APU Chugging

APU Chugging; (turning slowly making regular muffled explosive sounds, as of an engine running slowly.

ATA: 49-50
Fleet Type: 700
Applicable Reference: WDM 49-62-12, 49-52-31 FIMM 49-55/806
Possible Fix: Replace splice in APU wire harness (3888449-1) for Delta
pressure/Total pressure sensor. To enviromental style Part # D436, It is
recommended to use a enviromental sleeve over the splices to provide further
Tip: While APU is operating and underload shake down wire harness on APU,
pay special attention to the splices that are located L/H side of the APU at
the lowest part of the harness. The wires can check good using a digital
mutlimeter, but
break down due to corrosion intermittently while in service.
The pressure sensors are instumental in the ECU’s control of the APU during
operation. This breakdown of the harness has caused problems with APU
operation, newer aircraft come with a 388449-2 wire harness that utilize
enviromental splices in this location
Inlet Temperature Sensor
The inlet temperature sensor supplies inlet air temperature data (T2). The
ECU uses this data for these functions:
Fuel control
IGV control
SCV control
EGT trim schedule (ECS and MES modes)
Inlet overheat.
Functional Description:
The inlet temperature sensor is a resistive temperature device (RTD). When
the air temperature into the inlet of the compressor changes, the resistance
changes in the sensor. The ECU senses this change in resistance. The ECU
adjusts the APU operation due to that temperature change. The ECU also sends
the temperature to the control display unit (CDU) in the flight compartment
Purpose: The three pressure sensors measure load compressor pressures and
convert these pressures into a electric signals. The electric signals are
sent to the ECU.
These are the three pressure sensors:
– Inlet pressure (P2) sensor
– Total pressure (PT) sensor
– Differential pressure (DP) sensor
Location: The P2 sensor is attached to the APU inlet. The PT and DP sensors
are above the surge control valve (SCV) on the APU inlet.
Operational Description: The P2 sensor measures the APU inlet pressure of
the load compressor. The ECU uses this data to control APU functions. The PT
sensor measures load compressor total discharge pressure. A DP sensor
measures the difference between the total discharge pressure and the static
pressure in the diffuser. Differential pressure equals total pressure
minus static pressure. The ECU uses PT and DP to calculate load compressor
airflow. The ECU uses load compressor airflow, inlet temperature, and IGV
position to operate the surge control valve.

Thanks to Tom S. for his input !


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