The coil gets 12volt with key on
The ICM also gets 12v with key on
The ICM powers the 12v Hall Effect sensor in the distributor(but not the coil)
Hall effect sensor produces a 12v on/off pulse as "vanes" in the distributor shaft spin passed it, that pulse goes to ICM
ICM then pulse the Coil's - wire to unGround and ground it, which causes it to spark, based on the timing pulses from Hall Effect sensor
These timing pulse are also sent to the PCM from the ICM, not directly from distributor
PCM will turn off the CEL when it "sees" valid timing pulses from ICM
Unhook your coil's wires
Key on
With key on you should see 12v on one wire and GROUND on the other, the ground is important, thats from the ICM
Ground is the default state of ICM for its coil - wire, so coil can "power up"
An ignition coil is a pretty simple device
It has a Primary "Coil" which is what it sounds like, just a coil of wire but wrapped around a metal tube
When the Primary coil has 12v and a ground then voltage/current can circulate thru the coil of wire and that makes the metal tube an electro-magnet so creates a Magnetic Field around it
In the center of the metal tube is another coil of wire, the Secondary coil
The Magnetic Field in the Primary coil causes the Secondary Coil to build up a "charge"
Easy version
If Primary Coil has 100 windings/loops around the metal core
And Secondary has 10,000 windings
The 12v will become a 12,000volt charge, low amps but high volts
To release the 12,000v charge you simply disconnect the 12volts from Primary Coil and Magnetic Field goes away and the "charge" is released, you have a spark
You can cut the Coils + or - connection and get a spark, but the - side is usually used
Back in the "old days" we used Points in a distributor, it was hooked to the Coil's - terminal, just safer to use ground side
Distributor shaft has a cam with lobes match cylinder number, i.e. 6 lobes for V6
The points would open and cut coil's - connection when lobe push it open, then reconnect coil's - between lobes to generate another 12,000v spark in Secondary coil
You can test any ignition coil by powering it up and then disconnecting the power, it will spark if its working
Electronic ignitions replaced the points, which always wore out and needed to be adjusted(so bad old days really)
They use a sensor in the distributor that had no contact with spinning distributor shaft, so wouldn't wear out or need adjusting, it read vanes, 6 vanes for a V6
And a electronic module the would cut the coil's - terminal same as Points did, and reconnect it in between sparks
Start of the good old days, lol