At 02:45 AM 1/22/2002, Laurence Smith wrote:
>I am looking for advice on how to test the green coax wire coming from
>the O2 sensor.  In the archives there is discussion about this wire
>and the possibility of an internal short that causes the ECU to get
>upset, run the engine rich and cause poor idling.  However, the
>archives were not very clear on a test procedure for this shorting
>situation.

The coax braid is grounded -- if it shorts to the internal wire you'll see
a constant low voltage on the sensor and the van will run *very* rich.  If
you see this condition, unhook the sensor from the green wire and measure
the voltage at the green wire with engine running -- it should go to about
a half volt and stay there.  If it does, prolly bad sensor.  If it doesn't
prolly bad wire.

>Another question...the O2 sensor voltage is definitely produced by the
>sensor - correct?  Not the ECU.  The ECU picks up this voltage and
>looks for a nice consistent flip-flop between .45 and .55 volts -
>correct?

The ECU supplies a very high-impedance (easily overpowered) bias voltage to
hold the system at about a half-volt until the sensor comes on line.  The
sensor is actually an odd sort of battery that supplies close to a volt
(still at high impedance) when its temp is above about 300C *and* there is
oxygen on one side of it and not the other, which is the case when a rich
mixture burns up all the oxygen in the charge.  The ECU notices that the
voltage has moved away from the initial half-volt, and starts paying
attention, i.e. goes into closed-loop mode where it adjusts mixture based
on the sensor reading.

If the sensor indicates rich (> half volt) the ECU leans things out until
the sensor indicates lean (< half volt).  The ECU then richens until the
sensor flops over to rich again, repeat forever.  If you look at the sensor
signal with a scope you can see that it makes a sloppy square wave between
about .2 and .7 volts in response to the ECU's fiddling -- however with an
ordinary digital meter all you will see is that the reading jumps around
above and below a half volt with no visible pattern.

You must use a high-impedance meter to measure this signal, or you'll
distort it and make the engine run richer than it should.  Ditto when
checking the green wire with the sensor disconnected.

david

--
David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"