At 07:16 AM 8/4/2008 -0700, Mike Elliott wrote:
>Some comments to the below:
>
>>>That said, your situation with disconnecting the O2 sensor instead of the
>>>gauge and the resulting response tells me that the gauge is a problem in
>>>itself.
>>Only if disconnecting the gauge makes the symptoms disappear...
>
>That will be tested today. Yesterday's conditions were: O2 sensor
>disconnected from ECU input and meter input. Meter input and ECU
>input still connected together, meter sat steady at midscale.
>Unless, as was mentioned, the signal line had a lot of high
>frequency noise on it that the meter could not display.
>
><snip>
>
>>True -- although Mike said that the meter stayed centered, suggesting
>>that the ECU was back in open-loop mode?
>
>I don't see how that indicates the ECU was in open loop, only that
>the bias voltage at the ECU input did not change -- right? Pin 5 (O2
>sensor signal) is just a high-impedance input, yes?

It starts in open loop and stays there until the O2 sensor wakes up
and overrides the bias signal.  I don't know whether reasserting the
bias signal will put it back in open loop, but my understanding is
you started from cold with the sensor unplugged.

>Possibly useful would be to disconnect the sensor signal from the
>ECU input and connect it to the meter to monitor what the engine is
>doing when the ECU can't see the sensor. David said something about
>biasing the sensor with a half-meg resistor?

No, one meg, biased to half a volt from a voltage divider.  Ten meg
might be better but I have a sort-of memory that what comes from the
ECU is about one meg impedance.  The only point of doing this would
be to be able to tell when the sensor comes alive, because otherwise
it will sit at lean until it warms up.

d

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