Can somebody please explain for me the purpose of the Crankshaft Position System Variation. Does it have anything to do with how the PCM detects pinging and retards the timing? If not, what does it do?
If you install a new PCM a relearn is needed. I will have to do a relearn since i have bought a new PCM.

I feel like a complete :conehead having to ask this question but hey, you gotta learn sometime and i thing the best place to learn is here! :thumbs:

Thanks!

 

Lap,

the main reason for crank position variation relearn is compression.
As an engine ages, various things can affect the compression of each cylinder.
Piston ring and cylinder wear, valve seat affectiveness, valve lift and duration which is affected by all the components in the valve train, etc.

Let's back up a bit first. The crankshaft on a Gen III small block has a large wheel attached to it (permanently). It has several notches in it, the notches are oddly spaced in a special pattern. Of course since it's a "disk" it's a complete circle. The notches I say are oddly spaced are done so in patterns that are different for each 90 degrees of rotation, that allows the computer to identify exactly where the crankshaft is by watching the signal from the crank sensor. Basically the sensor watches the wheel and as the teeth and gaps pass it sends a high or low signal to the PCM. The PCM watches this signal and since it knows the pattern it knows exactly where the crankshaft is.
Now more importantly for OBD-II, for '96 and up vehicles, the computer has to watch for and recognize a misfire. That is an EPA regulation for OBD-II.
In order to do this the computer not only watches the position of the crankshaft but how "fast" it is going.
In combination with the Camshaft sensor, which tells the computer which half of the cycle the engine is on, the computer knows EXACTLY what piston is firing and which should fire next. As you know in a 4 cycle engine you have intake, compression, power, and exhaust strokes. Now on a V8 you have 8 cylinders doing this in concert. The computer knows when the compression stroke of one cylinder will slightly decrease the speed of the crank and the power stroke will cause a burst of speed that is noted also.
This is miniscule to us in the driver seat but it's pretty noticeable on a computer watching sensors for milliseconds at a time.

So the computer watches for the "pattern" of speeding up and slowing down caused by normal engine operation.
If the computer does not see the correct pattern, as in if one cylinder does not fire, the crank won't speed up for that power stroke, the computer calls that a misfire and stores a count in history. Enough counts in a diagnostic cycle and it will set a code.

Now back to variation. The computer knows what the "pattern" that has been established should be. If you change components or replace the computer the computer may sense normal operation on a high mileage engine as abnormal and flag misfires, this would be bad.
The relearn procedure forces the computer to learn the compression characteristics of your engine.
This is done by performing a special routine with a tech 2 scan tool.
You follow certain steps and then push the throttle to WOT (or the floor).
When the computer cuts off the fuel the engine "stalls" out, you let go of the gas and then engine revs free and decelerates.
As this happens the computer watches the "speed" of the toothed wheel to "learn" the compression characteristics of your engine.

If you don't do it on most engines it will set a code P0336 (if memory serves)
indicating the relearn has not been done.
This is in order to prevent the false setting of misfire codes, P0300 thru P0308.

I hope this clears things up a bit for you.
Please feel free to ask more questions if it has not.
I don't get back too often but I'll look back from time to time.
If you don't hear from me, send an email to the addy on my profile.

EWill

 

great explanation :cool:

 

As I wrote before, we will get great infos from him, thanks. :cheers:

Frederik

 

Laplander

To perform the relearn:

1) Place vehicle in Neutral (Park) with parking brake set and hood closed and install scan tool.
2) Start engine and allow to warm until coolant is at least 150F
3)Turn OFF all accessories (A/C, radio, etc.)

IMPORTANT While the learn procedure is in progress, release the throttle immediately when the engine starts to decelerate. Engine control is returned to you and the engine will respond to the throttle after the learn procedure is complete.

4) Enable the crank position system variation learn procedure with a scan tool.
5) Slowly raise engine RPM to 4000
Immediately release the throttle when engine speed decreases - engine speed will return to idle at end of process

Scan tool should indicate: Learn Status: Learned This Ignition

6) Turn OFF ignition and wait at least 15 seconds while PCM stores the learned data

If not, you can attempt this up to 3 times, after that, there are further diagnostics that you will need to perform.

Given that you installed a new PCM, it should "learn: on the first attemp.

Good luck,

Steve


[Modified by BlackZ06, 6:45 AM 5/13/2003]

 

Wow! Thanks a LOT you guys! :hurray: :cheers:
This helped me understand the procedure and how the system works!
A friend of mine with his own Tech 2 will try to perform this on my car this Thursday.

Can driving around carefully before the learn is done cause pinging??
(Magnuson installed) :D

:cheers: :cheers:


[Modified by The Laplander, 5:19 PM 5/13/2003]

 

It might cause an check engine light but should not cause pinging to drive before it's done.

You are quite welcome...

EWill

 

GOOD STUFF!!! :)