Read PDF posted below for GM Diagnostics of IAC.
Some ECM PCM may have differant wire colors. Check your wiring diagram. Letters A B C D will be on IAC. A B will show but have to remove plastic clip holding wire pins in to see C D.
Blue wire white stripe to A on IAC connector.
Blue wire black stripe to B on IAC connector.
Green wire white stripe to C on IAC connector.
Green wire black stripe to D on IAC connector.
Use a jumper wire to cross A and B on ALDL diagnostic port!
Then use an OHM meter and check between A and B? Should be greater than 20 OHMS. Check OHMS between C and D should be greater than 20 OHMS? If not replace IAC.
Diagnostic procedure says to use test light and all termianls should light... but I do not have one other than 12 volt. They are refering to a test light that will light for any voltage. So I used a DMM (Digatal Multi Meter).
Testing wires from harness to IAC connector.
A should be about .5 volts.
B should be about 12 volts or batterey voltage
C should be about 12 volts or batterey voltage.
D should be about .5 volts.
If not check wires are connected to proper pin at ECM. Then check wire for break by connecting ohm meter to each end of wire for continuity. If all wires are OK and all wires are connected to proper pins on ECM? Replace ECM.
This would be exact GM test procedure from IAC diagnostics PDF.