AC GENERATOR SYSTEM.

TM 55-4920-424-13&P

Relay and Contacts

APU Components

K1, contacts A, B, C, D

Fuel pressure switch

K2, contacts A, B, C, D

Hour meter

K3, contacts B

Ignition exciter

K4, contacts B

Start fuel valve

K5, contacts B, C

Main fuel valve

6-8. Malfunctions are indicated by the low oil pressure indicator light DS1, high exhaust indicator light DS2, and

overspeed indicator light DS3, and controlled by the following corresponding relay contacts.

Relay and Contacts

Rectifier

K1, contacts C, D

CR1

K2, contacts A, D

K3, contacts A, D

K4, contacts A, D

K5, contacts A, D

If malfunction circuits indicate faults, or if false APU shutdown occurs, carefully check all malfunction circuit components.

6-9. AC GENERATOR SYSTEM.

6-10. Malfunctions in the ac system should be traced by verifying that the ac power switch S101, auxiliary contacts of

relay K101, and the circuit breaker closed indicator light DS5 are functioning properly.

6-11. If power is not available at the aircraft terminals, the ac generator G101, voltage regulator Z101, ac power contactor

K101, and the ac power cable should be checked.

6-12. Problems with proper readouts of ac voltages and amperes should be traced by checking the ac selector switch

S102, current transformers CT101, CT102, and CT103, voltmeter M101, and ammeter M102.

6-13. EXHAUST TEMPERATURE SWITCH.

6-14. Improper operation of the APU control system may be traced to the APU exhaust thermal switch. The switch may

be checked with a Jet-Cal test instrument, or equivalent. The normally closed switch contacts should open at 1120 to

1130F (604 to 610 C).

6-6