Relative Compression Picoscope waveform
I am delivering a class today (and tomorrow) that includes some (Pico)scope basics and today we ran a relative compression test on the above vehicle. See the attached file of the waveform. Any thoughts would be appreciated! Thanks ahead of time, too.
Was there any problem with this vehicle? Also if you would be so kind could you repeat this test with a cylinder sync? If so then we can at least tell when the anomaly is occurring.
Yes. What tanner said. Need a cylinder sync. The higher amperage is the engine trying to recover after a low cylinder
Oh.. Thats neat. Can you send a file? Its a 6 cylinder so it doesn't seem to be related to a single cylinder event. Only thing I can think of is an exhaust restriction at a certain frequency, but sure would like to see that file.
Nevermind. Its upside down as well. I get too easily excited. Still would like to see the file if that was possible?
-Is this an amp probe capture or v-drop capture?
-is the acquisition AC-Coupled?
Tanner, Chris, Bill, and Brandon.
Thank you for your replies. I will attempt to answer your questions . . .
Tanner, there was no known issue with the truck; it is just a year+ old and 25K on the odometer.
As it is a diesel, is there a way I can repeat with a cylinder sync? I don't know of one. help, please.
Bill Hamilton, as the truck was just randomly pulled from the (DOT) fleet, there is no indication from the driver of exhaust restriction . . .
Brandon, the capture was done using a 600 amp clamp.
Thank you again!
Can I ask why you are not using the Automotive software. Also appears your clamp is backwards as the current is going negative instead of positive. How are you keeping vehicle from starting? Sorry so many questions
Some scope basics are needed to polish this up a bit. Yes, I agree a sync must be needed to Identify WHEN a problem occurred but, lets analyze what we have as is.
Your amp clamp appears to be backwards. (Scope trace down = current rising. Also everything is going to be in negative numbers.) Not impossible to interrupt but its trickier to analyze inverted patterns yielding negative numbers.
The anomaly appears to happen every 11 pulses. What can cause that? How many cylinders does this vehicle have? 8? How repeatable is this? Its a diesel so a sync isn't the easiest thing to obtain. Is this vehicle broken? Is there an audible hiccup in the cranking cadence?
Also, is this a pico 4425? Why arent we using automotive pico 6 software? Also, i'd recommend getting a whole bunch more time on the screen to show how repeatable this hiccup is. At the same time, lets turn up that sampling rate a bit so there wont be a loss of resolution at longer timebase.
Personally I think if you are teaching this test, lets polish up the basics first and I would choose an easier vehicle to do so on.
On the diesel you could use an injector to sync off of. Its not ideal but it will work for what you are doing. You can either use the turn on signal for the injector or you could use one of the pico wands for parking sensors and touch it to the fuel rail pipe for one cylinder and get a sync that way. It is easier to do on a gas engine so that may be easier test to achieve. Also be sure that the battery is fully charged as that can skew the results.
I'm still learning on this. But engine has a grid heater. It may have cycled off causing a shift in amperage. Next time try with key being on for a minute before cranking.
Jess, Thank you - excellent point! As you know Cummins prefers that air intake grid heater over the glow plugs that others use. By the way, the new Cummins ISV5.0 in the Nissan Titan DOES use glow plugs. I added that engine to my coverage of Cummins diesels because of requests for it . . .
Jess, a question: Any ideas on how to provide a #1 cylinder sync? Tanner (above) suggests using an inductive probe off an injector but, as you know the injectors on a Cummins are inside the valve cover. I tried an inductive pickup intended for C.O.P.s on the wiring to the injectors but no joy on that . . .
I was going to suggest maybe a current clamp on injector wire. But won't work if fuel system is disabled. I believe the crank sensor has a reference notch. But haven't researched into notch position vs tdc. Pico is having a webinar on January 16th on this particular topic. Their newsletter sent out end of November has details.