Hawker 900XP APU High Oil Temp

Working on a Hawker 900XP with APU no start and High Oil Temp Light always illuminated.

In the photo im holding the light test button down just to show the light in question since it was taken after the fix.

 

My first time working on the issue but it did have some recent history, twice the high oil temp S/W was replaced and after each replacement the APU start just fine and about 1-2 weeks later dead again. Now it was my turn to take a look and see what I could find.

I found it a little puzzling at first after reading the WDM and the APU trouble shooting manual seem to contradict each other. As I read the print it looks to me like when the switch is closed the ground is provided to the light causing it to illuminate. Although when you read the trouble shooting manual it reads that the switch is in the closed position when cold and opens when overheated. I will just have to assume that some magic that is not depicted takes place in the APU ECU to make the light illuminate. 

 
   


The first thing I wanted to check was the Lamp test 1 relay in the APU control panel, I was thinking maybe a sticky relay was causing the High Oil Temp light to be on all the time, although thinking about it now even if that was the case it would probably still start just would have the light on but auto shutdown not triggered. Anyway still some findings that helped me out. I found from PIN 1 to ground of the relay was a reading of 3 Mega Ohms and when jumped to pin 5 the light would still illuminate right away. So my next stop is going to be the High Oil temp S/W.

 

Oddly enough once I depanled the APU shroud the light went out, at first I was thinking possible chaffed wire touching the shroud or something like that, also the way the back shell was laying the back shell and wiring were both touching the shroud. Before I got to do anything else I started to inspect the plug and was still getting my High resistance reading from the High Oil Temp S/W to the plug. Further inspection and removal of the connector back shell showed me the real problem.

   


I just started to unscrew the backshell and found the wires were disconnected from the pins. This should explain our intermittent issues and the high resistance reading earlier. After crimping on some new pins putting everything back together re clocking the backshell and wiring away from the shroud all was good and APU is now starting and running without issues.

I will have to assume that when the switch was replaced and the plug tightened down again it was just enough to keep it going until it was shook loose once again. 

Still slightly confused by the wiring diagram but just decided to trust the manual and continue forward and I was lucky enough to find the issue before sinking to much time into it.