Date: Tue, 23 Aug 2011 23:26:32 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: O2-sensor voltage problems '84 1.9L -- Help needed lead
voltages!
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The O2 sensor is a bit unique in that it actually produces voltage-current. Most other sensors change resistance, capacitance, or inductance based on some condition change.
In the wonderful world of electronics most things rely on semiconductor devices such as diodes and transistors. Really basic explanation but the most common are germanium and silicon type devices. Silicon devices have a fairly constant forward voltage drop of ~.6 volt. The O2 sensor lead to the ECU goes into an amplifier type circuit. With nothing connected there is a voltage between the lead and ground most likely the bias voltage or forward drop of the base-emitter junction. When the O2 sensor is connected before it generates it's voltage it acts as a load and sinks this voltage to near zero. So yes you can measure a voltage on the O2 sensor input connection.
So when the O2 sensor is not connected, this leakage or bias voltage is real close to where the ECU would try to adjust it anyway. As such it is satisfied and does not make any adjustment. This is why leaving the sensor disconnected will often allow the engine to run OK. Sink the voltage down and the ECU will continue to richen the mixture until a signal is present. Apply a voltage and the ECU will lean the engine out.
So if disconnect the O2 sensor from the ECU and put a meter on it, we should be able to ground the ECU lead, make the engine go rich and measure a voltage. We should be able to get 1.0 volts or more when there is no O2 in the exhaust. Now if we apply a positive voltage to the ECU lead (a 1.5V D cell works good for this) the engine should lean out and we should see the O2 sensor voltage drop. A variation of this procedure is in the Bentley. If the sensor checks out during this test then we should be able connect it to the ECU lead and along with the meter we should see the sensor voltage change as the ECU now controls the mixture. It should continuously vary from ~.2 to .8 volts. A meter with a bar graph display will make this easier to see.
It should be noted that O2 sensors do not work until they are heated. Lazy sensors take too long to go online and will create drivability problems during the warm up cycle. Do to the poor placement they can be effected by wind and weather and since they sense the O2 content inside and outside they can be fouled by contamination from both sides. O2 sensor killers are excess engine oil, (zinc, ash, carbon,) antifreeze, and road crap, salt. The output is actually created by the difference of the O2 outside as compared to in the exhaust. Originally there was a deflector on the sensors to keep the wind and dirt off them.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Scott Daniel - Turbovans
Sent: Tuesday, August 23, 2011 5:01 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: O2-sensor voltage problems '84 1.9L -- Help needed
hi,
would love to really understand ..
re
"Voltage from O2-cable coming from engine is +0.46 volts. "
'voltage from cable from engine' ..
what does that mean ?
there is no cable from the engine ..
there is the signal wire coming out of the oxygen sensor, which is screwed into the exhaust pipe or cat, that's bolted to the engine.
is that what you mean ?
doesn't make sense though ..
because ECU is not in the picture with that measurement.
are you saying ..
you are measuring voltage on the signal wire that feeds the 02 signal to the ecu ? Like measuring into the ecu for voltage with key on or engine running ?
It would never occur to me to do that.
on my own 87 Wolfsburg ..
just had a whole go-around with a burst hose ..
which caused a somewhat tired distributor cap to act up ...
and I have another coil on temporarily,
and she ran fine for 20 miles...then started acting up again ..
weak throttle response, and if I cycle the key off and back on quickly, even in gear...then she runs fine for a while.
To get home without that happening,
I disconnected the oxygen sensor signal wire,. and she ran fine.
I doubt anti-frz harmed the 02 sensor..
and I have seen anti-frz play total havoc with electronic and electrical components before ..
like rendered them unreliable from then on, once that hot juice sprayed on them.
my plan at the moment is try another 02 sensor, since I have a couple of new ones at hand..
and ...
before that, unplug ecu and measure resistance between 02 single wire and it's coaxial shield ,...see if perhaps there is a short there.
Scott
----- Original Message -----
From: "John Lauterbach" <jhlauterbach@BELLSOUTH.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, August 23, 2011 6:31 AM
Subject: O2-sensor voltage problems '84 1.9L -- Help needed
> Here are the details. O2-sensor not connected. Voltage from O2-cable
> coming from engine is +0.46 volts. Again, O2-sensor is not connected.
> I have tried with two different ECUs and get same result.
>
> With engine off, resistance between O2-cable coming from engine and
> ground is 140K ohms. I have a spare FI cable assembly and get same
> resistance from that cable when a ECU is connected.
>
> If I connect O2-sensor as intended, engine runs rich and stalls.
>
> John
