So we have departed from the Pico diagnostic platform
Where we did a simple relative compression check
And we have identified a weak cylinder
I have left the vehicle in the same condition and we had around a
30 percent drop on a particular cylinder
So let us go back to the oscilloscope
We are still connected to the vehicle battery
So we are going to measure voltage
Potential difference across the battery during compression
Which is what the Pico diagnostics platform is using
So that was the only connection we had
I am going to compliment the voltage measurement with current
And you will notice that we have connected the second channel
The red channel
With a current clamp over the battery negative lead
I have deliberately set the polarity of this device
So that the current appears to be taken out of the battery
Something I feel quite passionate about
Because I am using this as consumption from the battery
By the consumer the starter motor
And of course if the engine were to run
The alternator would then show a positive current flow
Not a negative flow out of the battery
So how are we going to set this up
20 Volts would be the most appropriate setting for channel A
Channel B I am going to choose 1 Volt
Some of you will have seen me doing this before
For a very good simple reason
It is a very simple view of how to interpret current using voltage
The sensitivity or attenuation of the current clamp is every mV is 1 Amp
There are 1000 mV in 1 Volt
So by setting 1 Volt I am establishing a 0 to 1000 Amp range in both directions
When I conduct the test you will see that aesthetically it look right
And the comparison of both voltage and current is much easier to understand
But we need a longer time base
So I am going to set a 2 second time base
Which gives us a sweep time across the screen of 20 seconds
Long enough for me to conduct the test and you can see the test running live
We will come back and do the analysis
Roughly a 10 second crank duration
Now in terms of assessment I have got quite a high sample rate 10 millie samples
Which gives me the opportunity when I zoom in
Dividing the amount of data that is stored in the buffer
Which is how the display
Acquisition storage display the three functions of an oscilloscope
We can extract the data out of the buffer with plenty of detail
Because I have got lots of data stored
So we get a very nice accurate image
The noise on signal is due to the battery support
Which I have kept on the vehicle as we have done a lot of cranking tests
So I do not want to diminish the results of the test by a decaying battery
So I have supported the battery with a battery support tool
Now let me just explain my take on this current measurement
If we use a cursor and we measure the peak current draw
That is represented by 695 mV
Which is 695 Amps it is that simple
Just move the decimal point three places
So I am happy with that I hope you are
However what we do want to look at is the results of this test
Over a period when it is stable
So I am going to choose
Now of course the last test we did looked at the voltage analysis
So let us take a look at that first
Voltage differential and you can see there that there is a differential in voltage
During the cranking process
And we can see that the pattern is not even
Now I find that a little difficult to establish good compressions
These are good compressions and these potentially are poor compressions
OK let us look at it from the current point of view
So let us mark the good compression events
Obviously the negative events
I would say that we have two healthy compression events
That is a little lacking
And this is seriously lacking
So that is the problem event
So we are pulling during this stable cranking event
In other words when the current has recovered and the engine is now revolving
At around 250 or 260 rpm
The amount of current consumption by the starter has recovered somewhat
And we are pulling just under 230 Amps of current
However this event here which is obviously a weaker event
Is only consuming
And we could do a little bit of filtering now guys
Let us call it 200 mV
So there is a 31 mV or 31 Amp difference in the work rate on that cylinder
Some cylinders are even worse than that
So that is how the relevant compression test is done
But you can do it using the scope in real time
If we choose to filter
We might get a little bit more of an accurate assessment
Let us filter a little bit more and see if we can improve this
Be careful with filtering guys because what we do not want to do is
A very high sample rate a lot of noise in the signal
I do not want to filter out detail
So I am going to hold at that point
I think that my assessment is reasonably accurate
If we look at the performance between a good compression
And a poor compression it is 27 mV 27 Amps difference between the two
Which is around 10 percent of the work rate done by the battery
So we can see that there is a mechanical problem with this engine
Now you may say how do I know it is not a problem with the starter motor
Well that is down to a different frequency
The rotation speed of the starter motor is much quicker than that of the engine
And the performance of the starter motor armature is represented
In these events here
We have a very high sample rate so we can actually look at the performance
If there was a problem with the windings in the starter motor
We can see that there are some slight differences here
Of the current consumption from the starter motor but not effecting compressions
Which are these events
So that is my take on a variation of the Pico diagnostic platform
We can do it and I often do this test with the scope
I think it has a little more detail in it
And we can measure the difference in terms of current draw
From the starter motor with a poor compression
So that is the first basic test
I am quite happy with the results of that test
I know there is a problem with the engine
Let us now try and develop a means of proving which cylinder the problem is on
And potentially what that problem could be
Now to do this what I am going to do is do a compression test
While the engine is running
And that means obviously that one cylinder will not have a spark plug in it
However we are not really interested in the spark and the fuelling
We are interested in in the mechanical efficiency of the engine
So we are going to be doing a compression test
With a WPS in cylinder 1 in real time
With engine running not cranking
You will see some very interesting results