TM 1-2840-252-23-3
G-6 FAULT ISOLATION
a. Effective fault isolation is the determination of the actual problem by an intelligent analysis of the symptoms of
the fault, followed by a systematic series of checks to isolate the fault and to correct the cause. Fault isolation
is not complete until the symptom(s) has been proven to be cured. Replacement of the fuel control
hydromechanical assembly (HMA) or the electronic control unit (DECU) for fault isolation purposes should be
at- tempted as a last resort. If the symptoms are not corrected by this method, reinstall the original HMA or
DECU on the engine. If the new components completely correct the fault, make certain to include complete
symptom data together with accumulated operating time on maintenance forms returned with the component.
If such data is not supplied, the item must be returned to overhaul, regardless of condition.
b. For faults related to signals supplied from components external to FADEC, it is recommended that these
external components be functionally tested per manufacturer specified procedures. Fault isolation procedures
for the FADEC system may not detect all problems occurring in external signal sources.
WARNING
THE EMC-32T FADEC FAULT DETECTION SYSTEM IS A LIMITED DIAGNOSTIC TOOL. IT
MONITORS CRITICAL INPUT AND OUTPUT SIG- NALS OF THE FADEC SYSTEM AND INTERNAL
FUNCTIONS OF THE DECU. THE FAULT LOGIC DOES NOT MONITOR ALL COMPONENTS OR
FUNCTIONS OF THE T55-L-714 HMA SYSTEM SINCE MANY TYPES OF FAILURES ARE NOT
ELECTRONICALLY (BIT) DETECTABLE. THE FAULT LOGIC MONITORS ONLY THOSE
COMPONENTS AND FUNCTIONS AS SPECIFIED IN THE CECO SYSTEM SPECIFICATION 109597.
THEREFORE, THE ABSENCE OF FAULT CODES DOES NOT GUARANTEE HMA SYSTEM
INTEGRITY. ANY HMA SYSTEM PROBLEM OR ANOMALY SHOULD BE FULLY INVESTIGATED BY
MAINTENANCE PER- SONNEL FOR CAUSE AND RESOLUTION PRIOR TO DETERMINING
READINESS FOR FLIGHT. READINESS FOR FLIGHT SHOULD NOT BE DETERMINED SOLELY ON
THE ABSENCE OF FAULT CODES.
CAUTION
DIRECTION IN THIS MANUAL TO REPLACE HMA OR DECU COM- PONENTS IS IN SOME CASES
BASED ON DEFAULT, THAT IS, IT IS CONCLUDED TO BE TO BE THE ONLY REMAINING
POSSIBILITY OF PROBLEM CAUSE WITHIN THE SYSTEM AFTER ALL OTHER CAUSES HAVE BEEN
ELIMINATED.
IF THE SYSTEM PROBLEM IS NOT ALLEVIATED BY REPLACEMENT OF THE DECU OR HMA, IT IS
LIKELY THAT THESE COMPONENTS WERE NOT DISCREPANT. IN SUCH CASE IT SHOULD BE
VERIFIED THAT ALL PRECEDING FAULT ISOLATION STEPS WERE CONDUCTED CORRECTLY. IT
SHOULD ALSO BE INVESTIGATED WHETHER THE PROBLEM HAS CAUSES EXTERNAL TO THE
HMA SYSTEM OR CAUSES DUE TO INCORRECT OPERATING PROCEDURE.
CAUTION
DIRECTION TO REPLACE AIRFRAME OR ENGINE
COMPONENTS
WHICH
ARE
NOT
MANUFACTURED BY CECO IS BASED ON THE LIM-
G-3
