m. If the UNDERSPEED lamp does not extinguish, or extinguishes before 90 per-
cent engine speed is reached, adjust the speed switch. Refer to TM 55-1520-
209-35 for the procedure.
If the LOW OIL PRESS lamp does not extinguish after APU cranking is initiated,
n.
the APU lubricating oil system or the low oil pressure switch has malfunc-
tioned. Stop cranking the APU and correct the malfunction before initiating
another start. Refer to TM 55-1520-209-20 to correct this malfunction.
o.
If the cranking cycle is accomplished without a lightoff, the fuel control unit
(fuel pump or acceleration control), ignition exciter, spark plug, or fuel
nozzles may be malfunctioning. Refer to TM 55-1520-209-20 for corrective
procedures.
CAUTION
Do not attempt to restart the APU after a malfunction
shutdown until the malfunction has been corrected.
If the APU shuts down because of overspeed, the OVERSPEED lamp will
p.
illuminate. The APU fuel control unit, the speed switch, or the start fuel
solenoid valve may be malfunctioning. Refer to TM 55-1520-209-20 or -35 for
corrective procedures.
If the APU shuts down because of high exhaust gas temperature, the HIGH EXH
q.
TEMP lamp will illuminate. The reason for this shutdown may be due to
loading the APU prior to attaining 100 percent rated speed, residual fuel
remaining in the combustor, a restriction in the air inlet or exhaust outlet,
or a failure in the engine (resulting in increased drag). Correct these condi-
tions accordingly. Also, the fuel control may be out of adjustment. Refer to
If the APU shuts down because of low oil pressure, the LOW OIL PRESS lamps
r.
will illuminate. This condition may be caused by low oil level in the APU,
malfunctioning oil pump, clogged oil filter, electrical discontinuity, or a mal-
functioning oil pressure switch. Refer to TM 55-1520-209-20 to correct these
malfunctions.
3-13. APU TESTING. After the APU has automatically accelerated to rated speed, it
is protected by three automatic safety devices; a speed switch, a high exhaust tempera-
ture device, and a low oil pressure switch.
3-10
