Lambda Sensor Guided Test ... must read ...

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Lambda Sensor Guided Test ... must read ...

Postby FioranoCars » Tue Nov 01, 2016 2:02 am

For those not getting the automotive newsletter from Pico, or simply missed the article, then you really need to pop over and read Steve Smith's guided test and detailed explanation of Broadband Lambda Sensor testing:

https://www.picoauto.com/library/automotive-guided-tests/bosch-lsu-4.2-broadband-oxygen-sensor

Excellent, read it a couple of times, as it sparked an old memory, and it raises a few thoughts and observations
1. On the basis that the resistance is so sensitive relating to the Sensor resistance calibration, would using backpins (still Pico one's of course!) not reduce any changes to the circuit? (Not piercing, just back pinning). :?:
2. While Channel C or Channel D could be reused, I'll agree once you have verified them, you only need one to continue, BUT, it would be really nice to allow RPM (and vehicle speed), to be "imported" via a scantool, similar to the NVH tool in PicoDiagnostics, to see the reaction to the WOT snap, in this instance. Yes, I know the data is much slower (often only 4 samples a second) but it would help, especially new users to include RPM without needing a maths channel. :idea: (I know I've asked before, but hey) :D .
3. There is a reference to "If the correct resistance value is obtained, move to 2.2 below." but no 2.2, just HTML formating issues I'm sure, but I'm nicer to be fixed. Also 2 points below I think this should be a heading? (bold underlined bit).
4. While the conversion of the voltage to amps for the pump is great, and it certainly makes me think of other circuits that I could measure using a 4425 floating scope and a resistance test (ok, with caution and maybe verified with a clamp the first time!), to allow current measurement yet free up the channel currently used by the clamp, while retaining voltage readings (and hence volt drop etc verse other channels!) ... BUT ... what does the current tell you/us about the pump cell that the voltage does not? Is the sepcification listed as a range of mAmps? Honest, I love it, thought provoking, but I'm keen to understand what it's telling me verse the raw voltage.
5. When many of our cars (Donkeys) went to Wideband sensors, they dropped Thermo probes and used the pump cell to calculate temperature (I'm sure we've discussed it here 3-4years ago), but never found a way to extrapolate and calculate this ... is this something that can now be achieved?
6. You have noise on Channel A and Channel B, which at first glance I thought was ignition, then :oops: ... WOT snap ... and no change in frequency ... so any ideas what the noise you have is ?

Anyway, thanks again for a marvelous article, can't wait to see more in the coming months

Best
Richard Lukins
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Re: Lambda Sensor Guided Test ... must read ...

Postby Steve Smith » Wed Nov 02, 2016 2:23 pm

Hello Richard, thank you for the post and the heads up with some of the formatting issues.
The publishing team will resolve this for sure

Interesting point on the back pinning suggestion and it could simplify the scope connection process too. I think using break out leads makes for a stable connection (thinking of road-tests) but in the “real world” we would go for the easy option.
The Guided Test serves a guide only and for sure, connection techniques can be amended by the user.

With regards to the influence on circuit resistance, there has to be some form of effect when introducing break out leads, but in this case, not to the detriment of the obtained measurement or PCM monitoring (No fault codes generated).

I agree with the suggestion of RPM imported into PicoScope via the OBD. We have looked at this feature and a New Feature Request has been handed to the development team.

Another interesting point that you raise is “what does the current tell us that the voltage does not”
I agree as its all Ohms Law and directly proportional. Often the Scan tools refer to pumping current as do fault code descriptions and so using the scope to graph current will help with direct evaluation/verification

Frank Massey has an excellent video on our YouTube channel that uses the voltage measurement technique which simplifies the measurement process but using voltage.

https://www.youtube.com/channel/UC0j4RP ... PdEL-Q7m9A

https://www.youtube.com/watch?v=UdbZfXMMTQs

Connecting as we do with this Guided Test allows for a full evaluation of the sensor, calibration resister, heater, PCM and circuit using PicoScope

I will look into the temperature calculation when I can and would be interested to read any posts that describe this technique (There has to be a relationship for sure)

I hope some of this information will help

Take care…….Steve
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Re: Lambda Sensor Guided Test ... must read ...

Postby FioranoCars » Wed Nov 02, 2016 3:52 pm

Hi Steve
Thanks for the reply, and glad those pesky HTML issues will get sorted!

I'm fixated by noise, so your thoughts on that (point 5 of original post) would be good please ... dog with a bone on that topic right now!

The use of voltdrop with fixed resitance, is sadly not got that much wide ranging potential as I thought ... as most volt drop will be full battery voltage (+/- a little for wiring) for most situations and then there is the issue of how many items have fixed resistance (light bulb, motors, etc all chaneg with heat, speed, blade resitance etc) the only similar components I can think of would be "Current based pressure sensors (mostly 4ma-20ma output ranges), but I think with my limited understanding of them, then I'd still be better using a "external resistor" between the +/- of the scope leads to earth and the transducers signal ? I have now got a micro amp clamp, so maybe I need to do some tests on this and see if I can corralate a result ... a separate post maybe!

So back to using the volt drop for current measurement, is there any other automotive circuit that comes to mind where the technique and components would be both work and be useful? I'm keen to hear ...

Ok, on the temperature calculation, I found the old post, on this Pico Thread, but thanks to Robski and KimAndersen who both contributed to the original discussion and without whose help I'd have been stumped before I started!

It all started as we had an exhaust temperature issue, but no thermo probe/cold joint and could not figure where the temperature was being obtained from ... the original article that started my research on the feature is here:
http://www.megamanual.com/PWC/wire.htm
One note, the sensor is only used above 300*c for temperature calculation, although how they calculate that, when they can't use it below is another story (yes, I'm sure lots of other factors like oil and water temp etc etc, and trial readings), but I like the chicken and egg conundrum :D

Kim provided Bosch Patent application on Temperatures etc - as a PDF file

Just for technical background, Lamborghini don't use the Lambda for temperature, but do have a great document, which they share here, all about front and rear sensors:
Lamboghini PDF on Lambda Monitoring

Please let me know if you get anywhere, and I'll be honest, I've not refreshed my memory on this topic for the better part of 3 years, so the answer might be staring out at me! :oops:

Maybe we can revert this Temperature discussion back to and post on the old thread, with any new findings? Keep things here related to your Guided Test and Lambda testing issues more generally?

Best
Richard Lukins
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