IAC Valve gets 12volt with key on
The computer controls the Ground to open and close it, this is just safer that running 12volt wires, can't short, but could open IAC all the way if it did, lol.
Anyway, the computer "Learns" how to control the IAC Valve, if you change to a new IAC Valve then you can force computer to relearn it by unhooking the battery for 5 minutes or so, this clears computers "learned" memory, like it does the Radios "pre-sets", same thing, both have KAM(keep alive memory) that has power all the time, unless you unhook the battery
So after unhooking the battery, for any reason, your idle may wander a bit, even with original IAC Valve, until computer learns that 7.5v, for example, sets 1,000rpm when throttle is closed, i.e. TPS under 1 volt
The Ground is Pulsed, which in essence, lowers the voltage, same as if you pulsed 12v side.
But the voltage in a vehicle is not stable..........12v engine off and then close to 15v engine running, then it drops down to 13.5v after a bit, WTF!
So computer uses a Reference voltage, its Power wires, for IACs learned RPMs, it does a voltage difference not a set voltage.
So, for example: 12v - 7.5v = 4.5v
Computer uses 4.5v difference to set 1,000rpm, so doesn't matter what vehicle voltage is(up to a point, lol) it just lowers it 4.5v in this example.
Pretty much all sensors or controls use reference voltage in vehicles
This method is very very precise.
The feedback for computer learning IAC Valves RPM/voltage is, of course the RPM of the engine, so pretty straight forward, but....1994 and earlier computers(EEC-IV) didn't have direct RPM feedback.
1994 and earlier used external self contain spark systems, these sent computer the Pulse for RPM, so it could time fuel injection and also act as Vacuum advance(for TFI and EDIS)
Computer has "target" RPMs programmed into hard memory, my 1994 manual trans idles at 625 warm, only varies by 5rpm either way, so tight control
Temp sensor(not sender) causes computer to increase idle RPMs when engine/coolant is cold, so 1,100rpm on cold startup is normal
As engine/coolant warms up computer will close IAC Valve more until it gets to Target idle for your vehicle.
Computer also opens IAC Valve all the way for startup, so key on = full voltage at IAC Valve.
When you start any fuel injected engine, without touching gas pedal, it should REV to 1,500+ RPM, then drop down to temp controlled idle level
That's because IAC Valve was open all the way
IAC Valve without power will close all the way but that doesn't mean 0 air flow thru that passage in most cases, which is why there is an "anti-diesel" screw on the throttle linkage.
With engine warmed up and idling you unplug the IAC Valve and then set the anti-diesel screw so idle is 500 or so, barely running.
You need to be careful because this adjustment also effects TPS(throttle position sensor) voltage, which needs to be under 1volt when throttle is closed, .69-.99v is spec