Speed Monopole Failure

We had an APU that would not start. The RPM indication fluctuated wildly on spin-up. A dual monopole fault was recorded on the ECU (Electronic Control Unit).
  
     
  
It seemed strange to have both monopoles fail. They don't use a common ground and their wires are completely separate. They both read around 7 ohms from the ECU rack back to the APU.

A new unit was checked, it came in just above 3 ohms. 4 ohms aircraft wiring resistance isn't bad for such a long run.
  
     
  
We pulled both monopoles. Neither had o-rings installed and the left probe was covered in oil.
  
     
  
The left probe was loose. It could be turned by hand. It was no where near close to being fully seated. It's threads showed signs of wear from moving around inside the case.
  
     
     
  
These probes are mounted 180 degrees out from each other. They are monitoring the same spool. If one wasn't fully seated I would guess it's output was slower (and different) from it's pair. The ECU received mixed signals. Not knowing the correct speed would be cause for a shutdown.

We replaced both (with the o-rings). All APU functions were normal after a good start.

As a note: the 767's APU is a single spool unit. Speed is a fuel control function. Bleed pressure is controlled by inlet guide vanes.

Some aircraft like the DC-10 and MD-11 are dual-spool units. There is a monopole for both N1 and N2 speeds.
  
  

I'm guessing that a single monopole failure would flag a fault on the ECU, but having the other sensor sending valid information would allow the APU to operate normally.