Crank() function in Picoscope may help you find the TDC

Dmitriy Analyst Toronto, Ontario Posted   Latest   Edited  
Tech Tip
Driveability
Tdc
Ckp
Waveform

This quote from diag​.​net/msg/m438upc23y… :

"I decide to get the cam/crank sync capture to compare to the known good I have for this engine. The signals are there and plenty strong enough to be seen by the ECM. ... I decide to use in cylinder pressure to determine where top dead center is. "

made me thinking -- information about where TDCs are is already there in the CKP waveform. As a cylinder approaches TDC at compression stroke, the crankshaft slows down, and this is reflected in the CKP waveform. For VRS CKP sensors and their digital siblings, Picoscope provides crank() function in the Math Channels. TiePie scope has a similar function implemented.

diag​.​net/file/f1phz2eyv…

The number of teeth should be estimated from the zoomed-in CKP waveform.

Here is an example, which is not perfect -- it is a capture of an engine start at just 13kS/s, as described in article picoauto​.​com/library/case-s…

If we apply crank() function for it, we get this:

diag​.​net/file/f6bz49hlw…

The valleys in crank speed show TDCs, and, not surprisingly, dips in the starter voltage on Ch. D (=inverted relative compression test) follow the TDCs as well.

The crank() function is not perfect; for example, it leaves area around the sync notch blank. It does not show which cylinder gets to TDC. Its precision is limited. But if it helps to identify the problem (e.g., the ignition event does not stay with one cylinder, but floats around instead) and allows to skip the in-cylinder pressure test setup, it's worth a minute or so it takes to apply it, right?

+4
Mario Diagnostician
Weston, Florida
Mario
 

Thanks for sharing this Dmitriy. Using the count feature coupled with this makes short work of grabbing tdc's. using in cylinder waveforms to attain tdc is only good if timing is good. Leaning towers can burn someone. I like this method. Thanks.

0 Ð Bounty Awarded
Francisco Diagnostician
Vienna, Virginia
Francisco
 

I just don’t get why would you want to use this?

-1 Ð Bounty Awarded
Justin Mobile Technician
Herriman, Utah
Justin
 

When I get back home tonight I will show how I can do this with the ignition voltage signal I captured in that study. I was looking for an exact point for TDC that could be identified as the exact cylinder I was checking. It also helped to see the intake and exhaust openings to verify the mechanical timing of the engine was correct. so two tests in one.

0 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
 

Thank you, Justin, your case study waveforms should be good for a demonstration whether this method works in practice or not. And I totally agree that it may not show everything one needs to know to make a conclusive diagnosis. It would be wonderful if you shared some psdata files to play with as well.

0 Ð Bounty Awarded
Justin Mobile Technician
Herriman, Utah
Justin
 

Here is the same waveform. The green trace is the ignition voltage to the coil on the same cylinder the compression is being tested on in the yellow trace. The picture with the green line relatively flat is the original capture. There was no ignition taking place during this part of the capture. The second picture with the green trace showing humps is the result of me manipulating the voltage

+3 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
 

Very nice captures, Justin! I won't be able to take a closer look tomorrow, but I will soon.

0 Ð Bounty Awarded
Dmitriy Analyst
Toronto, Ontario
Dmitriy
   

Alright, everyone, here is my take on what I saw in the waveform after applying crank() function in Picoscope. I found significant deviation of crank position angle from the values predicted by Picoscope Phase Rulers. youtu​.​be/NzxdS5Pu3Hw Please discuss! P.S. Justin, I hope it is OK with you to publish this video for general public. Please let me know. diag​.​net/file/fd22be4vz… ​

+2 Ð Bounty Awarded