When a water supply is established by either hydrant, tank water, or draft, the pump should recognize that there is water in the pump. This is done through the pump transducer.
These small sensors allow the computer system to determine if there is water or not. This then allows the pump in “PSI” mode to have its safeties engaged, including keeping the pressure where it needs to be for cavitation protection.
When the transducer is not working (common problem on many different manufacturers) the computer usually thinks there is no water in the pump. This typically won’t allow you to throttle up in “PSI” mode. Usually a message will appear saying "Lo water" or "no water". Something is wrong either with the wiring or the sensor itself.
At this point you need to switch to “RPM” mode. This removes the pump's “safety net” and allows you to run pressure up with no water.
That is why “RPM” is needed for establishing a draft. RPM mode allows you to control throttle, regardless of whether there is water in the pump. It allows you to throttle up and begin your prime to pull water in without having any water already in the pump.
When in this mode, you have just made yourself the safety net. You are responsible for ensuring proper water supply is actually going into the pump. You are responsible for not cavitating the pump as it may not shut itself down in this mode. You can’t set a pressure and walk away, because it will not keep the pressure where you set it. You are setting an engine speed and have to gate everything to proper pressures manually. You then have to babysit the pump because it will continue to keep that speed no matter how much water is coming in or out.
This is a temporary solution for on scene issues, and is not a fix. This issue needs to be repaired by a qualified repair shop; not in the station.