The ISM (Intercooler Spray Manager) uses an RPM input to let the user choose when water should be sprayed.
This article details the signal input the KSC & ISM can accept.
The signal must alternate between ground and 5-15 volts.
Once every two revolutions or once every revolution.
Minimum Voltage must be ground ie 0 volt
Maximum voltage can be between 4.5 and 15 volts.
RPM signal can be diagnosed in the KSC app. Check the RPM shown by the app matches the actual RPM of the engine.