the alternator contains diodes in the armature circuit, current  cannot reverse flow from the bus to the alternator. When troubleshooting an alternator system, be sure to  monitor the aircraft ammeter. If the alternator system is  inoperative, the ammeter gives a zero indication. In this  case, the battery is being discharged. A voltmeter is also a  valuable tool when troubleshooting an alternator system. The  voltmeter should be installed in the electrical system while  the engine is running and the alternator operating. A system  operating normally produces a voltage within the specified  limits (approximately 14 volts or 28 volts depending on the  electrical system). Consult the aircraft manual and verify the  system voltage is correct. If the voltage is below specified  values, the charging system should be inspected. Many aircraft employ an external power circuit that provides  a means of connecting electrical power from a ground source  to the aircraft. External power is often used for starting the  engine or maintenance activities on the aircraft. This type of  system allows operation of various electrical systems without  discharging the battery. The external power systems typically  consists of an electrical plug located in a convenient area of  the fuselage, an electrical solenoid used to connect external  power to the bus, and the related wiring for the system. A  common external power receptacle is shown in  Figure 9-89 .  Figure 9-90  shows how the external power receptacle  connects to the external power solenoid through a reverse  polarity diode. This diode is used to prevent any accidental  connection in the event the external power supply has the  incorrect polarity (i.e., a reverse of the positive and negative  electrical connections). A reverse polarity connection could  be catastrophic to the aircraft’s electrical system. If a ground  power source with a reverse polarity is connected, the diode  blocks current and the external power solenoid does not close.  This diagram also shows that external power can be used  to charge the aircraft battery or power the aircraft electrical  loads. For external power to start the aircraft engine or power  electrical loads, the battery master switch must be closed.  Virtually all modern aircraft employ an electric motor to start  the aircraft engine. Since starting the engine requires several  horsepower, the starter motor can often draw 100 or more  amperes. For this reason, all starter motors are controlled  through a solenoid.  [Figure 9-91] The starter circuit must be connected as close as practical  to the battery since large wire is needed to power the starter  motor and weight savings can be achieved when the battery  and the starter are installed close to each other in the aircraft.  As shown in the starter circuit diagram, the start switch can  be part of a multifunction switch that is also used to control  the engine magnetos.  [Figure 9-92] The starter can be powered by either the aircraft battery or  the external power supply. Often when the aircraft battery Starter Circuit External Power Circuit Figure 9-89.  External power receptacle. Figure 9-90.  A simple external power circuit diagram.  9-50 - + + + - External power receptacle External power solenoid Battery solenoid Starter motor Reverrse polarity diode Starter switch To Electrical loads To Bus Battery master switch Aircraft battery