Wrong reading of engine speed when Pico is connected

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Wrong reading of engine speed when Pico is connected

Postby victor2k » Tue Dec 27, 2016 9:27 am

Hello,
On a 2007 RAV4/2AD engine type with some staring issues I made some captures.
I used a 4425 Pico,MI007 cables and back-pinning probes for crank , cam and rail pressure signals,clamp on 20a scale for cyl 1 injector current.
Only for the rail pressure sensor I used the engine ground,for crank and cam I used the signal ground provided by ECU.
Both signals(cam and crank ) have some interferences,also with Techstream+Mongoose interface I can see a abnormal engine speed(150-300 rpm with engine off,ign on!).
Removing back-pinning probes the engine speed goes to 0.
My setup was wrong?
Thank you.
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Re: Wrong reading of engine speed when Pico is connected

Postby KimAndersen » Tue Dec 27, 2016 11:33 am

HI victor2k

I could you upload the psdata file to us. It's much easier see if anything is wrong or not. :wink:

Regards
Kim
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Re: Wrong reading of engine speed when Pico is connected

Postby victor2k » Tue Dec 27, 2016 7:54 pm

KimAndersen wrote:HI victor2k

I could you upload the psdata file to us. It's much easier see if anything is wrong or not. :wink:

Regards
Kim

Hello,
I tried but "Sorry, the board attachment quota has been reached." is the warning message.
Here is the link for my file:
https://mega.nz/#!oBIylKoA!g3A0a186MFAk ... 8MZ3V0rmOA
Thank you
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Re: Wrong reading of engine speed when Pico is connected

Postby KimAndersen » Wed Dec 28, 2016 1:04 pm

Hi Victor2K

When you are measuring a inductive sensor as Crankshaft Position Sensor (CPS) with no ground reference you should set up your Channel A as AC coupled and the same goes for the inductive CMP sensor.

There are much help to get if you use pre-settings guide from the top menu which goes as this - automotive/sensors/crankshaft.


Regards
Kim
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Re: Wrong reading of engine speed when Pico is connected

Postby victor2k » Wed Dec 28, 2016 6:19 pm

KimAndersen wrote:Hi Victor2K

When you are measuring a inductive sensor as Crankshaft Position Sensor (CPS) with no ground reference you should set up your Channel A as AC coupled and the same goes for the inductive CMP sensor.

There are much help to get if you use pre-settings guide from the top menu which goes as this - automotive/sensors/crankshaft.


Regards
Kim

Hello,
Thank you for your support.
I will try to use the guide,but then I can't see the injectors curent and rail pressure signal.Or on a bad starting car(at the first starting) cam vs crank is only for reference...we need a 6th or 8th chanell automotive osciloscope :shock:
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Re: Wrong reading of engine speed when Pico is connected

Postby Autonerdz » Wed Dec 28, 2016 7:46 pm

KimAndersen wrote:When you are measuring a inductive sensor as Crankshaft Position Sensor (CPS) with no ground reference you should set up your Channel A as AC coupled and the same goes for the inductive CMP sensor.



There is no reason to choose AC coupling with VR (inductive) sensors. In fact, it can be problematic because you can hide important diagnostic information from yourself. AC coupling filters out the DC component of the signal. Absolutely no reason to do this and the examples in the help menu are configured incorrectly.

What you do not want to do is hook up your scope across any VR sensor and attempt to use another channel. You risk crossing up your signals. This applies to the 4425 scope as well. When working with a VR sensor, always ground the channel to battery neg.
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Re: Wrong reading of engine speed when Pico is connected

Postby STC » Wed Dec 28, 2016 8:55 pm

Hi Victor

I would spend a few minutes looking at this from a different angle.

clamp on 20a scale for cyl 1 injector current.

I can see 4 injection events per 720 degrees so not sure where you have that current clamp.

The fact that you have injection occurring does to a point rule out Cam, Crank, RPM issues. If only for a short time ?
RAV 4 Delete.jpg


I am thinking that
1 - RPM at 160 is a bit slow but could be helped along with brake cleaner to prove or disprove the theory.
2 - All 4 Injectors are firing and that is evident from the corresponding FRP reaction. However, the injection events are making no contribution to exciting engine speed.

To me this looks like Compression, Valve Timing, Fuel Quality, Injector atomisation, Intake or Exhaust Restriction .... .... ....
Last edited by STC on Wed Dec 28, 2016 9:12 pm, edited 2 times in total.
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Re: Wrong reading of engine speed when Pico is connected

Postby KimAndersen » Wed Dec 28, 2016 8:57 pm

Hi Tom

I understand what you're saying, but I'm simply refer to the help guide that Picoscope provide us with. I don´t really know how victor did connect measuring probes other than he is experiencing some interference on the measurement he is trying to do.

The help guide from Picoscope should the first starting point, if your having some problems with a particular measurement.

Maybe Victor could give us some more information regarding his problems !!!

Regards
Kim
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Re: Wrong reading of engine speed when Pico is connected

Postby STC » Wed Dec 28, 2016 9:25 pm

There is no reason to choose AC coupling with VR (inductive) sensors. In fact, it can be problematic because you can hide important diagnostic information from yourself. AC coupling filters out the DC component of the signal. Absolutely no reason to do this and the examples in the help menu are configured incorrectly.


There is indeed Diagnostic value to be lost by using Sensor Earth rather than Battery Minus and that is with Inductive and Hall sensors.
Manufacturers do not always use a true Earth at these sensors, often they will pull it up by 0.5 / 1v for Diagnostic purposes. Coupling that signal to AC will hide that essential information.
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Re: Wrong reading of engine speed when Pico is connected

Postby Autonerdz » Wed Dec 28, 2016 10:35 pm

The help guide from Picoscope should the first starting point


We struggle every day trying to get techs to avoid using the automotive menu. We find that it creates more problems than it helps with. Those that start there have to re-educate.

Here is one recent example of when this spilled over into our public boards. Free membership is required to follow this link.

http://www.autonerdz.com/cgi/yabb2/YaBB ... 1470695090

Coupling that signal to AC will hide that essential information.


Absolutely. I learned this decades ago when working with an analog ABS problem. One wheel would code but all sensors displayed identical waveforms AC coupled. As soon as I chose DC, the problem was obvious. The coding wheel had a much higher bias voltage which led to the solution. Again, the automotive menu instructs you to AC couple these. Wrong.

AC coupling is often used for alternator ripple so you can filter out the DC and see the AC riding on it better but using a lower scaling. But with Pico's awesome zoom and filter capabilities, there is nothing that needs AC coupling.

I don't want to derail this thread so I will shut up now.... :wink:
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