Emblemdhc

Fault message Recordeed on APU FADEC 
1. SYSTEM B/U OVSPD ( Means backup OVERSPEED) caution
2. FADEC N1 DELTA ( means there is mistach in both channels of DUAL SPEED SENSOR
MESSAGE ON AIRCRAFT EICAS WHEN APU SHUTDOWN AUTOMATICALLY ( APU FAIL)
And AIRCRAFT CMC MESSAGE ( OVERSPEED)

SAME MESSAGE ALL TIME 
IN APU DC GENERATOR INSTALLED We sawpped the GENERATOR  AND GCU But no results
We swapped most of the harness but no results 
We also send data to the APU manufacturer they are saying BOTH SPEED SENSOR CHANNEL FAILED

Already DUAL SPEED SENSOR SWAPPED WITH TWO AIRCRAFT BUT NO RESULTS 
And AIRCRAFT original FADEC IS Also installed but no results 

Now we are clueless BUT WE CAN DO 
​​​​​sometimes APU Takes start but sometimes not 
Sometimes its work for hours and sometimes even shutdown in 3 to 10 minutes 

First time dealing with this type of snags
 

attached training manual documents 

On OUR GULFSTREAM G(IV) Facing below mentioned snags.


1.Pressurization IS INOP IN AUTO MODE

(WORKING FINE IN MANUAL MODE)

2.FLAP Disagree EICAS MSG During Auto throttle use

( Flap Working fine in Ground) 
3. L-R AC POWER FAIL message on EICAS

Have you ever faced this issue

Sir please guide us for troubleshooting