high way spark

[ony]

does the hyway spark only occur at a steady cruise or is it happening all the time once the speed parimeter is met?

[RobertISaar]

i assume the exact requirements will be mask dependent, but with the things I play with, have to meet the gear, TCC, MAP, coolant temp and RPM thresholds.

[84Elky]

As Six said -- Virtually all of the GM OBDI auto trans BCCs check Calibration Switches to determine the conditions under which Hwy spark will be invoked:
If TCC locked or unlocked
In HI (4th) gear only or in either LO(3rd) or HI(4th)

If the above conditions are met, the code checks the following 80 times a second. Hwy spark will be invoked if ('x' represents calibration item):
kPa <= x and
Coolant > x and
RPM >= x

If all above conditions persist for x seconds (generally 10 seconds) SA-TDC is advanced x degrees (x=kPa-based spark increase table value).