Jon, are you sure they are solenoids?
Most of the ones I have seen are operated using geared DC motors. They sort of sound like a solenoid in operation, but when you take them apart there is a DC motor and gear-train in there.
Not familiar with the Merc actuators, so if you have had them apart and they are solenoids, that is good enough for me.
EDIT: There would also be motors in the instrument cluster, to move the gauge needles, also, depending upon the engine speed regulation (cable or drive-by-wire) method, there could be some form of idle air control motor, or an electronic throttle body motor.