KI525A glide slope bar intermittent

[Fritz1]

424SB KI525A - KFC225 glide slope wiring.pdf424SB KI525A - KFC225 glide slope wiring.pdf

 

I need your help to figure out the cause of my intermittent glide slope bar on the KI-525A.

 

History:

The intermittent glide slope bar started a couple of years ago after a GTX345 was installed. The glide slope bar sometimes disappears when jolt goes through airframe, like landing gear coming down or aircraft touching down. GNS530 was overhauled, KI-525A was overhauled twice. A couple of months ago I realized that the glide slope bar was disappearing when pressing on a wire bundle in the copilot footwell. The autopilot and the flight director were never affected, always stable, so the autopilot was always getting good data from the 530.

 

My conclusion: 

Something is intermittent between the autopilot P2251 connector and HSI connectors (marked up diagrams enclosed)

 

During my last flight about a week ago the glide slope bar was working fine I took the HSI and and the autopilot control unit out to get to the bottom of this intermittent problem and checked continuity between HSI plugs and autopilot plug P2251:

 

GS down:   HSI B connects to AP 52

GS up:        HSI E  connects to AP 12

GS Flg+      HSI J does not connect to AP 14 but connects to AP 15

GS Flg-      HSI W connects to AP 15

 

This would mean that the GS Flag circuit is shorted at the AP plug, however the HSI glide slope bar is working fine in test mode and I cannot make the glide slope bar disappear no matter how I move the wire bundles in the copilot footwell. Something does not add up.

 

I have enclosed a scan of all wiring diagram that I am using, the KFC 225 installation manual is revision E which I think applies to my airframe made in 1999 SN280.

 

Before finding the disconnect 14-J I was planning to parallel the four wires for the glide slope between the HSI and the autopilot. Now I am wondering if I have the wrong wiring diagram or if not how the glide slope bar can test correctly and appear half up when the 530 goes through its startup test in spite of the GS flag circuit being shorted at the AP plug.

 

Soo, what is it that I am not getting??

 

Your insights are greatly appreciated!

 

 

[Fly Boomer]

4 minutes ago, Fritz1 said:

424SB KI525A - KFC225 glide slope wiring.pdf424SB KI525A - KFC225 glide slope wiring.pdf 2.72 MB · 1 download

 

I need your help to figure out the cause of my intermittent glide slope bar on the KI-525A.

 

History:

The intermittent glide slope bar started a couple of years ago after a GTX345 was installed. The glide slope bar sometimes disappears when jolt goes through airframe, like landing gear coming down or aircraft touching down. GNS530 was overhauled, KI-525A was overhauled twice. A couple of months ago I realized that the glide slope bar was disappearing when pressing on a wire bundle in the copilot footwell. The autopilot and the flight director were never affected, always stable, so the autopilot was always getting good data from the 530.

 

My conclusion: 

Something is intermittent between the autopilot P2251 connector and HSI connectors (marked up diagrams enclosed)

 

During my last flight about a week ago the glide slope bar was working fine I took the HSI and and the autopilot control unit out to get to the bottom of this intermittent problem and checked continuity between HSI plugs and autopilot plug P2251:

 

GS down:   HSI B connects to AP 52

GS up:        HSI E  connects to AP 12

GS Flg+      HSI J does not connect to AP 14 but connects to AP 15

GS Flg-      HSI W connects to AP 15

 

This would mean that the GS Flag circuit is shorted at the AP plug, however the HSI glide slope bar is working fine in test mode and I cannot make the glide slope bar disappear no matter how I move the wire bundles in the copilot footwell. Something does not add up.

 

I have enclosed a scan of all wiring diagram that I am using, the KFC 225 installation manual is revision E which I think applies to my airframe made in 1999 SN280.

 

Before finding the disconnect 14-J I was planning to parallel the four wires for the glide slope between the HSI and the autopilot. Now I am wondering if I have the wrong wiring diagram or if not how the glide slope bar can test correctly and appear half up when the 530 goes through its startup test in spite of the GS flag circuit being shorted at the AP plug.

 

Soo, what is it that I am not getting??

 

Your insights are greatly appreciated!

 

 

When you said "intermittent" you have half the problem.  The other half is "where".  There are some super-talented people around here that will have some suggestions, but you may have to wait until after the holiday.