1G radiator fan won't turn on

[scheides]

Having radiator fan troubles on the 1G. Checked the fan, it spins if I wire 12v to it, no problemo. This is the stock fan on the passenger side, wired into the stock plug. What is it that makes this fan spin when the car gets hot, is it the ecu, triggered by the lower temp sensor? Sensor feeding the gauge seems to work just dandy, have not hooked up a logger to verify the other sensor is working. I have not tested the voltage at the plug, either. Just wondering if anyone has any suggestions to get this thing spinning!