TM 1-2840-252-23-3
FAULT ISOLATION TREES
G-13 FAULT TREES
a. The maintenance procedures required to isolate the fault codes displayed on the DECU to line
replaceable units (LRUs) are depicted in the following fault tree diagrams. Fault trees are listed according to
fault codes. The procedures are biased against removing the HMU until other more accessible possibilities
have been eliminated.
b. In the fault trees, "DECU #1" is used to indicate the FADEC under investigation, be it the right-hand
or left-hand engine. "DECU #2" is used to indicate the FADEC on the opposite-side engine.
c. Each of the fault trees is accompanied by a facing page that contains expanded instructions for
some of the steps. The expanded instructions contain more detail for certain steps, reference to helpful
diagrams (which are at the end of this manual), and resistance limits.
d. The term "manufacturer" as stated in this manual refer to either Textron Lycoming or Boeing
Helicopters. dependent on whether the context refers to an airframe component or an engine component.
G-14 DIAGRAMS
The following diagrams appear in this manual.
a. Figure 201, Fuel Control System Harness Connections. This diagram shows the physical location of
each harness connector and component connector. It can be used to locate the connectors as called out in the
charts.
b. Figure 202, Electrical Connector Pin Locations. This diagram can be used to identify the physical
location of each connector pin of the component (DECU or HMA) connectors. Note that the socket locations for
the harness connectors are the same except that they are reversed to form a mirror image.
c. Electrical System Schematic (Ref. APPENDIX D). This diagram can be used to trace the
termination of each terminal on each connector. For instance, find the N1 A SPEED PICKUP at lower right
corner of the diagram on page D-3. The diagram shows that it is in the HMU at J4 connector terminals R and
S. Follow up to top to trace through harness P1 7/J75 connector to harness PL1 connector to DECU PL1
connector terminals MM and FF.
d. Figure 203, HMA Schematic Diagram. This diagram gives an operational overview of the HMU.
G-11
