The first test I have chosen is one of those tests that is often omitted
It is a very important test and that is the battery itself
The battery can no longer be considered just for the storage of electrical energy
It is an integrated part of the electronic control circuit
The purpose of the test is to observe voltage drop against load
Measured as current by the consumer
In every good circuit all of the load should be consumed by the consumer
In our case the starter motor
I mentioned the word simplistic approach dont complicate the use of a scope
So what is the function of the starter motor
It is to rotate the engine to create enough rotation speed to enable efficient and prompt starting
A good example of why the rotation of the engine is important is starting
Not just to get good efficient compression which on diesel is very important
Equally so on direct petrol these days
But also because we are driving the high pressure common rail pump
Unless we achieve a certain amount of pressure which applies to direct petrol also
Then the injectors are not released and the vehicle will not start
So rotation speed of the engine is vital
The correct specification of the battery
The health status and the consumer are all tested with this process
That is the focus of this test
Let me explain what I mean by simplicity of choice when using the scope
In my opinion there are only three voltage ranges we need worry about
0 to 5 volts for sensors
0 to 20 volts for anything that uses battery power or current
0 to 100 volts
The scope can go to 200 volts for piezo injectors
So this can be increased anywhere the induction process takes place above 100 volt threshold
But these are the three ranges we need to consider
Time bases are also quite straight forward I would choose three
1 m/s for intermittent or switched events such as injectors
Ignition and other events that happen at a regular high frequency
0 to 100 m/s for repetitive events such as duty cycle and actuators
0 to 50 s that sounds a long time which it is
But for profile events over time such as lambda or air mass meter
Common rail pressure sensor testing and cranking tests
Providing your sample rate is high enough then the number of events that have been
Taken as a reference point represent the wave form as displayed on the screen
Will then give you the accuracy you need if you want to zoom in and
Examine that image in great detail
So un-triggered free run which will report any events that which place
With a high sample rate gives us the accuracy
That we require for these kind of tests
Let me briefly explain the connectivity which has already taken place
Voltage is electrical pressure so we have
Channel A connected to the positive and negative terminals of the battery
The current clamp is around the circuit which goes to the consumer
The starter motor
I would just like to point something out
I have selected a 1 volt range for this tool not a current range
Speed is the decision here
The sensitivity attenuation of this clamp is 1 mV/Amp
I set 1 Volt as a range on channel 2 red which I have
1000 mV in 1 Volt 1000 Amps range it is that quick that simple
Remember accuracy speed efficiency
I also and this can be controversial
Like to see current going down for this test
Because I am taking it out of the battery
I am not measuring the consumption as a positive from the starter
But as a negative out of the battery
So I want to see both voltage go down and current go down
So I can visually compare them much more easily
Which you will appreciate when we run the test
We are set up and ready to go
I have disconnected the camshaft sensor which should prevent the injectors
From being phased or triggered so the vehicle wont start
I want to run this over an extended period
The scope is running
Let us reset the timebase so the sweep begins from now
Lets conduct the test and view the results
This is a timed out test it will prevent cranking after a limited period
ENGINE TURNS OVER
OK so around 10 seconds of test duration
Initially what I want to look at is voltage drop
You might be surprised at sometimes how much that voltage drop represents
The tool I am going to use here
We have got a fairly high sample rate 4 meg of samples over the sweep time
I want to focus on the voltage under load
You never test a circuit unless it is under load
I am going to use that particular tool to allow me to expand into that part of the image
And you can see we still have a very high quality image because of the high sample rate
I want to look at maximum voltage drop
We can see from the scaling
Channel A 7.67 sounds quite dramatic but that is a static engine
Lets have a look at the average
Now if you use a specialised tool perhaps other than a scope you will look at averages
Averages always worry me because you never are quite sure what the absolute values are
But the average voltage drop during cranking and these images represent compression
If we take an average through there we have 10.32 which is a respectable figure
We are looking for around 10.5
So how can we judge the voltage drop
Well actually we cant because it depends on what the consumers drove
Before we can assume that the battery is in good condition or not
Lets take a look at the consumer the actual starter motor current
Once again lets use that tool and expand into this part of the image
And you can see quite a dramatic consumption of current here
Once again lets measure the current
Peak current appears there
Channel B minus 917 that is a little bit generous lets go up a touch about there
905 m/V 905 Amps note how quickly that decision can be made
Because I have left that as scaling the voltage it means that when I have finished
That current measurement I can go straight back with my voltage probe
And carry on doing voltage checks
For me it is about speed
OK so that is an enormous amount of current
Does that mean the consumer starter motor is faulty
Not necessarily no
Is the engine cold how thick is the oil
How much physical work has it got to do in turning the engine over
These are all factors that effect the performance of the consumer
What is the average current consumption once the engine starts to turn
Lets take the cursor once again and take an average right through the compression events
Which are displayed there and we have minus 268 m/V 268 Amps
Is that reasonable
Well that depends on the size of the battery
The consumer should take about 3.5 times the Ampere hour rating of the battery
So we are looking for about 250
This battery has an ampere hour rating of 70
210amp plus a little bit more
We have 268 we are pretty much on the money
So the size of the battery should determine the average consumption of the consumer
Both of those are within specification so despite having what appears to be
A high voltage drop and a fairly high initial current draw
I have no worries about the amount of current taken from the battery
By the starter motor
Now the vital part
All of this will effect rotation speed
Now we also have formulas for rotation speed
This is a fairly old vehicle it needs to rotate at a minimum of 200 rpm
250 would be good
Tempered with that we now have modern stop start systems
And on those systems with more advanced battery designs we should be looking at 300 rpm
So I am expecting somewhere between 200 and 250 rpm
To measure rotation speed and calculate it is quite straight forward
If we use the horizontal cursors found in the left hand bottom column
Vertical cursors I should say
We take two events as it is a four stroke engine
So any two compression events is sufficient
The more accurate you make these events and they are reasonably accurate
And the calculation of rotation speed is 245.78 rpm
Absolutely on the money
So what have we proved here
Voltage drop
Current draw
Rotation speed
Based against the Ampere hour rating of the battery
Which all form that magic triangular performance which proves
The circuit and the consumer are correct to specification