The APU uses a single EGT thermocouple placed in the exhaust/tail pipe and when heated, the thermocouple emits a millivolt signal to the ECU. The ECU, in turn, sends a zero to 1 vdc signal to the cockpit indicator. The electronic control interprets an open thermocouple circuit as a 732 C overtemperature fault. This will automatically shutdown the APU, prevent starter engagement, and the EGT indicator will display maximum EGT (1,000 degrees on an analog gage and, 9,999 degrees on a digital). An electrical short in the thermocouple or harness will actually create another thermocouple and display a much lower than actual EGT. The ECU will now control EGT to this faulty indication. For example, if the erroneously indicated EGT is 75 C, the ECU will allow the APU to operate at full load, or accelerate, with an actual EGT much higher than shown. This obviously will cause severe distress to turbine components.
Some installations use a DC starter motor and clutch assembly. A slipping or damaged clutch can cause high EGT during acceleration. A damaged clutch can seize, forcing the starter to be motored by the APU. If rotated by the APU, the starter will fail causing severe damage to the APU.
A caution in the maintenance manual tells us whenever a DC starter failure has occurred, the starter clutch needs to be checked in the overrun position. Once the starter is removed, the internal spline in the clutch is visible; by inserting a screwdriver or spline adapter into the clutch, one can rotate the drive. The clutch should rotate in one direction smoothly without the feeling of contamination or damage. When rotated in the opposite direction, the APU should start to rotate immediately without hesitation or slippage.
If the clutch needs replacement, use extreme caution when removing as there are several washers between the outboard bearing and the adapter. When the adapter is removed, the washers can fall into the gearbox and there is also a spacer between the two inboard bearings. Use care when removing the clutch from the gearbox, the back bearing sometimes sticks in the gearbox exposing this spacer. You guessed it — it is also easily dropped into the gearbox. After the clutch and starter have been replaced, removing the inspection cover from the rear of the starter will facilitate checking the starter and clutch. Insert the appropriate size screwdriver into the slot in the shaft to rotate the engine, starter and clutch. The starter should stop immediately upon removal of the screwdriver and the APU should rotate freely. There are clutch inspection and shimming requirements, so refer to the maintenance manual when replacing the clutch.
The hour meter is energized when the RPM reaches 95 percent plus four seconds and starts counting time. On the 2118474 series ECUs (Some Gulfstream installations), if the hour meter is shorted, an overcurrent protection shutdown is annunciated. Like many other components, disconnecting the hour meter is an easy way to troubleshoot a fault as the hour meter draws very low current.
Troubleshooting an overcurrent fault can sometimes be very difficult. Learning when the components are energized will be an asset to you. Think about it; what could cause a shutdown at approximately 10 to 20 percent RPM? If you guessed that the ignition unit, FSOV, or SCV might cause the problem, you are on the right track. The easy part about this ECU is that in most cases a cannon plug can be pulled to isolate a component.
Remember that every APU is an individual and will operate a little different from another. Accurate fuel pressure and temperature values can only come from your APU — so don't make comparisons to another unit.
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