Electric fan control confusion

[NC_John]

Originally Posted by 87chevy.com

How do you know your getting 12v fan signal?
How are you measuring it?
The pcm supplies ground to the relay. The + wire is a strait power or ignition depending on how the harness was made. I would measure battery post - to the " + " from the harness and see what you get. Try no key, key ignition on. Do that for both " + " wires. Then battery post + to the wires marked " - " and then use hp tuners or equivalent to activate each one to verify pcm is giving the on signal.
Post up what you find.
Clint

I ran the 12V signal from the speartech harness but ran a ground to the other side of the coil directly from the chassis. The fan came on. So that's what the issue is- despite what I am reading on my meter, the PCM is NOT calling for the fans to come on.

The only thing leaving me puzzled is how I am getting a 12 VDC reading off my meter if, by my understanding, the ground should be switched off at the PCM. Where is it getting the ground to read the voltage?