How to diagnose root causes from faulty waveforms?

badtruckshop · Post by **badtruckshop** » Thu Aug 20, 2020 1:27 pm

I am new to all things picoscope but I am very interested in accessing its full potential. I have been using the 4425a every day for the past week and can agree with Steve's introduction post that I have more questions than answers. My question arose from a fuel pressure test on 2012 Ford F-450 6.7l Diesel. I have a waveform that has some deviations from the reference waveform outlined in the guided test. So I know I have a problem (also because I have p0087) but how do I accurately analyze that waveform to determine what part of the fuel system I should be looking at as a cause of my problem? I see the benefit in having known good waveforms but verifying a problem is one part of the equation. Finding the root cause of the problem is what I really want to know. Or at least a direction to move to further the accuracy of the diagnosis.

Iver · Post by **Iver** » Sun Aug 23, 2020 7:10 pm

The scope will not always give you root cause of the fault. Neither will the scope make one a better Technician or help to fix more cars. It is Good Knowledge and Good Tools !!

My question arose from a fuel pressure test on 2012 Ford F-450 6.7l Diesel.

I'm guessing you attached the Scope to the Fuel Pressure Sensor ?? If so then you have not a Fuel Pressure test !!
Your Sensor Could be faulty !!!

I have a waveform that has some deviations from the reference waveform outlined in the guided test.

Don't go by the reference waveform, use the actual Technical Data for your Vehicle.

So I know I have a problem (also because I have (P0087)

P0087 (I think) is Rail Pressure Deviation (Negative)

That means that the Pressure Sensor is Sensing Less Pressure than Expected at a certain time !!
A good Scan tool will give you Actual & Specified Pressure. Again with a Good Scan Tool you can GRAPH that data and look for the deviation. Then you will have facts about why the code is being set.
Not every job is a Scope Job

You CANNOT scope Specified fuel pressure, that's within the software of the ECU. So here a Scan tool is of more use than the Scope !!!

but how do I accurately analyze that waveform to determine what part of the fuel system I should be looking at as a cause of my problem?

To be fair if your working on Diesel Engines you should know that ??

In General Your DTC can be caused by:

An incorrect Fault Code from a Bad Scanner - Verify the Code is accurate !!
Lack of Fuel.
Fuel Tank
LP Fuel Pump & Control Circuit not Delivering the correct FLOW & or Pressure to the HP Pump.
Fuel Lines
Fuel Filter
Inlet Metering Valve & it's Control Circuit.
HP Fuel Pump
Rail Pressure Regulator & its Control Circuit.
Over Pressure Blow Off Valve
Fuel Pressure Sensor & Circuit.
Injectors.
An ECU Input from another Sensor
A Faulty ECU.
(I may have missed something)
(Not all systems have all the components listed above)

All the above tests need to be done and need to be done at the point the failure occurs !! The DTC will only tell you that the system failed, Freeze frame will give you more information.

I would look at the fuel Supply (pressure and flow) to determine if the fault is before or after that HP Pump Inlet.
If the supply is bad work on the LP Side, If it is Good then move forwards to the HP Side.

The Scope will assist you with all of the above tests, you will need a pressure transducer WPS500 & A flow Meter, Amp Clamp .... as Accessories.
A few other tools & tricks too !

I see the benefit in having known good waveforms but verifying a problem is one part of the equation.

Correct. Waveforms & Scopes are not a substitute for Physical Testing and the correct data for your vehicle.

Finding the root cause of the problem is what I really want to know. Or at least a direction to move to further the accuracy of the diagnosis.

You have a great Scope (box ticked) But we all need to have nailed system strategies & Function, mechanical knowledge, and test methods.

There really is no point in any testing unless you KNOW what the result should be before you carry out the test, that will come from Training, Training, Training.

There are some great videos on Youtube, Books, Training Facilities etc to learn from.

badtruckshop · Post by **badtruckshop** » Mon Aug 24, 2020 12:01 am

Thanks Iver for the reply. I appreciate your statement "Not every job is a Scope Job". That is definitely what I needed to hear. I think I got too wrapped up in the initial excitement of having a new and capable tool that I was trying to center my diagnostics around the tool, instead of using the tool as a part of the diagnostics. I should have clarified about the fuel system fault example. I have the factory scan tool (dealer technician) so I actually did diagnose the truck already using my normal methods. I was attempting to duplicate my diagnosis with just the scope in an attempt to see if I could come to the same result faster or gain some insight/learn how to use it. I could have used another example of a smart alternator diagnosis in which I attempted the same thing.

I will continue to pursue learning and growing deeper in my understanding of how to use the scope to its fullest potential. I also got the WPS500x which is great, but I think I was just initially discouraged because I put too many diagnostic eggs in the picoscope basket.

If you or anyone knows of any resources that was extremely valuable in becoming more proficient, I am all ears. I have been watching youtube tutorials non-stop but I am always looking for more opportunities to learn.

muttnjeff · Post by **muttnjeff** » Mon Aug 24, 2020 2:33 pm

My experience has been that a example wave form is helpful but not something i would normally use to make a decision... It is just that an example.

What I find most useful when looking at systems that are unfamiliar are design & function info & wiring diagrams, once you have a understanding of how the system is supposed to operate you can then decide which tools are best for the job.

As Iver said, not every job is a scope job, however since this is a new tool it wont hurt to use it more often if only for becoming more comfortable in the setup process.

Good luck.

Rhothgar · Post by **Rhothgar** » Thu Sep 03, 2020 8:19 am

badtruckshop wrote: ↑
Mon Aug 24, 2020 12:01 am
If you or anyone knows of any resources that was extremely valuable in becoming more proficient, I am all ears. I have been watching youtube tutorials non-stop but I am always looking for more opportunities to learn.

I second this. It would be great to hear whose videos are worth watching especially for newbies to Picoscope.
Obviously there are the Frank Massey ones. Also ScannerDanner is worth a watch but unless someone explains FULLY in a video whether the sensor they are testing is hall effect, inductive or whatever, I think it is almost impossible to find a video that is 100% comprehensive (although the Picoscope ones are nice and clear).

Watched a ScannerDanner one recently in which he finds a short. It was about 50 minutes. He's clearly very knowledgeable which doesn't necessarily make it easy for a newbie to diagnostics to follow and when he didn't bother to read the instructions on how to use his new short tracer I very nearly switched off!

ronw38 · Post by **ronw38** » Thu Sep 03, 2020 9:37 am

All of the professionals that put there fixes out there do a great job they are not running in depth training courses, as others have commented nothing replaces understanding system operation and basic test procedures, with out these the scan tool and scope more likely add to the confusion. I consider my self very proficient in the diagnostic and repair battle, I often pick up tips and information they add to my skills. Silver bullets are a rare thing. Cheers Ron W

PicoScopes
& kits

For vehicle manufacturers

EV kits and
accessories

Noise, Vibration and Balancing

Milliohm tester

Pressure sensors

Current clamps

Breakout
leads/boxes

Accessories

Automotive Forum