Date: Tue, 7 May 2002 12:11:03 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: A damn OS question again, yea!!! now i need some help,
it's my turn....
In-Reply-To: <NBBBLKPACPEEKLBIBDMMAEBHEEAA.lsmith@cogeco.ca>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 09:34 AM 5/7/2002, Laurence Smith wrote:
>Take the reading with the O2 sensor plugged in and the engine warmed
>up. Touch the +'ve of the VOM to the connector where the green O2
>wire meets the white O2 wire (or is it black - can't remember).
>Attached the -'ve of the VOM to an engine ground. You should see a
>fluctating voltage between .4 and .7 volts. It never stays steady -
>always fluctuating above and below .5 volts.
Agree with this, but I have some quibbles with stuff below:
>If there are more readings below .5 then you are a bit lean. If most
>readings are above .5 then you are running rich.
I have two problems with this; first, you really cannot tell this with a
regular meter that takes 2-3 readings a second -- the ECU changes the
mixture on its own schedule that does not correspond to the averaging time
of the meter. If you want to know what the actual values are you have to
use a scope. Second, the feedback loop between the O2 sensor and ECU is
very simple: if the ECU sees > 500 mV it leans the mixture; if it sees <
500 mV it richens the mixture. It does not attempt to hold exactly at the
500 mV mark but rather is constantly flipping back and forth past it,
therefore the mixture is constantly going from *very* slightly lean to
*very* slightly rich and back. It runs in a tiny range that is too lean
for best power and too rich for best economy, but exactly right to allow
the catalytic converter to take oxides of nitrogen, carbon monoxide and
unburned hydrocarbons and recombine them into carbon dioxide and water.
>If your reading is way higher than .5 (say 1.5) then you are running
>VERY rich and have either:
The sensor itself cannot generate a voltage higher than one volt, so a
higher reading indicates a grounding problem or leakage from the sensor
heater into the sensor itself (there is no heater on the 1.9l sensors).
As a side note, the range of the sensor is so small that it goes offscale
before you reach either the best-power or best-economy mixture, so as a
practical matter it can only tell you "lean" (low reading) or "rich" (high
reading) but not how much -- that's if it's not hooked to the ECU. If it
*is* hooked to the ECU and you have any long-term steady reading, the ECU
is probably driving the mixture to an extreme -- rich if the reading is
low, lean if it's high.
>(1) a sensor that is bad which is enrichening the entire system and
>the ECU can't adjust for, or
The ECU cannot adjust for a bad sensor -- it uses the sensor to adjust
everything else in the system. If the sensor goes open circuit the ECU
will shift into open-loop mode and depend strictly on the AFM and its
internal mixture map, but if the sensor gets stuck with either a high or
low reading the ECU will drive the mixture as far as it can in the other
direction in a futile attempt to get the sensor to cross over.
>(2) you have a bad O2 ground wire (the outer sheath of the green coax
>O2 wire from terminal 19 on the ECU).
A shorted ground will cause a low reading, not a high one. Seeing the low
reading, the ECU will think the mixture is too lean and will drive it
extremely rich instead. An open ground will allow the other wires in the
harness to pass interference into the O2 sensor line, with unpredictable
results.
david
--
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
