One waveform One picture

Ask for and share advice on using the PicoScope kit to fix vehicles here.
Steve Smith
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One waveform One picture

Post by Steve Smith »

There are a number of posts on our forum that are most certainly time consuming to digest yet invaluable when “researching” or developing a deeper understanding of the topic under discussion

These are the posts that require absorption in bite size chunks with breaks in-between whilst you mull over the content

I wanted to add a number of posts here where we add one waveform and one picture that I hope will help with relating the information captured in one waveform, to a physical mechanical fault with a component captured in one picture

Below we have a VW Passat Diesel 2.0 TD PD with slow and labored cranking from cold

Using the PicoScope 4823 I wanted to cover all avenues associated with poor cranking in one capture and so we have:

Battery voltage measured across the 12 V battery (Channel A) and at the starter motor solenoid output terminal (Channel E) Here we ensure battery condition and positive circuit volt drop (Inc. solenoid contacts) are correct through the poor cranking period

Channel B captures voltage drop in the starter ground circuit
Channel C captures engine speed/rotation using the crankshaft sensor
Channel D captures cranking current (work done)
Channel F Starter motor energizer wire to ensure no dropout of the energize signal
Image 1
Image 1
Above we confirm the voltage drop within both positive and negative circuits to be acceptable

We can see the energizer circuit to remain live throughout cranking

We capture the moment where the engine almost grinds to a halt (thanks to the crank sensor signal) during cranking, yet the current flow increases into the starter motor

The picture reveals worn starter motor brushes and more importantly, one positive brush is sticking within its housing and only periodically making contact with the commutator.

During the above test I hesitated about capturing crankshaft signal data as connecting to the crank sensor required more time and a little struggle. Looking back at the capture above, it proved invaluable to the diagnosis. Note that the mechanical condition of the engine was fine and turned over by hand with no issues or “tight spots” A replacement starter motor of course was the cure here.

I hope this helps and please feel free to add similar captures and pictures which bring the waveform to life and help with interpretation

I will add more as we go+

Thank you once again to Kevin @ Ives Garage for his invaluable support each and every year

Take care……Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

One of the great things about working for Pico is meeting people from all over the world (often virtually) and sharing in their success stories.

This is great capture from Renato in Bergamo who I met whilst supporting our Italian distributor

Here we have a Skoda Fabia, Engine code AMF which is a 3 -Cylinder Diesel power unit.
The vehicle has illuminated the MIL light and fault code P0321 has been stored (RPM Signal implausible)

Image 1
Image 1

There can be no doubt as to why the crank signal was “flagged” as implausible in the scenario above given the damage to the “reference” point of the pick-up ring.

Having knowledge of the pick-up ring configuration i.e., number of teeth and reference points about the entire circumference is essential to conclusive diagnosis as this is quite a unique arrangement (3 groups of missing teeth per 1 revolution)

Take care….Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Many thanks to Stuart White at CCM for this “support request” which we tackled collectively back in August 2019

Here we have a Volvo XC90 Diesel with popping into the intake and general instability at idle speed. Using the WPS500 pressure transducer located @ cylinder 3 we can immediately see an increase in the cylinder pressure during the exhaust stroke (180° to 360°)

The pressure decays at 360° ATDC compression when the intake valve opens (IVO) and approx. 1.9 bar of cylinder pressure is vented into the intake manifold (Here we have our “popping” in the intake)

So, what is the cause and why?

Using the phase, partitions and time rulers of PicoScope we can see a reduced exhaust valve duration thanks to compressed hydraulic valve lifters

As a rule of thumb, the EVO event is typically 25-30° before bottom dead center of the expansion stroke. (around 150° ATDC compression)

In the image below the EVO event is retarded by approx. 30° and opens around 181° ATDC compression.
The EVC close event is advanced by approx. 38° and closes around 322° ATDC compression
Image 1
Image 1
The EVC and IVO events are often a challenging area of diagnosis and very difficult to capture due to multiple events taking place around 360° ATDC compression (i.e., valve overlap and change of piston direction)

However, the above fault reveals no overlap & a crystal clear IVO event thanks to the uncharacteristic pressure build up in the cylinder due to an advanced EVC event

Such interpretation of the 4-Stroke cycle waveform will help identify the areas to focus upon when dismantling an engine, if indeed dismantling is required!

I hope this helps, take care……..Steve

ben.martins
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Re: One waveform One picture

Post by ben.martins »

We often talk about how CAN is so fault tolerant. Never is this more evident when it comes to large amounts of EMF that is caused by an electric motor.

For those of you that follow our case studies you'll know that I've been lucky (or unlucky!) enough to visit some less mainstream machines that we wouldn't expect to see in an automotive workshop. None more so than a visit to see a Hinowa tracked spider crane. Electric driven machinery has been around in access long before hybrid and electric drives were common place in the automotive world.
20200113_132839.jpg
Powering them is an electric motor that drives a hydraulic pump from which hydraulic fluid is used to operate the machine. Battery packs can vary between manufacturers but the lack of emissions make these perfect for using in enclosed spaces and indoor places where people could be exposed to fumes.

I was there to see if we could decode the CAN data to better understand a way of identifying nodes to help with diagnosis. When connecting to the network though and running a service it was clear that the level of noise present could well have an effect on communication.
CAN tolerence.png
As you'll notice though, when you perform the A-B math channel, the interference from the motor has gone and the decoding can be performed on the math channel.

This is classic example of when looking at communication issues and using the scope to view a differential network, don't assume that interference or noise is the cause of the problem without doing an A-B math channel as the chances are it will cancelled out. This is what makes CAN so fault tolerant!

Rfmotors1
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Re: One waveform One picture

Post by Rfmotors1 »

Hello,

We very much like the short case style idea, one page, one picture.

Here is one simple example where Pico helped us to find potential issue on the V6 Diesel Truck engine. The task was to carry out a check on the crank cam alignment after repair carried out on previous visit, just to make sure all is OK.
There was not any complaint from the customer and the engine runs without any problem according the customer.
Here is what we have concluded from the captured waveform of engine at idle.
Crank&Cam Correlation Analysed report.jpg
We believe there is a problem with "out of balance" flywheel which does not cause any problem at this time, no problem was reported by customer and no DTC was logged in. However, the power of being able to find the problem before it becomes a complaint is amazing. We will try to capture another vehicle waveform and compare it, to see if our theory is correct or not and will post the good known waveform here later.

Thank you for the reading,
Roman & Hameed
Attachments
Crank and Cam Correlation Analysed 1 waveform.psdata
(2.37 MiB) Downloaded 188 times

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Thank you for the feedback and for taking time to create these posts

Thanks to the helpdesk support feature at Pico (which you can access using the email support@picotech.com) we are privileged to share in diagnostic challenges from around the world

Here we have an inquiry from our Canadian customer looking at involuntary ABS operation of an Audi A4 as the vehicle speed changes

Such faults are often associated with damaged teeth about the circumference of the ABS pick-up ring, cyclic interruption of the ABS wheel speed signal as we have discussed here https://www.picoauto.com/library/case-s ... -low-speed and of course this illusive gem that proved to be incorrect tooth count https://www.picoauto.com/library/case-s ... -operation (Its all about the frequency)

With the integration of magnetic pole ABS pick-ups into wheel bearing assemblies, physical inspection of the pick-up is most certainly challenging by comparison to a physical inspection of a toothed wheel either pressed onto the hub or drive shaft assembly

In the scenario below, our customer has captured the speed signal from 4-wheel speed sensors and then applied a “math” channel in order to graph the frequency.

As we can see, there is a momentary, cyclic interruption to the frequency of the wheel speed signal connected to channel D, so confirming further inspection and dismantling of the relevant hub to find the cause.
Image 1
Image 1
I have to say at first glance above, the signal looks fine in Scope view 1 but the math channel reveals the subtle changes in frequency as a result of a deformed magnetic pole pick up ring.

Whilst we are on the subject of magnetic pole ABS pick-ups, the following link topic19361.html looks at an alternative use for our Keyless entry detector when it comes to the inspection of such wheel speed pick-ups

I hope this helps, take care….Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Following on from our forum post here viewtopic.php?p=70211#p70211, measuring cooling system pressure with the WPS500 pressure transducer not only allows you to capture coolant pressure, but also plot each pulsation against other events such as combustion.

Within the forum post we use the pressure transducer to identify excessive pulsations with the coolant thanks to a cylinder head gasket failure.

We also plot the problematic rise in coolant pressure against manifold boost pressure under load conditions during a road test!

The example waveform below captures an uncharacteristic rise in coolant pressure of 0.6 bar shortly after cranking and the pulsations are most certainly sequential
Image1
Image1
Using injector number 1 as a synchronization signal in conjunction with the phase rulers and firing order, we can identify one of two scenarios.

Based on the occurrence of the peak coolant pulsation, either the power stroke of cylinder 3 or the compression stroke of cylinder 4 is blowing through into the coolant jacket.

As can be seen from the included image, cylinder 3 combustion (power stroke) was the offender and the head gasket the reason for failure.

Please read though the full forum post here viewtopic.php?p=70211#p70211 as the information contained within carries a number of further tips and opinions

I hope this helps, take care……Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Here is a topic we discussed in our April 2014 Newsletter which surrounds budget/inferior fuses. (Our Newsletter archive can be found here https://www.picoauto.com/library/newsletter-archive )

The case in question surrounds fire damage to a vehicle wiring harness which routes from the cabin fuse-box up to a vanity light mounted within a sun visor

Long story short, the customer had repeatedly replaced the “Dome” fuse due to an intermittent short circuit which resulted in no cabin illumination. Having exhausted a supply of included 10 A fuses, a pack of budget fuses were purchased online and then installed.

What followed can be seen in the image below as the vanity light wiring has burned through the harness and roof lining! On this occasion it was quite helpful as it led me to the offending component but not so pleasing for the customer
Image 1
Image 1
Given the cause of the short circuit proved to be the combined vanity light/ sun visor assembly, it raised the question why did the fuse not blow and protect the wiring and trim?

Channel A above captures the current flow through one of our customers budget fuses, short circuited across a 12 V battery. Note how the fuse continues to pass current until I can not longer hold the test leads which were getting very hot! The momentary drop in current shortly after the trigger is due to connection issues. (I was not expecting current to flow for so long)

In contrast, the magenta reference waveform is an OE 10 A fuse which peaks at nearly 60 A for 34 ms before blowing

I hope this helps, take care…….Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Whilst we often measure for the presence of voltage to determine “control signals” or sensor outputs, it never ceases to amaze me just how much information is contained with a single current measurement.

Measuring current provides an indication of “effort” or “work done” and the ability of the circuit to carry such current under load conditions (I am thinking here of measuring injector current as an example)

There is a valid argument suggesting if the current flow is correct throughout a circuit, then the power and ground are automatically verified

Here is another example of how measuring current flow through a turbocharger actuator revealed a fault with the actuator motor assembly.
Image 1
Image 1
Above we have captured sporadic current flow through the turbocharger actuator motor and the included image explains why. Not only do we have poor commutator to brush “contact” (creating arching) we also have a number of commutator segments short circuited thanks to a build-up of brush and commutator material

The above captures were taken from this case study here https://www.picoauto.com/library/case-s ... power-loss which has the full story describing the diagnostic technique followed along with the post fix captures

I hope this helps, take care…….Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

We often discuss “non-intrusive” measurement techniques during the initial phase of engine diagnosis.

The results obtained inevitably lead onto the next level of intrusion in order to provide further “direction” or hopefully a conclusion

Cam and crank correlation tests may be considered “next level intrusion” (depending on accessibility etc.) where such tests provide “real-time” dynamic valve timing measurements with respect to the timing gear arrangements.

Carrying out cam and crank correlation measurements have been discussed here topic18471.html and here topic21754-10.html

Dynamic valve timing tests (i.e., with the engine running) provide real world evaluation of the camshaft drive system (belt, gear or chain) under varying load conditions

Free revving the engine during correlation tests allows for analysis of the camshaft drive system under “push/pull” conditions which therefore takes into consideration the following:

Chain and belt stretch/flex
Inadequate tensioning (whip)
Timing gear backlash
Idler gear wear
Slipper and guide wear

The waveform below has captured one such event where an initial manual inspection of the valve timing revealed no error!
Image 1
Image 1
The waveform has captured a cam and crank correlation error from a Quad Cam engine which you can read about here https://www.picoauto.com/library/case-s ... ning-light

Once you have obtained correlation data, the question then is “what does a good one look like?” This is where the waveform library comes into its own……..
More information on the waveform library can be seen here https://www.picoauto.com/library/waveform-library along with Bens video here https://youtu.be/8ePHLmRI5YY

“Food for thought” should your cam and crank correlation measurement confirm no fault; whilst valve timing can be correct at the timing gears, the valve train and in-cylinder valve timing can still deviate.

I hope this helps, take care……Steve

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