Q: When do the primary and secondary fans kick in?
A: According to the 1990 service manual, the primary fan is controlled by the ECM and will come on when the coolant temperature reaches 190 degrees F. The secondary fan has its own temperature sensor in the bottom of the engine block (passenger side) and will turn on when both of these conditions exist:
The coolant temperature is greater than 222 degrees F
When the A/C control head pressure is greater than 1600kPa and the vehicle speed is less than 40 mph.
The Auxiliary Coolant Fan is controlled by the A/C Pressure switch and the auxiliary coolant fan switch. It is not controlled by the ECM. It should have black and black/pink wires going to it, and the coolant fan will have black and black/red wires going to it.