That lead comes from the ECU/PCM. It is a bidirectional data bus that the FCM and ECU use to communicate. Measuring it's voltage tells you (almost) nothing. You must look at the width of the pulses coming from the ECU to know what the ECU is telling the FCM.

OR, you can do this. Start engine when COLD. Fan should not run. Turn on A/C. Fan should run at low speed.
Now, CUT the gray wire, the fan should take about 30 seconds to warp up to very high speed (this is emergency mode, the FCM knows it is not talking to the ECU so it is trying to protect the engine). What also happens, is the ECU knows it is not hearing from the FCM so it lights the little "radiator" or "coolant" light on the left end of the instrument panel.

Now, that this information and continue your diagnostics.

One more thing, there is a SMALL wire (red/gray I think) that is 12 volt switched directly from the ignition switch - does it have power on it when the key is on?

I do have a criticism for you: Troubleshooting the electronics of the car without using the service manual is, generally, a waste of time. We can help you a lot, but even after reading my text, the manual will help you a lot with wire colors, connector pinouts and theory of operation (tho I admit the service manual was written very light on theory as they assume the average "mechanic" cant understand electronic theory anyway).