That is because the IAC valve gets it's instructions (in terms of
voltage) from the ECM, not from a hot ignition. You should see the
valve open partly or full in fast-idle mode when you first turn the
engine on, until kick-down. The fast-idle warms up the cold engine by
allowing more air into the intake, increasing the idle speed to shorten
the time it takes to get the engine up to operating temperature. That
is the basic concept, anyway. Some of the Chilton manuals have great
descriptions on how the ECM controls the different states of the intake
components: idle, fast-idle, wide-open throttle (WOT) and limp mode.
Simply turning the ignition on won't actuate the valve, the engine must
be running. A catch-22 if you are trying to determine how much the
valve is opening, because the engine won't run without the valve
One question, what do you mean by "doesn't start, but idles OK"?