As the motor builds up speed, the current draw of the motor  begins to decrease. As it decreases to less than 200 amps,  the undercurrent relay opens. This action breaks the circuit  from the positive bus to the coils of the motor, ignition, and  battery cutout relays. The de-energizing of these relay coils  halts the start operation.  Air turbine starters are designed to provide high starting  torque from a small, lightweight source. The typical air  turbine starter weighs from one-fourth to one-half as much  as an electric starter capable of starting the same engine. It  is capable of developing considerable more torque than the  electric starter.  After these procedures are completed, the engine should be  operating efficiently, and ignition should be self-sustaining.  If, however, the engine fails to reach sufficient speed to halt  the starter operation, the stop switch may be used to break  the circuit from the positive bus to the main contacts of the  undercurrent relay. The typical air turbine starter consists of an axial flow turbine  that turns a drive coupling through a reduction gear train and  a starter clutch mechanism. The air to operate an air turbine  starter is supplied from either a ground-operated air cart, the  APU, or a cross-bleed start from an engine already operating.  [Figure 5-20]  Only one source of around 30–50 pounds per  square inch (psi) is used at a time to start the engines. The  pressure in the ducts must be high enough to provide for a  complete start with a normal limit minimum of about 30 psi.  When starting engines with an air turbine starter, always  check the duct pressure prior to the start attempt. The procedures listed in  Figure 5-19  are typical of those  used to repair malfunctions in a starter generator starting  system similar to the system described in this section. These  procedures are presented as a guide only. The appropriate  manufacturer’s instructions and approved maintenance  directives should always be consulted for the aircraft involved. 5-13 Figure 5-19.  Starter generator starting system troubleshooting procedures. Starter Generator Starting System Troubleshooting Procedures Engine starts but does not accelerate to idle Engine fails to start when throttle is placed in idle Probable Cause Isolation Procedure Remedy Engine does not rotate during start attempt • Adjust voltage of the external power   source or charge batteries. • Replace switch. • Replace switch. • Place switch in OFF position. • Replace relay if no voltage is present. • Replace relay if no voltage is present. • Replace the starter if voltage is present. • Replace relay if voltage is not present. • Replace the engine. • Use larger capacity ground power unit  or charge batteries. • Clean or replace spark igniters, or   replace exciters or leads to igniters. • Check voltage of the battery or external   power source. • Check switch for continuity. • Check switch for continuity. • Check position of generator control   switch. • With start circuit energized, check for 48   volts DC across series relay coil. • With start circuit energized, check for 48  volts DC across starter relay coil. • With start circuit energized, check for   proper voltage at the starter. • With start circuit energized, check for 28  volts DC across the relay coil. • Listen for sounds of starter rotation   during an attempted start. If the starter   rotates but the engine does not, the   drive shaft is sheared. • Check starter terminal voltage. • Turn on system and listen for spark-  igniter operation. • Low supply voltage to the starter • Power switch is defective • Ignition switch in throttle quadrant • Start-lockout relay is defective • Battery series relay is defective • Starter relay is defective • Defective starter • Start lock-in relay defective • Starter drive shaft in component drive gearbox is sheared • Insufficient starter voltage • Defective ignition system Troubleshooting a Starter Generator Starting  System Air Turbine Starters