My grandfather's 91 Nissan Maxima radiator fans will run continuously.
I'd unplug the fan switch near the thermostat and the fans still run.
There are three relays and all seem to be fine because when I unplug
them one by one they will make a click sound and the fans will stop.

The diagram that I have off the Chilton manual shows a fan switch
wired to the ECM. The ECM controls the three relays. I have no diagram
which shows an AC button or any other coolant temperature sensor
connected to the ECM, which I suspect may be the ones keeping the fan
on. I'm not familiar with a Maxima. I would like help finding out why
the fans are running even after the fan switch is disconnected. Any
help is appreciated. What are some of the possibilities that will
allow the fans to run continuously?

Thanks