PRV Exit Pressure sensor

PRV exit pressure sensor reading -7 psi,  and very very infrequently reading normal pressure but only for a few snap shot frames of the AHM data. Duct pressures are normal so this is an AHM hit. Multiple sensors replaced, leak check line, air supply bite module changed. Is it possible to bench test this? I have no manufacturer data for the sensor, could the r/b twisted pair be reversed resulting in a negative reading.  
  
  

Can you tell the the aircraft type and system?

Is this an ongoing fault?

Also, I'm not sure what AHM means.

767 at purple promise, ahm is the "real time" boeing aircraft health monitoring system. There are no flight deck effects for this sensor, it just senses the bleed pressure after the pressure regulating valve but before the prsov. It will show -7 psi just about every single leg, with occasional psi spikes. Even with multiple sensors replaced it will still show -7 psi but the cockpit duct pressure uses the same 28vdc, I am just not sure if it is getting to this particular sensor.

I can see where this signal is only used by the AIR SUPPLY BITE  unit and that the crew would not have a cockpit indication.

I have never metered one of these with pressure on so I can only guess what the output might be.

You've got 28VDC coming in. I would expect a value from 0-28 coming out and you would think that value would be positive for a positive pressure.

You're getting -7 PSI..... are those output wires swapped?

That's a shielded twisted pair. It would be worth pulling the box and shooting those wires. Red to Blue or either to ground (with sensor disconnected).

You have a blueprint of the proper resistance values (with sensor connected) from the other BITE box.

  
     

Hang a light bulb off that pin 3 at the sensor. It should come on bright if the voltage is good and the wire can handle the current.