Mark

Servo valve explanation.....
  

25+ years of aircraft avionics/electrical work..... "0" power feeder wire issues. Last 8 years..... 10+ power feeder wires.

Another one. PW 4000 on a 767. BPCU (Bus Power Control Unit) showing left DP (Differential Protection) Fault and Overlap Fault.

This particular problem reacted differently than most. Instead of just a generator disconnect, it dumped the whole left bus also. "THIS IS NOT NORMAL". The system should dump the generator feed and then the Left Bus Tie Breaker should have closed to feed power from the right to the left.

To isolate the issue we disconnected the three main feeds (and the smaller connections) from the generator breaker (upper left wires).
    
       
  
We disconnected the feed wires from the IDG and fed 1000V from the megger into each lead. As a note: when using the megger, all wires have to be isolated. If we megged forward from the IDG without disconnecting on the other end, we could fry some downstream components. Not so much the breaker itself, but the sense wires going to the BPCU and GCU (Generator Control Unit).

Anyway.....
  
      
  
This was at the pylon disconnect box at the top of the engine. What sucks here is that I was originally shaking the wires and my co-worker on the ground is hearing a buzz. I'm right there and I'm not (it sucks when your hearing is dropping off). After swapping positions, he saw the spark.
  
  
At the forward feed-through of the box is a flange with a short tube. Inside that tube there is supposed to be a thick rubber grommet to center the wires. Where's the grommet???
  
     
  
All items underlined are part of the grommet assembly. The nut and bolt on either end are tightened to squish the assembly inside the tube. With the rubber sealed against the inside diameter of the tube, all the wires are isolated from the edges.
  
     
  
We had one wire shorting on the tube. You can see a pile of molten metal below.
  
     
  
Flange replacement, new wire, and properly positioned grommet.
  
     
  
Removed wire and flange.
  
     
 

Test!

A schematic print would be nice.

You're dealing with older 737's if I remember correctly.

The interphone system on this aircraft would be considered analog.

On the 767 there are nine total auto-pilot actuators, 3 each for pitch, roll, and yaw.
  
  
On each actuator 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.
  
     
     
 

Leach relay P/N BACR13CJ2 causing a cycling "FLAP/SLAT ELEC" warning on the lower EICAS display.
  
     
  
This is different from the posts above because this one has a 115 VAC coil. All previous intermittent relays were direct current.
  
  
This particular one was failed for about 90 seconds and energized around 10. (I wouldn't have caught it if I wasn't looking at the EICAS screens for another reason.)

When failed the message was in view.

I pulled the relay and checked for power (X1) and ground (X2). These inputs never dropped off. I then jumped the 115VAC and a separate hard ground to the removed relay. It dropped off within 10-15 seconds after power was applied.

A new relay fixed the problem.

In the schematic below, the EICAS computers do not want to see a ground on "F3" of the "D" plug. If a ground is present..... the warning will display.
K730 has 115 VAC available all the time. It's looking for a ground to energize. That ground comes from the Flap Slat Electronics Unit on pin "K13" of the "B' plug. If the FSEU is operational, it will provide that ground and there will be no warning present.
  
  

Old, but it still gives some good information.
Same system on 767.