|Here is what i got so far, this
post is from Jay of the The Vanagon Mailing List
How to check the OS sensor
The voltage comes from the sensor, not from the ECU
and through the sensor. It should run OK with
the sensor un-plugged. On a cold start, the sensor won't put out any
voltage until it gets hot. After the ECU gets a reasonable signal
from the O2 sensor, it uses the voltage to start adjusting the FI
pulses. Un-plug it when the engine is cold and watch the meter. It
will tell you how soon it starts to send a good signal to the ECU.
You can plug it back in at anytime--it won't hurt anything. Again, I
only have the single wire. You have additional wires, which I believe
just speed up the heat up time. Just be careful, one or both of the
white wires may be hot (12V+)
The voltage is generated by the O2 sensor. Put
the voltmeter (+) on the green/black wire from the O2 sensor and ground
the voltmeter (-) lead.
Once the sensor is hot, it will produce a voltage
based on how much oxygen is left in the exhaust. A higher voltage (.7 to
1.0 VDC) indicates a richer condition. Lower voltage (0.6 to 0.0 VDC)
indicates a leaner condition. My '85 only has the one wire. I have
adjusted the idle mixture (hex screw on AFM) by doing this, and it
seems to work pretty well.
If you read the voltage with it still hooked up
to the ECU, it tends to jump around as the ECU is trying to constantly
correct the mixture. With it un-hooked, the ECU goes open-loop and runs at
the default settings.
You can see how your AFM is doing by reading the
voltage off of idle.
Run a wire up front and set the meter up so you
can see it as you drive.
You are still running open-loop, and the meter
will show you how well the AFM is tuned. Again, you should see about
0.7VDC. There is a spring inside that can be turned in notches to adjust
leaner or richer mixture off of idle. Be careful if you do this. Mark the
starting setting with paint.
It is spring loaded and when you move the wire
clip, it can un-wind on its own. Tighter spring is leaner (needs more
airflow to move spring), looser is richer (less airflow to move the spring). To
get at this, you have to remove the black top on the AFM. Re-seal when
you are done. I ended up pretty much back where I started, but it is
interesting to fool with.