allow higher frequency noise to get
coupled into the OXS signal.  You may end up with a car that runs well
in the day time but misbehaves at night.  Believe me, ground induced
issues can be a PITA to isolate and resolve ... 

Steve B
San Jose, CA (USA)
> The gray wire is the extra, it needs to go to the other 
> ground. I ran mine 
> to the ground for the aux fan on the drivers shock tower. If you are 
> planning on keeping this car, I'd also recommend getting the 
> plug from a 
> Ford. Any of the 4.6 cars use four of them, so it shouldn't 
> be hard to find 
> one. The colors aren't the same on the plug, but just line them up by 
> position, and connect like wires. The two whites are the 
> same, so they can 
> go to either corresponding white. same for the black wire.
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