OBD1 d16z6

When the coolant temp switch clicks on it provides ground to the radiator fan relay which completes the circuit and powers on the radiator fan. It also grounds pin A12 of the ECU.

My question is: Why does the ECU need this signal? What does it do? Do I really need it?

Right now I just have my fan hardwired and A12 hooked to the cts.

 

The ecu needs to know when the cooling fan is running because the ECU bumps up the RPMs by 100 or so to compensate for the Instantaneous Amp draw of the cooling fan when it first turns on. The car will function almost normally without the ECU getting the ground signal the same time your cooling fan turns on, you might notice a slight dip in RPMs for a second as the fan turns on, that is all.