APU OIL PRESS LOW
(Caution)

The amber 'APU OIL PRESS LOW' caution CAS message indicates the APU oil pressure is low.
NOTE: This message will only post on the ground. This failure triggers a protective shut-down of the APU. The shut-down is inhibited in flight.

• The ESU has determined that the sensed oil pressure from the oil pressure transducer is out of specifications, below 26 psig (179 kPa).

• APU Oil Pressure Switch
• APU Oil
• APU Low Oil Quantity
• APU Oil Filter
• Packing, (Filler Cap)(Liquid Level Gauge)
• Associated Wiring

Advisory Wire/Service Bulletin:

• AW300-49-0268 - Potential damage to the Auxiliary Power Unit (APU) Low Oil Pressure Switch Wire

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): None

NOTES:

• A loose or leaking oil filler cap will allow outside air to be drawn into the sump. As the air is drawn out through the ejector (and into the exhaust area) it will carry the lubrication oil with it.
• The filler cap incorporates a packing that should be checked for cuts or damage. In some cases a leaking accessory pad seal or any other source of air into the gearbox sump can also cause this.
• APU oil may break down if it is older than 800 hours of APU operation, changing the oil could fix the discrepancy (low oil pressure shut-down). Verify oil system servicing. If the oil is close to 800 hours, drain, and service.
• If this occurs on first flight of the day or APU start after landing when the oil is cold, replace the oil filter first.

1. Perform the operational check of the low oil pressure switch (P/N 3876001-8) as follows:

   1. With no oil pressure applied to the low oil pressure switch:

      LRU	From	To	Expected Result	Result
      Oil pressure switch	S110 - A	S110 - B	<5 ohms
      Oil pressure switch	S110 - B	S110 - C	Open

   2. With 35 psig (214 kPa) oil pressure applied to the pressure switch:

      LRU	From	To	Expected Result	Result
      Oil pressure switch	S110 - A	S110 - B	Open
      Oil pressure switch	S110 - B	S110 - C	<5 ohms

   3. Slowly decrease oil pressure to 26 psig (179 kPa).

      LRU	To	From	Expected Result	Result
      Oil pressure switch	S110 - A	S110 - B	< 5 ohms
      Oil pressure switch	S110 - B	S110 - C	Open

2. Replace the low oil pressure switch if inspection requirements are not met.
   Select an MDC Fault Code/Message:B3-006355 - OIL PRESS LOW [APU]