Waypoint A.5 is where we become detectives. We are discussing Required Wiring Checks using Troubleshooting Procedures. EWIS faults are rarely as obvious as a leaking hydraulic pipe. They hide. To find them, we need a systematic approach defined in AMC 20-22.

Your primary map is the Troubleshooting Procedures Manual or Fault Isolation Manual (FIM). This is not a suggestion guide; it is a logic tree. You must start at the beginning. Skipping steps because “you know what it is” is the fastest way to replace a $50,000 computer when the problem was a $5 pin.

We rely on the AMM and Illustrated Parts Catalogue (IPC) to physically locate components, but for logic, we need the Wiring Schematics. These show the system logic—how the electrons flow from the bus to the switch to the load. However, schematics don’t tell you where the wire goes. For that, you need the Wiring Diagrams. These show the physical routing, connector pin-outs, and splice locations. You need both: Schematics to understand what should happen, and Diagrams to see where it happens.

The process of troubleshooting involves testing continuity, isolation, and bonding. But here is a critical warning: Testing of LRU connectors. Never jam a multimeter probe into a connector socket. The probe is often larger than the pin it mates with. You will splay the socket, creating a poor connection. Once you reconnect the LRU, you have created a new intermittent fault. Always use a breakout box or the correct mating test probe (a “sacrificial pin”).

We must also address the “No Fault Found” (NFF) phenomenon. This is the scourge of avionics. You remove a unit because of a fault code, bench test it, and it passes. You put it back, and the fault returns. Why? Because the fault wasn’t in the box; it was in the EWIS. It was a loose backshell, a pushed-back pin, or a chafing wire that only shorts when the aircraft vibrates. Before you swap a box, verify the wiring integrity. Check the ground points. Check the backshell tightness.

Effective troubleshooting is about ruling out the EWIS first, not last.