Tracking Down Broken Circuit

Engine vibration is normally monitored from two locations, the fan case and core section. Accelerometers create a signal that is fed to a Vibration Monitor Unit.
  
     
  
This particular aircraft had a recently replaced engine. There was no indication of fan vibration after the change.

Knowing that the issue was most likely on the engine side, we still approached it as the problem could be located anywhere on the aircraft.

We discovered that accelerometers cannot be read with a VOM. The sensor in question read open.... but so did all the others.
  
     
     
  
On the CF6-80A engines, the fan accelerometer is located engine right side forward under the thrust reverser half. In the second image, a secondary unit is installed as part of the deferral process (it didn't work either, as we're soon to find out why).
  
     
     
  
Since the sensor couldn't be read directly, a jumper was installed on the plug.
  
     
  
The circuit still read open from the monitor unit. So we disconnected the pylon plug, installed a jumper and read resistance from the unit. The reading was good.
  
     
     
  
As already suspected, the problem is engine side. We checked from the pylon plug down to the accelerometer plug. Which read????? Open!
  
  
 
  
  
Now..... comes the harness shake-down while watching meter. The circuit remained open until right around the pylon plug itself. Narrowing it down even further revealed a broken crimp wire at back of pylon plug pin.
  
     
  
This procedure took us about 15 minutes to find the issue. It is a typical method for chasing down wiring issues. Segmenting down to the problem area. It sounds simple, but sometimes that wire break (high resistance, short to ground.....) is located on a 50+ foot wire run, that's when the "real" fun starts.