OK, I have finally gotten back to it. Putting power to the terminals resulted in smooth sound of the motor, and an even change in resistance between terminals 1 and 2. So I proceeded to take the upper part of the servo apart, per instructions in the Mercedes Technical Book published by the MBCA. It was clean inside, with only assembly grease evident - no coolant. The shaft of the water valve seemed to move very freely - too freely in fact. So at this moment, I suspect it as the cause of the fault, and I will be in touch with Performance Analysis regarding a remedy.