One waveform One picture

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renato torri
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Re: One waveform One picture

Post by renato torri »

Steve Smith wrote:
Mon Jan 04, 2021 9:12 am
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)


CRANK SIGNAL.png


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
many thank Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Measuring power supplies under load conditions provides conclusive evidence about the integrity of the circuit under test

The following case study https://www.picoauto.com/library/case-s ... -displayed is a typical example where measuring voltage (under load) revealed a concern with the circuit that later turned out to be a broken wire within the wiring harness.

The waveform below captures the 12 V “Crash signal” voltage measured at the battery management ECU under load conditions during vehicle “power up”

Note the stability of the 12 V signal until load is applied.
Image 1
Image 1
For the first 2 seconds (approx.) of this measurement, the voltage is stable as no load is applied and this is where we can fall over during diagnosis with the wrong scope settings!

When using the scope to monitor and capture voltage dropout: Select a slow time frame (2 s/div) where-by the waveform is drawn across the screen in “real-time” which can be interpreted by the human eye during events such as ignition on/off, load application or wiggle testing.

While the scope can also be used to capture these events at ultra-speed with triggers, it is difficult to interpret and link waveforms to physical events, such as wiggle tests that will only be displayed on-screen for a split second!

In such cases, the user would then have to scroll back through the waveform buffer to find an event that may or may not be linked to ignition on/off “loading” or wiggle testing.

Given the “Crash signal” is not typically used for “powering” devices and serves as an indication of the presence of voltage, we may need to apply load from an external source.

In such a scenario the use of a test bulb or “loading probe” will provide the load required whilst we simultaneously monitor the voltage level with PicoScope

Of course, is goes without saying, we must know the circuit and wiring under test before applying load; especially at wiring connected to ECU’s

I hope this helps, take care……Steve

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

An interesting case here surrounding prolonged cranking after the installation of a new starter motor. (The Starter motor was replaced for intermittent failure to crank)

Whilst the intermittent cranking nature proved to be unrelated to the starter motor (see here viewtopic.php?p=101532#p101532) when the vehicle would crank, cranking time would often exceed 5 seconds

Below we monitor the crankshaft sensor signal and cranking current which I now believe should always be measured simultaneously in order to relate cranking errors to mechanical or electrical fault conditions as we have here viewtopic.php?p=101377#p101377
Image 1
Image 1
The waveform above confirms prolonged cranking with very odd current peaks during the rotation of the starter motor. Note how the current appears to be switching on/off with periodic gaps where no current is flowing. For every revolution of the starter motor the current is switching on and off approx. 23 times which equates to the same number of segments on the commutator!

Thanks to feedback from Liviu, this is often the case with new starter motors where the “bedding in” process between the brush and commutator is incomplete. From the included image we can see the uneven patina/film coating about the surface of the commutator which I must confess I have never experienced or fallen foul of until now!

It would appear to be typical phenomena with reconditioned starter motors for numerous reasons such as “final finish” of the brushes & commutator, material properties, environmental use, brush spring tension and alignment

Once again, thanks to Liviu for the link to the Mersen site here https://www.mersen.co.uk/ which is well worth a visit when/if you have time. Click on NEWS & MEDIA >Technical documentation and scroll down through a feast of articles to find commutator/brush publications

Whilst the internet has its faults, from an educational point of view it is heaven sent and the majority of study material is free!

Moving onto prolonged cranking, note the condition of the crankshaft signal during starter motor operation. The inductive crankshaft sensor signal has been adversely affected by the electromagnetic interference (EMI) emitted from our new starter motor! Such interference may “consume” our inductive crank signal via the wiring harness where there is a capacitive link between the main starter motor positive cable and adjacent wiring or indeed, via the engine bay environment itself.

The irony in all of this, refitting the original starter motor cured the problem!

To digress a little; I remember a modification years ago on Ford Escort/Orion CVH with a similar prolonged cranking condition (Forgive me if I have the model incorrect) Often, aging or inferior starter motors would emit high levels of EMI where the cure was to separate the crankshaft sensor signal wire from the main starter motor positive cable and include shielding to prevent the conditions described above!

Returning to the capture (Scope view 2) note the sinusoidal appearance of the cranking current peaks which resembles conventional cranking current where each peak denotes a compression event.

Finally, after prolonged cranking we seem to have a reduction in peak cranking current, combined with an increase in engine speed and no doubt a reduction in EMI such that the formation of the crankshaft signal is restored sufficient for interpretation by the PCM, so allowing the engine to start

Hindsight is a wonderful thing and after discussing the above with Ben it came to light that he would always bench run a starter motor prior to installation which is something I have never done! It is certainly food for thought and I will consider on the next adventure

I hope this helps, take care…..Steve

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PicoKev
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Re: One waveform One picture

Post by PicoKev »

Certainly an "Interesting Adventure....." :)

Replacement fuel pumps are another example of horrible looking WF's measured immediately after fitting there does however seem to be a distinct correlation between the "Quality" of the pump and the quality of the WF! :roll: .

Measure them after a bunch of miles and they look perfectly "Normal" whatever that is?

I do not remember the CVH issue but I did a shed load of Vauxhall cam sensors, back in the day, that were modified from a 3 to a 4 wire. The new sensor was supplied with a plug and pigtails.

The extra wire had to be added to the foil shielding in the the cable. The issue was that the wiring and to some extent the sensor itself were practically on top of the alternator......!!! :roll: :roll:

It was always amusing to see the number of vehicles coming in that had had new sensors fitted (after all that was what the code said! :D) that had the "Spare" wire just hanging next to the loom (made a lovely EMI collector aerial).

Kev.

ben.martins
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Re: One waveform One picture

Post by ben.martins »

One of the first HGV case studies I created with Pico was the looking at a Scania R480 with poor idle, black smoke and misfires. This particular engine was developed by Scania and Cummins and contained 2 crankshaft sensors.

I’ve still yet to find the exact reason for this with some Pico users offering some insight where it could be a back up sensor, more accurate fuel timing and engine balance being suggested. Either way, what was clear is that they both sensors have to provide the correct signal for the engine to run correctly.

From the initial capture we could see that the signal from one of the crank sensors was giving a faulty reading. Fortunately, being in a yard of similar vehicles we found a known good which we could then use as a reference and by adding these to a view port in PS7 we can keep the known good whilst we captured the faulty sensor signals.

With enough evidence, the sensor was removed and found to be in a poor condition. Further investigation found that a number of the teeth from the flywheel were damaged and chipped which may well be the reason for the damage to the sensor.
1pic1wave.png
A new sensor was fitted and the waveform confirmed to be correct with the known good and vehicle was back on its way. You can read the full case study here - https://www.picoauto.com/library/case-s ... ault-codes.

I hope this helps,

Ben

RyanH
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Re: One waveform One picture

Post by RyanH »

After a decade of using a Picoscope at work, I find that I have fallen into a bit of a trap. On late model vehicles, scan data and the PCM's ability to identify a fault has greatly improved. I have realized that I am relying on my scan tool more and not pushing myself to scope newer vehicles. This is a mistake. I specialize in diesel engines and this post is centered around the glow plug system.

Monitored glow plug systems are not new, modern diesels use glow plug controllers that can monitor the current draw of each glow plug and set a DTC if there is a fault. Last week, a 2004.5 Chevrolet Silverado 2500 HD with the 6.6L Duramax engine code LLY came in for a hard starting issue with excessive exhaust smoke after a cold start. This engine was the first Duramax to use Beru's Instant Start System (ISS). Unlike older system that used a relay to energize the glow plugs, this system uses power mosfets for each glow plug and will PWM voltage to limit current and maintain glow plug temperature. In this system the glow plugs are rated for 4.7 volts and will reach 1000°C in 1 to 2 seconds by being fed 12.6 volts.

Now the vehicle in question had no codes for the glow plug system and in the past I would have moved on to compression or the injectors for the root cause of this issue but I decided to measure the current draw instead. To my surprise the glow plugs were only supplied full current for 11.74ms. I also noticed that the current draw peak was slightly lower than normal around 137.6 amps. This was obviously an issue but why did the system not trigger a code?

I have replaced many of these modules but never for a fault like this. I decided to pull a few glow plugs and take a look. The first two I removed where an aftermarket glow plug and the third was the same but had the voltage printed on it.....10.5V. I checked the part number on the manufactures website and sure enough they were all 10.5 volts and the application was from 2001 to 2005. The glow plugs in a 2001-2004 LB7 Duramax have the same external dimensions as those found in a 2004-2005 LLY Duramax but operate very differently. The LB7 glow plug is self regulated and does not start to glow until 4-5 seconds of system voltage has been applied.

The lower than normal current draw is a characteristic of this brand of glow plug. I have seen this is in other applications as well and to be honest they normally don't last 6 months without several burning out. In this case the glow plugs would never have heated up from the day they were installed. I suspect the low current draw triggered the control module to PWM the voltage thinking that the glow plugs were already at operating temperature. My takeaway is not to rely on the PCM to diagnose a system for you even if it appears to be accurate 99% of the time.
LLY-Glow-Plug-Current-PS.jpg

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Hello RyanH, thank you so much for taking the time to add your study here and sharing what will save others hours of "heartbreak" before arriving at this conclusion

Take care.....Steve

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Re: One waveform One picture

Post by Steve Smith »

Diagnosis is challenging as we all know and often there is only a very small window of opportunity to capture an intermittent event.

Choosing what to measure and when can be a lottery and usually led by experience based on the symptom. For example, “Investigate intermittent misfire from cold with a petrol car”

Assuming you have a 4-channel scope, we have to choose 4 signals to capture and hope that one will present a clue as to the cause, or if not, eliminate a number of components in the process

Based on the example above, we might go for:

Ignition
Injection
Crankshaft signal
Exhaust pulsation

Again, the above is an example and not a recommendation as our approaches are all different but have the same goal. From these signals we hope to qualify ignition and fuel “events” combined with crankshaft acceleration/deceleration and any combustion anomaly “felt” within the exhaust gas pulsations. (Not a bad start for sure)

The ability to include more signals in those precious seconds where the vehicle misfires become invaluable and this is where the 8-Channel scope comes into its own

We can now add to the example above with:

Camshaft position sensor (often more than one)
Intake manifold pressure
Fuel rail pressure (GDI)
MAF

With the additional signals above we can now evaluate dynamic valve timing, intake pulsations, fuel delivery pressure (if applicable) and MAF voltage values or indeed pulsations/disruptions within air flow. Of course, the list goes on and on depending on engine design and configuration

The following case study https://www.picoauto.com/library/case-s ... -from-cold tackles one such vehicle where a momentary misfire was captured with an 8-channel scope so providing the evidence required to make an accurate diagnosis on a costly component
Twin Air actuator
Twin Air actuator
Above, we capture the momentary failure of the inlet valve to fully open resulting in low compression, a reduced intake “pull” (within the intake manifold) and of course, a combustion anomaly fondly referred to as a misfire

When you look at the entire capture spanning 10 seconds, the engine misfires just once which you could attribute to any number of reasons (without objective data) However, the capture above confirms why the engine misfired and more importantly eliminate a number of other components in the process.

There is no doubt the above can be diagnosed with 2 or 4 channel scope, however, given the nature of the symptom (momentary misfire from cold) once the engine had started it was required to return to cold before further tests could be carried out. This will no doubt increase diagnosis time and introduce possible correlation/measurements errors as we try to align signals between multiple captures

I hope this helps, take care……Steve

ben.martins
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Re: One waveform One picture

Post by ben.martins »

Without fail, the relative compression is by far one of my favorite tests. Not only can it bring you a lot of information with one measurement but it's typically non-intrusive, quick and easy to spot an issue. It also helps to settle your mind. I've often found myself approaching a machine thinking where do I start which is where I turn to the relative compression test. At the very least it will give you some direction!

When it comes to a misfire, the relative compression test is always a must and by combining additional measurements you can quickly build a picture of what is happening within the engine. To keep things as easy as possible using a current clamp around the starter/battery cable, WPS500X in the exhaust and if possible a way to identify a cylinder. Not easy on a diesel as preventing the injectors from firing results in us not using injector current as a way of identify. CAM sensors are usually the next best thing. More often than not though the relative compression check is enough to give the direction for the next test.
pushrod snapped.png
The machine in the above capture had reports of a misfire. The relative compression test shows us a dead cylinder where no compression event has taken place. By having WPS500X in the exhaust we can also see how this dead cylinder contributes to the exhaust pulse. What we can see is that there is a sharp change in pressure which when using an overlay tool, or by changing the phase rulers to 4 partitions to highlight the 4 stroke cycle for each cylinder, we can identify that this exhaust pulse anomaly is directly associated with the dead cylinder.

We could leave the diagnosis here and just start removing parts but with this information we could further determine the cause of the misfire. We know that the exhaust pulse has a sudden change in pressure which in this case is being pulled down towards a vacuum. This can happen if the cylinder has a much lower pressure than the exhaust manifold causing a rush of air to fill the cylinder against the normal flow of air. The only way we can achieve a lower pressure in a cylinder just before the exhaust valve opens is by having a vacuum before. This is achieved when the inlet valve does not open.

As no air can enter and charge the cylinder, during the inlet stroke a vacuum starts to form. When the piston starts to come back up for the compression stroke this vacuum helps to drag the piston back up the cylinder and as there is only a small amount of air to compress, less current is required by the starter motor in order to push the piston over it's compression stroke. However, as this then enters the power stroke the pressure changes from positive to negative and when the exhaust valve opens the air in the exhaust is pulled into the cylinder before being forced out again as the piston travels up on the exhaust stroke.

As you can see in the image this machine had a broken push rod for an intake valve. In my career I've never seen a push rod snap like this and can say that this had broken 3" from the top. A new push rod has been ordered and then a full reset of the valve clearances will be carried out before re-capturing to ensure verification of the fix.

I hope this helps.

Kind regards

Ben

Steve Smith
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Re: One waveform One picture

Post by Steve Smith »

Product knowledge is a wonderful gift to hold and often reinforces why specializing in one product has many benefits

Here we have a Jaguar X-Type with the engine MIL illuminated and fault code P1699 stored within the engine management ECU

DTC P1699 refers to CAN network communication error with AC control module & Engine ECU which was interesting given the AC control panel was also inoperative

In addition to the above, the serial diagnostic tool could not communicate with the AC system and displayed “No system found” after a vehicle scan

Long story short, after inspecting the wiring diagram, our AC control module was missing a +30 power supply due to fuse 75 blown at the fascia fuse box behind the glove box (RHD vehicle)

The time it has taken to type the above does not do justice to the time spent to arrive at this conclusion but that is another story!

I am sure we are all familiar with the above when diagnosing vehicles with minimal product knowledge & generic technical information which I touched upon here https://www.picoauto.com/library/case-s ... ure-danger

Moving on, after replacing the fuse, all appeared well until operating the door mirror fold feature where they began to labor in operation followed by fuse 75 failing again.

A further review of the power distribution circuit from fuse 75 revealed our +30 supply is shared by AC control module, Alarm siren & Door mirror control module

Using the technique of measuring across the fuse board contacts to determine the location of our short circuit, the Alarm siren was identified as the cause by simply disconnecting and noting the voltage fall across the fuse board contacts

The fault was rectified and vehicle returned to the customer, however I was intrigued why the fuse did not fail immediately when replaced (Remember the AC & folding mirrors functioned as normal on initial replacement of the fuse)

Bench testing the siren whist monitoring power supply and current flow revealed the cause as water ingress and corrosion of the internal “battery pack”
Image 1
Image 1
Note how the current instantly rises to approx. 4.3 amps (below the 7.5-amp rating of fuse 75).

Had this been a direct short to ground, the current would have climbed exponentially as we have discussed here viewtopic.php?p=101571#p101571 and blown very quickly indeed as the fuse temperature increased

In the scenario above our current flow continues to rise steadily over approx. 3 seconds where the heat generated by the current flow (for a relatively long period) would result in a temperature rise and fuse failure

The nature of the current flow suggests the siren is acting like a consumer (possibly recharging the battery pack) rather than a short circuit which fits perfectly with the symptom during diagnosis of the vehicle

With regards to fuse “head room”, i.e., how much current is required to blow a 7.5-amp fuse, this is dependent not only on current, but time. Referring again to viewtopic.php?p=101571#p101571 we had an OE 10 A fuse peaking at nearly 60 A for 34 ms before blowing which is no surprise given the rapid rise in fuse temperature associated with a dead short

Above we have a 7.5-amp fuse blowing at approx. 10.4 amp which suggests the fuse has minimal “head room” above its 7.5-amp rating. However, the continual progressive increase in current flow (approx. 3 seconds) allows for the temperature to increase to the point of fuse failure. “Time” being the contributing factor in this scenario

I hope I have not broken the “One waveform one picture” rule using a thermal as well as a conventional image of the siren!

I hope this helps, take care…..Steve

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