APU Faults

From member Emblem DHC.....


Currently on OUR Embraer legacy 600 we are facing a serious problem with APU ( APU SHUTDOWN AUTOMATICALLY)

Aircraft is serviceable it's APU FADEC Is removed for another aircraft and when we installed new APU FADEC APU start shutdown automatically after 1 hour of operation
So we check APU FADEC FAULT Message
1. SPEED SENSOR FAULT ( AS per this SPEED SENSOR CHANGED)
2. SYSTEM B/U OVSPD ( BACKUP OVERSPEED) AS per this  FCA Fuel control assembly changed , fuel soleniod valve , APU Engine harness swapped)
But actual there is no OVERSPEED

Now problem becomes worse APU Start shutdown after 3-10 mintues,
Sometime in start cycle , sometimes work for 1 hour

One thing we notice RPM is increasing slowly after reaching 95
It's slow down  its like 96, 97then back to 96 and then increase to 100

I'm somewhat confused.
You pulled a working APU FADEC (or ECU - Electronic Control Unit) from the aircraft in question and installed it on another aircraft. Now you have issues.
Did you try putting the original ECU back in?

RPM is a fuel control issue. You stated you changed it. RPM's are fluctuating.

I'm I to assume it has a DC generator installed? If it's an AC generator, you can verify RPM's by watching the 400Hz output. If it's climbing and diving along with APU speed..... you still have a fuel control issue.

You swapped the harness, so rules out wiring.

Any other faults showing up on the FADEC?

Section 49-00-00-810-801 of the manual you sent me shows all the failure codes that can be read.

I also found this in the manual.....
  
     
 

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
 

I'm not going to be of much help, but have you looked at the aircraft side wiring?

Twisted pair shorts, either to each other, to shield, or to ground.
High resistance on any wires from ECU to APU. Especially look at the speed sensor wiring.

You seemed to have covered all the APU related items quite thoroughly.

As a follow-up to the problem listed above.

The prints and manual were forwarded to me and I responded with; 

I think you stated that you had a secondary speed sensor fault.
From the print, it looks like both sensors are exactly the same.
Can you swap pins from one to the other?
1&2 - 10&11
If the problem stays the same, obviously it's not the sensor and I
would still suspect wiring. If it "does" follow, you have a mechanical
issue inside the APU. (Sensor loose, backed off, positioned wrong???)

Did you verify both sensor resistances match?

Emblem DHC responded with:

I hope this finds you well. I wanted to express my sincere gratitude for your invaluable support in the recent APU troubleshooting process. We Swapped the APU on another aircraft and snag also transferred to another aircraft,we also installed Serviceable APU in our aircraft it's working fine  . It's internal Problem with APU