How Could This Work???

Pressure Switch fault ball showing on inbound "Stabilizer Trim" EICAS message.

What is the fault ball saying?

The Stabilizer Trim/Aileron Lockout boxes verify they have two viable hydraulic systems (in this case left and center), by comparing on-side to off-side pressure switch signals. This is the left box, so it looks at the left pressure as on-side and center as off-side.

The right box would look at the center hydraulic system as on-side and the left as off-side.

Either way...... there's a mismatch. The meter confirmed this to be true. With hydraulic pressure applied to left and center, only one (of two) input pins showed a ground (both should have).

The left pressure switch was at fault.

We shot the switch by pinning directly into it. No issues.

We verified wiring..... Pin 3 was showing a hard ground. Pin 2 wasn't.

No frickin way. Sure as shit, pins 2 & 3 where swapped in the plug.

This pressure switch feeds numerous other units which are looking for the same ground signals. All six spoiler boxes, the yaw dampers, and rudder load limiters.

It also plays a roll in automatic functions of the hydro pumps and the overhead low pressure lights.

How in the ---- wasn't there some other issues with the aircraft???

Too tired to follow them all out. Either way, after the wiring was corrected, all systems (that we knew of) checked out.  

Correct wiring verified by wire numbers.....  

  ..... and continuity checks from the P37 panel.  

With the pins in wrong positions, with no pressure - the grounds were correct. With pressure, both pins on the unit had continuity to each other. I really just don't know how anything worked right!