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Really Senior Member II
Really Senior Member II
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Original and STILL best

Joined: 26-March-2004
Location: East Sussex/Kent border
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Points: 2098 		Direct Link To This Post Posted: 03-May-2005 at 04:11
Originally posted by autofix autofix wrote:

I assume you are trying to monitor the oxygen sensor signal on your car, and your car is fitted with a Zirconia sensor. 

The first thing that you should be aware of is that the sensor should very rarely show a steady voltage. A zirconia sensor should switch between about .2 and .8 volts when the mixture is correct. This switching should happen at a rate of about 1 hertz (once per second), very often much quicker. No cheap multimeter will be capable of displaying this, they are much too slow. The Pico is a great tool to use for something like this as it is so fast. The only reason it is so cheap is that you use a lappy or desktop iin place of the usual hardware you would need to buy in a dedicated scope.

The O2 sensor (oxygen sensor) measures the amount of oxygen in the exhaust gases. This is directly related to the mixture (air/fuel) being burned in the combustion chamber. The min and max signal voltage as well as the frequency of the switching rate are all very important to the ECM (engine control module) as it uses this to choose the level of fueling required by the engine. 

Thanks for that, and just in from the pub too - either you had a quietish night or you eat, sleep and drink auto electrics!

Latest developments are (an apologies for being very slow with this job) are that I discovered that I have 2 sensors. Which means that my thoughts now are that to use a single meter i will need to either do each in turn or find where the signals are 'joined' which is probably the ecu, or use 2 meters.

rapidly looking like it would be easier to buy one of those AFR meters from the likes of lumenition... not convinced of the accuracy of these though from my research on the web...
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Really Senior Member I
Really Senior Member I


Joined: 23-August-2003
Location: Kildare
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Points: 294 		Direct Link To This Post Posted: 03-May-2005 at 06:02

Hi,

""  either you had a quietish night or you eat, sleep and drink auto electrics!  ""    True on both counts

I have just noticed that you have listed your cars at the end of your post, so you do not have zirconia sensors. Yours will be Titania which work in the range of 0 to 5 volts.

You are correct, there are two sensors. One is pre cat and the other post. On the E36, the post cat sensor is not used for anything other than to monitor the cat. The pre sensor is used for adapting fueling. The signal wires do not join up at all, each is a seperate signal for the ECU.

I am wondering why do you feel the need to check these signals?

Regards

Alan
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Really Senior Member II
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Joined: 26-March-2004
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Points: 2098 		Direct Link To This Post Posted: 03-May-2005 at 06:25

Hi alan!

umm, although I bow to your superior knowledge ;-)

My sensors are both pre-cat (puddy gone bye-bye!) and there is one for each manifold (1/2/3 & 4/5/6).

On my ETK system, the post cat sensor is for 'national vehicles - japan', and hance is not present on my car???

As i suspected with the signals then, dammit.

The reason for me checking the signals is I need to keep an eye on my mixtures. I suspect there is some weak spots as I have intake and exhaust mods on a standard program. (chipping is £££ for my vehicle)

What would you suggest?

Thanks.
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Really Senior Member I
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Joined: 23-August-2003
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Points: 294 		Direct Link To This Post Posted: 03-May-2005 at 10:26

HI,

Silly me, The E36 does of course have two pre sensors as the closed loop system works on 2 banks of 3 cyls. Your intake and exhaust mods would be likely to effect both banks the same, if at all, so I guess you only really need to look at one signal.

When the engine is lean, the signal voltage should react immediately and go high. When it goes high (or low) it stops switching until the mixture comes right again. In this case you would not need to measure with any accuracy the switching rate of the sensor, just if it stops switching and if it goes lean or not, so any meter will show you this. You should see the digital readout change constantly even though you will not know what the true min and max figures are. It would probably be enough for you to know that it is switching. One problem here though. The sensor does not switch all the time. Once you accelerate, even a small amount, the required fuel to lift the revs will cause a natural and correct rich condition and the sensor will react as such. The same is true on deceleration as the injectors shut down and the engine dumps big gulps of air into the exhaust without any fuel thus showing a lean mixture. The sensor only switches well at idle and cruise.

If you are suffering what you suspect to be a very lean flat spot then watching the signal could prove this to you. As you open the throttle the signal should drop low to show the acceleration enrichment and then go high to show the lean problem BUT it would have to go very lean for this to work. 

If you want to see how effective this can be, connect your meter and allow it to idle. Watch the way the reading floats all over the place and then give it a few quick revs. This should dump in fuel and cause a rich condition for a few seconds. you can also pull off a vac pipe to induce a leak and see the opposite effect.

HTH

Alan

 
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