sensor inputs to the ECUs through a 50-pin connector. The  harness connects to the engine-mounted pressure sensors  via cannon plug connectors. The 25-jpin connectors connect  the harness to the speed sensor signal conditioning unit.  The low-voltage harness attaches to the cabin harness by a  firewall-mounted data port through the same cabin harness/ bulkhead connector assembly. The bulkhead connectors also  supply the aircraft electrical power required to run the system. This microprocessor-based system controls ignition timing  for engine starting and varies timing with respect to engine  speed and manifold pressure.  [Figure 4-21] PowerLink provides control in both specified operating  conditions and fault conditions. The system is designed to  prevent adverse changes in power or thrust. In the event of  loss of primary aircraft-supplied power, the engine controls  continue to operate using a secondary power source (SPS).  As a control device, the system performs self-diagnostics to  determine overall system status and conveys this information  to the pilot by various indicators on the health status  annunciator (HSA) panel. PowerLink is able to withstand  storage temperature extremes and operate at the same  capacity as a non-FADEC-equipped engine in extreme heat,  cold, and high humidity environments. The ECU is at the heart of the system, providing both ignition  and fuel injection control to operate the engine with the  maximum efficiency realizable. Each ECU contains two  microprocessors, referred to as a computer, that control two  cylinders. Each computer controls its own assigned cylinder  and is capable of providing redundant control for the other  computer’s cylinder. The computer constantly monitors the engine speed and  timing pulses developed from the camshaft gear as they are  detected by the SSA. Knowing the exact engine speed and  the timing sequence of the engine, the computers monitor  the manifold air pressure and manifold air temperature to  calculate air density and determine the mass air flow into the  cylinder during the intake stroke. The computers calculate  the percentage of engine power based on engine revolutions  per minute (rpm) and manifold air pressure.  The low-voltage harness connects all essential components  of the FADEC System.  [Figure 4-21]  This harness acts as  a signal transfer bus interconnecting the electronic control  units (ECUs) with aircraft power sources, the ignition switch,  speed sensor assembly (SSA), temperature and pressure  sensors. The fuel injector coils and all sensors, except the  SSA and fuel pressure and manifold pressure sensors, are  hardwired to the low-voltage harness. This harness transmits  Low-Voltage Harness 4-13 Figure 4-21.  PowerLink system components. Low voltage harness High voltage harness Electronic control unit (ECU)