I just went through the exact same problem with our 150 l/hr Dessalator on our 54. I contacted MARTIN the main tech for Dessalator and we diagnosed the problem to the control box inside the high pressure pump. I'm not sure which HP pump you have, ours has the LaFert 2,2 HP pump (if I remember correctly). Inside the box on top of the pump there are two capacitors and one proprietary controller. One of the capacitors is for startup and the other one is for making sure the motor spin direction is correct. The controller controls the time and which capacitor is charged and discharged. I did not get into the Intricate details of exactly how everything works.
Once I changed the controller, everything worked fine. Martin is a very knowledgable person and knows the Dessalator system inside out and I would suggest you call him to debug the problem. He is also a very nice person that speaks fluent English. His number is +33 6 07 34 65 11.
Good luck and let us know how everything goes.
Mohammad and Aty
AMEL 54 #99