Myth - CKP Signal Missing Tooth = TDC

Brian from Willoughby Diagnostician Posted   Latest   Edited  
Discussion
Driveability
Propulsion
Myth

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

+8

Keith from Collinsville

   

Instructor
   

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

Mike from Mount Pleasant

 

Mobile Technician
 

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

+4

Scott from Claremont

 

Manager
 

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

Chris from Commack

 

Diagnostician
 

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

+1

Ward from Saskatoon

 

Technician
 

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

0

Geoff from Lahaina

 

Diagnostician
 

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 still be on time.

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Ward from Saskatoon

 

Technician
 

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 I welded to my crank sprocket using the TLAR method (That Looks About Right) so my offset is 38 degrees.

A missing tooth at TDC would be doubly troublesome in a design using variable reluctance pick ups as the voltage also drops off significantly at low RPM. I used Hall effect pick ups for crank and cam in my build. I used cam sensors for a 2005 (ish) Dodge Caravan 3.3 as they are hall effect, run on my 5V Ref (no need for a second 8V Ref) and are fairly bullet proof in service.

My truck cranks over dead slow in cold weather if not plugged in. We see -30C (-22F) virtually every winter and every couple years or so we colder than -40C (-40F)

Ward

+3

Geoff from Lahaina

 

Diagnostician
 

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

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