Myth - CKP Signal Missing Tooth = TDC

Brian Diagnostician Willoughby, Ohio Posted   Latest   Edited  

Myth or Fact? The missing tooth, circled in the capture below, in a Crankshaft position sensor signal, represents Top Dead Center on # 1 cylinder. 

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Keith Instructor
Collinsville, Oklahoma
Keith
   

It is almost never TDC, just a location that is marked and its position in degrees is known by the PCM for timing strategies. I'll see your question Brian and raise you another: WHY is it not TDC? (FYI, Brian knows the answer, I want others to chime in)

+3 Ð Bounty Awarded
Mike Mobile Technician
Mount Pleasant, South Carolina
Mike
 

I would guess it's not at any cylinders TDC because you would need as many teeth possible to detect a misfire accurately?

+4 Ð Bounty Awarded
Scott Manager
Claremont, California
Scott
 

Good point, thinking about this further, if you were tasked with measuring crankshaft acceleration after each firing event, what would you need? Lets see if we can get Brian to show us how to find the number of crankshaft degrees the missing tooth occurs at.

+2 Ð Bounty Awarded
Chris Diagnostician
Commack, New York
Chris
 

This is one of my favorite myths. I hear this all the time that people think this is number one tdc.

+1 Ð Bounty Awarded
Ward Technician
Saskatoon, Saskatchewan
Ward
 

I built a Mega Squirt injection system and put it on my '82 dodge truck, so I know why. You want me to spill the beans or do you want some one else to get the answer? Ward

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Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

It's been three days, may as well spill it. I have honestly never thought about it before. I don't design them, so I never needed to know, but now I am curious. As long as the PCM software is designed to match the installed crankshaft I can't think how it would matter where on a CIRCLE you place an extra space. You could set my watch two hours ahead, and tell me we open two hours late, and I'd

0 Ð Bounty Awarded
Ward Technician
Saskatoon, Saskatchewan
Ward
 

Hello Geoff! I didn't know you were on here! When I was building my Mega Squirt system the ECM had a parameter for the number of degrees the missing tooth was from TDC. They recommended placing it between compression strokes (45 degrees for a V8) as the cranking RPM is faster and more linear there, allowing the ECM to pick up the missing tooth more easily. I built a 36 - 1 toothed wheel that

+3 Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

Ward, yep, just signed up. "TLAR method " - I love it! Makes perfect sense about avoiding compression strokes. Thanks.

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