Oxygen Sensor Voltage Question

[ctaylor738]

I recently bought a 1984 280SL road warrior - 181K, sitting for a while due to clutch problems. Plan A was to use the drive train in my 280C. But I have fixed the clutch, and found that this is actually a nice car. Plan B is to make it my driver, so I need to take it in for a smog test next week.

Since this is a euro car, a converter, O2 sensor, computer, and frequency valve have been added. All after market. The sensor was unhooked when I got the car - looked like a rodent chewed through the wire. I tested the voltage, and got a nice fluctuating current around .098v. So I connected the wire and found that the idle smoothed out a bit, but it was pretty good to begin with. The frequency valve also started buzzing.

The sensor is a three-wire model with only the sensor lead connected.

The engine starts instantly, warms up well, goes great, and idles smoothly. If anything, it smells a bit rich when it's hot.

Everything I have read about O2 sensors is that they operate in the .15 - .85 volt range. Is this always true? Or do they come in different voltages and calibrated to the computer/valve? I have had opposing answers from two experienced techs. I am obviously trying to figure out if I have a bad sensor or the engine is running very lean, or if the system is just calibrated differerently and everything is OK.

TIA for any information.

[myarmar]

Chuck, did you measured with sensor disconnected? If so, the system was in open loop. Voltage .098 means lean condition. Now you need to measure with sensor connected. You should see oscillations from approx. .070 to .75 Volt.
Reg. Mike

[manny]

Chuck

Yes, any O2 sensor has a working range of about 0.1 - 1.0 volt.
Except some of the newer Hondas.
I understand they are using a " wide band " O2 sensor ( 0.1 - 5.0 volt ).
If you are running with only the " signal " wire connected ( heater element power & ground not connected ), there is a good chance the sensor will cool off ( below 600 deg. ), in which case the ECU may even go back into open loop.
Open loop usually means a slightly richer running engine, which may explain a slight smell.
You can verify this theory by running the engine at @ 2000 rpm for a couple of minutes, and then check your signal.
It should show varying voltage as long as the O2 is still hot.

[ctaylor738]

Thanks for the responses. I will re-test with the sensor lead connected to the brain and see what I get. I don't think the mixture is off-the-scale lean, so either the sensor was not hot enough or it is bad.

You did answer my question about standardization of voltage - thanks.

[ctaylor738]

Connected the DVM to the sensor lead with the sensor connected to the brain.

After warm start at idle: .41v

Held at 2000 RPM for 5 minutes: gradual drop to .26v and stabilizes

Idle for 5 minutes: slow rise to .38v and stabilizes

Very little fluctuation.

?

[manny]

Looks like the O2 sensor is screwed.
Normally, a reading of 0.1 - 0.2, maybe even 0.3 volts, with no fluctuation, indicates a dead sensor.
If you can somehow make the fuel mixture VERY rich & the voltage remains low, it would also verify a worn-out sensor.

[Arthur Dalton]

Easy 02 test for DIYer is to hook the meter to sens sig wire and ground ... unplug sens sig wire feeding back to ECU ... leave sens heater wires connected ... warm car to normal...
Now , pull vac hose , creating a vac leak.. the sens should go lean
[ low v].... correct vac leak ... cuff the air intake, choking the engine and creating a rich mixture ... the sensor will see rich and read high.
When doing either of these , the meter should move as you open/close both vac leak or choke intake air..
All is you are doing is using the engine as a mixture tool and seeing if the sensor responds to your manually changing the a/f mix...that is why you do not want the wire feeding back to the ECU , as the ECU will try and correct your manual a/f mix inputs .........
you are looking for .1 to .9 volts, lean/rich , with a 1/2 sec change time...
that would indicate a decent sensor

[ctaylor738]

Thanks for the help on this. Here's an update.

I replaced the sensor with a single-wire Bosch. After the engine warmed up it appeared to get into closed-loop and had just a slight rythmic surge. The sensor output was fluctuating around .5v. When I drove the car, it felt a little perkier.

So today I am going to see if I can get a duty cycle reading and tweak the mixture if necessary.

[ctaylor738]

... measured at the leads on the frequency valve. Getting a fluctuating 12-20%. So I decided not to mess with the mixture, since the voltage out of the sensor seemed about right.

Got a smog test today and passed! A little close on HC at idle (183 ppm with a limit of 220), but otherwise fine. The tech was surprised.

So thanks again for the advice on this.

[manny]

Chuck

Good for you.
Btw., 37 ppm is NOT close..........2 or 3 ppm IS close.