I've got my prototype PCB and should be installing it soon in my car. I've made my DIP switch select setting the fan on temps in 5 degree F steps. The fan off temps are set at 5 degree F below that setting. I set the alarm (buzzer) at 10 degree above the fan on temps. These are all just my guess of what should work so testing in my car will determine if those values need to change.

I was also thinking of adding a voltage alarm in this unit looking for a sudden drop below 12.5 volts since that would result if your fan belt broke and the alternator and water pump stopped turning. If the water pump stops, then the coolant flow would stop and therefor the coolant temp would not rise to trigger an over temp alarm.

The "buzzer" seems pretty loud on bench testing. Not sure when mounted in the car with the relay cover plate on if the volume will suffice. Also needs to overcome radio background music.