but this is on a transmission driven jet drive for a boat.
Ok, that diagram helps. I don't really understand what the middle connection is doing for the system. That's another proportional valve and when you activate the relays, it must be off?So is the existing controller a proportional controller? That means it's already producing a PWM or analog drive suitable for the solenoids? If you still want it involved in the final Arduino-controlled system then you need to find out what it is doing electrically.You didn't answer the question about how you expect PWM to mix the two controls.