Coachman Laser water problems

Hi

i have a coachman laser with a whale on board cold water tank, truma barrel pump, and a separate control panel.  The pipe work includes a pressure sensor and solenoid valve.  

The issue I have is that the pumps rapidly switch on and off, creating a clicking noise which seems to come from the solenoid valve. The strange thing is that the clicking from the solenoid happens even if I disconnect the wires from the solenoid. I have already replaced the pressure switch but still have the same issue.

The solenoid is fitted with the direction of flow towards the on board tank. The pressure switch is installed in a bypass pipe around the solenoid, also with a direction of flow towards the on board tank.

I’m at a loss as to what the issue is, not helped by the fact that I can’t understand how the system is supposed to work. Any suggestions would be appreciated.

TIA

Mark