Auto-Pilot Actuator Servo Valves

On the 767, all auto-pilot actuators are the same part. There are nine total. 3 each for pitch, roll, and yaw.
  
  
On each servo there are line replaceable electrohydraulic servo valves and shutoff valves.
  
     
  
     
  
     
  
We recently had an aircraft with a center auto-pilot that was not engaging.  The MCDP checks showed a failed command circuit for the rudder (or directional) servo. The rudder servos are inside the vertical stabilizer.
  
     
     
  
When the valve was checked from the Center FCC, a reading of approx 1100Ω was seen.
  
     
     
  
You would think "I've got continuity so the valve is good", but that would be wrong.

There are actually two coils wired in parallel. 1100Ω is the resistance of "one" of the coils.

When two coils of basically the same value of 1000Ω are wired in parallel, total resistance is about half the value of one.
  
     
  
When the replacement valve was plugged in, the value read at the FCC rack dropped to about 500Ω.
  

       
  
It should be noted.... the servo (or shut-off) valves plug into the case of the actuator. If we had the same resistance reading with the replacement valve, we would most likely have a open wire internally "in" the actuator. That would suck as now the whole unit would require replacement.