Re: Bosch O2 sensor

[David Masters <david@xxxxxxxxxxxxxxxx>]

[]  John asks ...

I've read in recent digest about verifying if the O2 sensor is
working properly using a DVM to check for oscillating voltage...

Does this apply to a Bosch 3-wire O2 sensor as well? Can
someone please give details on how this is done and what
to look for.
[]
It applies to any O2 sensor that produces a voltage.  On the twin spark it 
was easiest to connect to the sensor in the engine bay where there is a 
connector on the firewall.  The shielded wire is the + and you grab an 
earth from a nearby screw.  I used speaker cable and ran it over the bottom 
of the A pillar and into the passenger side of the car, where I connect up 
the DVM.  Any DVM will do, they all have huge impedance which makes them 
suitable for any automotive work.  (ie, it doesn't have to be a Fluke!) 
 Set the DC voltage to something like one or 4 volts DC.  Do not use the 
resistance setting, I'm told it will ruin the O2 sensor.

What you will see is an oscillation of the voltage from .2 to .9 V.  If 
everything is right, you will get this oscillation at idle, and at steady 
cruise as the computer continuously varies the amount of fuel.  What it 
does also allow is the tuning of the mixture using the little screw on the 
AFM.  If you can get some data on what is a good setting for your car, say 
following a professional tune that leaves it running sweetly and 
oscillating nicely at idle, you can correct this base setting yourself as 
things wear.  The problem with this is that some cars/models just don't 
always do what you expect, and setting up a good mixture at idle can leave 
you rich or lean at cruise.  If you make an adjustment, ensure that you 
still get the same characteristic voltage output at cruise.  Prolonged 
periods or lean running will cause overheating at best, or engine failure 
if you don't notice it.

Simple tests like watching the colour of your exhaust pipe are always 
useful.  It should never go white, or get too many white flecks.

YMMV.
Cheers,
david masters.
Newcastle AUSTRALIA.
1989 ALFA75 TS.

Thanks in advance,

John Davis
Satellite Beach, Florida

89 Spider Veloce

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