Upon initial crank, the ICM (ignition control module) starts sending DRef pulses to the ECM (5v square wave)
Bypass will be low (0 volts) and EST will mirror DRef with the ICM pulling EST low (500mv square wave)
through a resistor.
Wow... either no one is reading the page or this one is too easy or too hard...
Lets review....
Upon initial crank, the ICM (ignition control module) starts
sending DRef pulses to the ECM (5v square wave)
Bypass will be low (0 volts) and EST will mirror DRef with the
ICM pulling EST low (500mv square wave)
through a resistor.
Once DRef reaches a specified rpm (approximately 200 or 300
rpm) the ECM will apply 5 volts on the bypass line
which will cause the ICM to internally switch circuits and no
longer pull the EST line down. EST will now be a 5 volt
square wave that leads DRef (timing advance). Now you should have
enough background information to figure out
what is going on.... right?
BTW, no need to inform iATN support that I'm stealing their
images. I am the one who drew and uploaded them there...<bg>
If you are a sponsor of iATN, you can do a TDF forum search using
the phrase "Code 42" and find not only alot of
information, but also an animation based on the frames you see
above.