767-200 with a right wing duct overheat system that would not test.
The duct overheat systems (Left, Right, Body) use detector loops and/or overheat switches that provide a ground signal in an overheat condition.
During a system test, integrity is verified by providing a ground signal that must pass through the entire length of loops and wires.
Signal path during test.
This aircraft had a broken wire on a terminal at one of the overheat switches in the pylon. The switch(s) only use two connections. A single wire hooked to ground and a double wire connection used in the loop path. If the single (ground) wire was broke, the system would test normally, but that switch would have no ground path for warning if it happened to overheat. The broken wire was on the double wire connection. With the signal path open, a proper test could not be confirmed.
The PRSOV (Pressure Regulating and Shut-off Valve) is located in this area. It is VERY
difficult to remove this valve and it is almost certain that the terminal in question was broken during a valve change.
A note about the functionality of these systems: Even with a broken loop path wire, in an overheat condition, a warning will still be shown. Each end of the loops are connected together at the test switch. The card only needs to see a ground. That ground can come from either side of a broken loop system. This same theory is used on engine fire systems.
767-200 WDM 26-18-21