Idle Air Control
The early type is a simple solanoid which allows a vaccum to pass to a set of bellows that move against the throttle plate stop.
Most people remove these when going to a stand-alone system, as they do not work very well after 25years of service.
The newer 20v engines have a two coil IAC (marked as A in this diagram)
One coil opens the IAC valve, the other closes it, with neither coil active the IAC valve sits at 50%.
A simple solution is to use a Relay to ground one coil or the other
In order to PWM the solanoid for the best results a FET driver with one inverting and one non-inverting output (such as the TC4428) may be used:
