ATA 32-41, Parking Brake

We recently had an aircraft that would show "AntiSkid Off" when the parking brake was set.




After some digging we found an open wire from relay K419 (Pin 6) to the parking brake valve (pin 4). Excitation power for the relay goes through the brake handle switch, through the closed contacts (2 & 4) of the valve, to the coil of the relay.




There was no ground present at D15 of the AntiSkid/AutoBrake box.

Using the Fox & Hound , we were able to trace the wire to a bundle on the left wall of the forward lower pit.

The wire was isolated by reading the wire number stamped on the wire.



I'm not a big fan of cutting wires, but in this case we had to. With the wire cut, we had continuity forward, but not aft.

After gutting the pit, we isolated and cut the wire near the back right wall before it went over the center fuel tank.
We were good aft, not forward. We had to choose..... keep cutting the wire until we found the problem or just replace it. We chose to replace it.



Now the question. D15 of the box is going to see a ground if the brake is set or not. If the handle is up, the valve closed, and the relay energized..... the same ground signal (as with the brakes released) is still being seen at the box. How does the box know the brakes are set???



767 SSM 32-41-01

I'm thinking when the valve opens, the circuit to EICAS is grounded. EICAS sees ground=parking brake open, or off. Handle pulled up, EICAS loses ground, parking brakes set.

Just a crude drawing. The red lines indicate the circuit with the brakes being set.

The handle up, valve now closed, and the relay energized.

The same ground is being seen as with the brakes not set.

I verified this on several schematics and wiring diagrams for different aircraft effectivities. They're all wired the same.

Confused as heck...... but the brain ain't clicking like it used to.

Totally agree that D15 is grounded either way, I'm thinking there is a circuit not shown between EICAS and ANTISKID UNIT that lets the logic know it's released?