I had the O2 sensor replaced in my '82 a couple of months ago.  Immediately afterwards, the car began hesitating from standstill and surging at constant throttle openings.  It was suggested that I have the CO checked which I finally got round to doing last weekend.  The CO and hydrocarbon meters clearly showed the mixture varying from rich to lean every 3 to 4 seconds. My mechanic explained that the O2 sensor was bad.  Before I mess with replacing this again, I ran a voltmeter on it and got the following: engine cold - 0.525v gradually increasing to 0.530v once warmed up.  My understanding was that the O2 sensor simply produces a constant voltage for a given volume of oxygen.  The readout was not oscillating but maybe my voltmeter was not sensitive enough to see it.  Is this OK or not? Trying to rule out other possibilities, can someone tell me the expected voltage from the outside temperature sensor at say 80 degrees F? Last possibility is the Air Flow Meter but I want to rule out the cheaper alternatives first! :-)
Thanks as always
John McDermott
1982 928 AT

John & Erin McDermott wrote:

> The readout was not oscillating but maybe my voltmeter was not sensitive enough to see it.  Is this OK or not?

I use a digital voltmeter to read the O2 sensor and it is pretty easy to read.  The O2 sensor  transitions from "high" which is .5 volts to maybe .9 volts to "low" which is about .1 to .18 volts at a stoichiometric mixture.  It should make this transition pretty much all the time with a warm engine driven at a steady speed.  The numbers on the voltmeter should be constantly changing.  If not, something is wrong.  I'm not sure which models are which, but some generic O2 sensors have wires for heaters in them and some cars don't use them.  It is up to the installer to pick the right wire.  Wrong wire?  Just a guess.


At 11:41 AM 4/4/00 +0000, Eric L. Johnson wrote:
>Can anyone give me the pinout for the 3 wire connector at the O2 sensor?
>I think it is as follows:
>green - controller for O2 signal
>blue - reference voltage (12v???)
>brown - ground

Pretty close - the blue is actually the heater for the sensor.



I've played a lot with my O2 sensor, using a oscilloscope, DVM and an Air fuel meter.

> >> now I found it, I put an Oscilloscope on it and measured it's output while
> >> driving, this is what I discovered:
> >>
> >> Idle:  seems ok, steady voltage

It should only be a steady voltage during warm-up,  about 60 - 90 seconds.
It should be at  ~.5 volts.
After that it should start fluctuating below and above .5 volts.

> >> Boot it:  I assumed that booting it would make the fuel richer, mine
> >> seems to indicate the opposite
> >> it goes real lean.

This could indicate that your mixture is not adjusted correctly.
After the car is warmed up,  Shut down the engine, unplug the O2 sensor with the A/F meter connected.
Restart the engine, the voltage should be just above .5 volts.  Higher if you want the mixture richer at WOT.  I wouldn't try to crank it above .9 volts.   The mixture allen screw is on the AMS.

In closed loop mode the O2 sensor will try to keep your mixture around a half of a volt.  At WOT you'll go into open loop mode using the MAP to program the mixture.  If the mixture is adjusted to low,  you'll get lean conditions.

> >> Puttering  along at 40mph,  the voltage goes up and down and sometimes will steady out it seems like the computer is chasing the O2 sensor around (or visa-versa), is this normal ? or am I having problems with fuel/air ratio ?, any suggestions welcome

That part sounds just like it should.  The computer is continually adjusting the mixture to the varying conditions.

Ken Postma '86 928S

Lambda sonde 3-Polig

Fitis PORSCHE 928 S4 GTS / 5.4

MJ: 86 - 95

Thread: M18 x 1,5

Operating voltage 0-1Volt

Resistance of the heating element 4 Ohm