Ignition timing determination?

[Jerryl]

I have been thinking: How is the ignition timing determined?

This is my understanding.
The crank sensor determines timing BTDC and the cam sensor determines cylinder fired. Sure, 2 fire at the same time in a ate spark system.

The balancer has a interupter ring with 3 windows.
Which tab/trailing edge (?) determines timing and is it always the same edge/tab?

If not, does the tollerance/variation of the spacing between the edges have any impact?

 

[RmvBfrFlght]

During cranking (non-EST mode) the trailing edge of the shutter is used to trigger the ICM for spark plug firing. This is done at 10* BTDC.

During running (EST mode) the leading edge of the shutter is used by the ICM to send a signal to the ECM for spark plug firing. This is at 70* BTDC.

The width of each shutter and each opening (window) is 60* of rotation. This places each leading edge 120* apart, same as the crank throws for the rods. Same for the trailing edges, 120* apart.

The GM shutter wheels are fairly accurate, the placement on the damper not so much.

RemoveBeforeFlight

 

[Jerryl]

Thanks for the reply.

So if timing for cyl 1 is verified and the leading edge is off of the other tabs, will the ignition timing will be off on the other pairs of cylinders?

 

[RmvBfrFlght]

Thanks for the reply.

So if timing for cyl 1 is verified and the leading edge is off of the other tabs, will the ignition timing will be off on the other pairs of cylinders?

Yes, any variance of the 120* from the leading edge of a shutter will cause a timing shift of the plug firing.

I have to edit this, unless the balancer/hub has been accurately marked every 120*, the following won't work. Not sure what I was thinking when I posted this:

This is easy to check once cylinder #1 has been verified. Move the timing light to cylinder #3 and check the timing, then on to cylinder #5 and check the timing. They each use a different shutter, covering all three by checking those cylinders.

Edit: this is correct:

On the stock ECM (and the EBL SFI-6 ECM), shorting A&B on the ALDL connector while running forces the spark to be at 15* BTDC. The timing tab has a pointer for that value.

RemoveBeforeFlight

 

[Jerryl]

Thanks a lot for responding.

PS:
I need to do a better job reading my typing before posting. My post made no sense to me. Lol