https://www.aulro.com/afvb/images/im...12/01/1069.jpg
So what I have done is measured the duration of each 10* of rotation (the small distance between each hole in the flywheel) of the crank and marked those time's in milli seconds in their repective spot. So my first 10* of rotation took 14.19ms. Where there is a 20* gap I have split the duration in two.
Now by comparing each duration to the previous I can tell if the crank is speeding up or slowing down. And for those who eyes are a bit shot I have shaded the areas of crank acceleration in red and deacceleration in blue. Alot easier to decipher.
You will also see as speed goes up or down so does the amplitude of the signal as well as the frequency.
In the first area, section A, which contains at least five holes does not have enough info for the PCM or me to tell where in the rotation the crank is.
In section B, five holes, we now know cylinder #3 is passing TDC (#3 injector fires at five holes every other time on the compression stroke (when diesels inject), otherwise when five holes past the sensor it's on the exhaust stroke). What we don't know is weather it's on the exhaust stroke or the compression/injection stroke. The crank is still speeding up, so no info there neither. And if all is OK #4 will be next at TDC.
In C, six holes. So that is cyl #4 at TDC, again we can't tell if it's exhaust or compression as that is all the info we have. But it is a good confirmation we are getting a correct sequence. So we can (as well as the PCM) be pretty sure that cylinder #1 is on its way up, exhaust or compression. But what we can see is the piston speed slows down, the duration between holes is increasing. The fast compressing air is resisting the starter. That tells me it should be on a compression stroke (I don't know if the PCM uses this info now or not) of number 1.
Section D, we see #1 piston is on its way up and the PCM decides now to inject fuel in number one cylinder. It's taking a stab it's got it right. After all it has no idea, for sure, what stroke, exhaust or comp, number one is on. But it could get lucky.
(I'm not sure why the PCM waits for #1 cyl to come up before it fires and doesn't have a crack at #4. It could be because it wants to use it as a confirmation of a sequence, it could be because it wants to see a certain crank speed, possibley for injector fuel pressure or air temp. Or is it looking for the piston slowing down to indicate a compression is occurring, I actually doubt this reason. My guess woul be fuel pressure but I just don't know.)
