The next test that I have chosen to demonstrate using the scope
Is a new evolution if you like
With the introduction of digital control from sensors or digital output from sensors
There is a need to convert a digital value to an analogue value
The full importance of this will only become established with the second part of this presentation
It is in two parts this one is looking at the conversion of a digital mass meter
Into an analogue event
The second part in a separate presentation
Is interpretation of a wide band sensor and to make it interesting
In terms of diagnostic process I have combined the two
As there will be some analysis between the performance of the mass meter
And the wide band lambda sensor that is why I have made two separate presentations
But coming to a joint conclusion
So what are the objectives
To determine the type of air mass meter under test
Converting a digital signal into an analogue display
And evaluating its performance
I have got something a little bit special in this event as well to throw something in the works
I have connected to the car which is a Skoda 1.8 direct injection engine
Fairly late generation engine with a digital air mass meter
Which is pretty run of the mill now with everything
I have connected the probe
Usually but not always the out put signal is nearest to the engine
So we are dealing with that probe with a reference from the battery negative
We are dealing with a digital signal so I am not going to do a
Close coupled or floating measurement in this case
I am quite happy to take ground from the battery
And the ignition is on so we have a value
I have not done any scaling I have simply gone to my home address
If you click on the house that is where you live
And there is the update
It is your house so you can put your furniture where you want
I know exactly where my furniture is
And I have a number of ranges it is free run with a pre set timebase
In the appropriate range of 200 Volts
I do not need 200 Volts so lets bring the voltage down
I will scale that to 10 Volts
The reason that I chose 10 is that most digital air mass meters
Work on a 5 Volt range so a 10 Volt scaling is about right
In other words the scale of the image is important
Now what has happened is what is called envelope or compression
Within that is a very high number of events taking place
Frequency the component works off frequency change not duty
We are going to use that sort of timebase later on
But if we wanted to measure specifically frequency
Then remember three timebases
1 ms 100 ms and 5 s/div
I want to choose 1 m/s
Which should give us enough events
I think you will agree that is quite clear
It is in free run I am not using triggers
I want to switch channel 5 off for a moment lets leave 5 out of the equation
That is the little special bit for the end
So we have a timebase of 1 ms/div 10ms across the screen
You can see the event is digital and we need to know what the frequency is
So lets call up the measurements box
Add measurements on A the channel of interest
And we want to call up frequency across the whole trace
Apply it and average frequency is 1 kHz 1.08 just over a 1000 Hz
A 1000 cycles a second
Most of these digital air mass meters increase in frequency
With the increase of air flow across them
However there are an increasing number of them
And I came across it on VAG the other day which surprised me just last week
That decrease they start at a high value and then come down hill
Generally it is the French variety that do that
And you also fine that some of the test capabilities of these air mass meters
Do also vary
The test I am going to show you on this car works perfectly
Regrettably the test I am about to show you does not work too well on
Certain types of air mass meter
And I which I knew the answer but I do not so any of you out there
Do know it please tell me
We are going to convert that into an analogue representation
But we need more time profile so I am going to go back and choose 5 s/div
That is going to compress this is envelope
So we have compressed many thousands of events in there
I am now going to select this box
At the moment all the measurements I have made so far are what we call
DC coupling with reference to battery negative
I am going to open that box where we have a choice of AC or DC or frequency
We can only do this with one channel
In other words the computing capabilities limit this test to one channel
So you have to test each digital device singularly
Now that I have got frequency we need to check range
I am going for the upper range which is a 20k range
And we should have a value
Because the way the software is written it now is using full screen
And you can see down here the zero baseline is here
And this trace that is being drawn
If I get my cursor and place it on that line
Left mouse channel A 1.053 kHz
So the measurement now instead of looking at frequency from the original image
We are now measuring it as an analogue representation
So lets run the vehicle and look at the trace
Everything is set up so we are OK to start
I am going to wait until the trace just starts to draw
Start the engine and let it idle
Let it idle for a short while checking stability
Maximum attainable RPM with a static vehicle
Engine off and ignition back on
I can not increase the RPM any more this is the DSG box
And freeze the image and just move the image up the page a little bit
Not that one
It is the blue image that we are interested in
Let me get that back down to the baseline there we are
Lets move the blue image up to a central baseline
So we now have if you are familiar with an air mass meter
A fairly recognisable output from the air mass meter
Can I use it diagnostically
Well in its singular format like this no
But in the second part of this presentation
I am going to bring in the wide band lambda sensor
The we can make some sense of it
And you will see the reason why I want to convert digital to analogue in the first place
This is some research we did a long time ago
And we are doing some currently which is not yet complete
Which we will discuss in a moment
But let us look at he event
Can we use this on its own no
We have already said that but we have got here is a representation of frequency as a visual image
Looking at the increase in air flow across the air mass meter
But we need to compare it with something
And what I want to compare it to is feedback
From the wide band sensor the oxygen sensor
That enables me to prove fuel trim analysis in real time
So you have seen that test I like the accuracy of it
But regrettably you do not get that same performance across all digital air mass meters unfortunately
What I would like to do now is switch channel 4 on
Channel 4 I have converted from yellow to black
Black is a nice strong colour I just prefer it over yellow
I will repeat the test we have just done
ENGINE REVS
And put ignition back on
That is OK
Let me bring this down and do that
If you do not find that interesting I find it fascinating
Because what we are actually doing is a development we have been doing over
A number of years the results are inconclusive across all air mass meters
So I can not produce it as a standard test profile which is why I don not talk about
it too much
It is incredibly accurate on the air mass meters that it does work with
And this is one of them because what we are looking at on the black trace
Is current flow across the airmass meter
The really interesting fact with this
Every engine we have tested so far irrespective of petrol or diesel or engine size
Produces exactly the same results of current flow
And what we have is around 100 mA of current at idle represented by 1 Volt
Which is the attenuation of this particular device I am using
So if I bring that down
890 mV that is near enough for me
And with about 170 mA of current at peak
So I will bring that up to the peak value about there
Current value is 1.636Volts which is 163 mA of current
Every one of them produces the same figure 100mA 170mA
With performance software you get a little change in that of course
But that is a standard current flow through an air mass meter
There has got to be something in this
I am trying to develop this test across all air mass meters but I have not concluded this yet
But we are still working on it
I think you will agree that I find this incredibly convincing in terms of the possibilities
Once again it proves how accurate an oscilloscope can be helping assess the true
Load value across a component because the air mass meter uses current in part of its
Functionality in converting a mass of air into a measurable value
And we have converted this from frequency
So we have gone from frequency to analogue
And we have also gone effectively from frequency to current
So there you have it
Once again demonstrating how powerful an anaclitic tool the scope can be
That concludes this part of the test
We will take a short break while we set up for the second part
Which is a separate presentation looking at wide band sensors