Fault Message:

SHORT-APU OIL FLTR DIFFPRESS SW/WRG

Fault Code:

4960253APU

Associated CAS:

APU FAULT
(Advisory)

Before the Auxiliary Power Unit (APU) is started, the Full-Authority Digital Engine-Controller (FADEC) receives an indication of a possible clogged APU Oil Filter Element. This shows a possible defective Oil Filter Differential Pressure Switch.

• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) (A108)
• Oil Filter Differential Pressure Switch (S2)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

1. Re-rack FADEC and verify both the tray and FADEC connectors for damage or FOD.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

2. Inspect oil filter differential pressure switch and connector S2 for damage or FOD.

   1. If fault remains, repair as required and do close out.
   2. If system checks are good, continue with next step.

3. Verify that the resistance within oil filter differential pressure switch connector S2. The resistance should measure as follows:

   From	To	Expected Result	Result
   S2P1-A	S2P1-B	<50 ohms

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

4. Perform a continuity check and resistance check between oil filter differential pressure switch and the ground shell. The resistance should measure as follows:

   From	To	Expected Result	Result
   S2P1-A	Ground	>1 megohm
   S2P1-B	Ground	>1 megohm

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

5. Swap the oil filter differential pressure switch.

   1. If system checks are good, replace defective oil filter differential pressure switch and do close out.
   2. If fault remains, continue with next step.

6. Verify that the resistance between connector P802 and oil differential pressure switch connector S2P1. The resistance should measure as follows:

   From	To	Expected Result	Result
   P802-26	S2P1-A	<1 ohm
   P802-27	S2P1-B	<1 ohm

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

7. Verify that the resistance between connector P802. The resistance should measure as follows:

   From	To	Expected Result	Result
   P802-26	P802-27	>50 ohms

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

8. Verify that the resistance within oil filter differential pressure switch connector S2. The resistance should measure as follows:

   From	To	Expected Result	Result
   S2P1-A	S2P1-B	>50 ohms

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

9. Verify that the resistance between connector J802 and APU tray connectors A108BP1. The resistance should measure as follows:

   From	To	Expected Result	Result
   J802-26	A108BP1-K3	<3 ohms
   J802-27	A108BP1-K2	<3 ohms

   1. If measurements are not good, repair defective wiring as required and do close out.
   2. If measurements are good, continue with next step.

10. Verify that the resistance between connector J802 pins 26 and 27 is more than 50 Ohms.

    From	To	Expected Result	Result
    J802-26	J802-27	>50 ohms

    1. If measurements are not good, repair defective wiring as required and do close out.
    2. If measurements are good, continue with next step.

11. Perform a continuity check between connector J802 and the ground shell. The resistance should measure as follows:

    From	To	Expected Result	Result
    J802-26	Ground	>1 megohm
    J802-27	Ground	>1 megohm

    1. If measurements are not good, repair defective wiring as required and do close out.
    2. If measurements are good, continue with next step.

12. Verify that the continuity between FADEC tray connector A108AP1 and the ground. The resistance should measure as follows:

    From	To	Expected Result	Result
    A108BP1-K3	Ground	>1 megohm
    A108BP1-K2	Ground	>1 megohm

    1. If measurements are not good, repair defective wiring as required and do close out.
    2. If measurements are good, continue with next step.

13. Replace the APU FADEC.
14. Close out.