OS Sensor

A= Sensor wire

B-C= heating wires


Here is what i got so far, this post is from Jay of the The Vanagon Mailing List Server

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.