Where is the eletronic spark timing circuit?

 

The PCM controls the ignition module that tells the coil when to fire for proper timing. Crude but hope that helps.

 

Give more detail on what you are chasing. If it is an ESC trouble code, it's for the knock sensor, not ICM.

 

Yep.... I'm guessing you got DTC 43, looked it up on a generic code list, and found it referred to the EST circuit. If that's what you did, always use Shobox's list, which is specific to the LT1. See link in his signature.