WAAS Glideslope indication issues, King 525A

The trouble I am having is that the glideslope indicator is staying hidden and then all of the sudden it drops to just above middle (perfect on glideslope) usually about one bar above. Yesterday on one approach it dropped to just barely below but still captured. With my autopilot KFC150, it seems like I am supposed to press the approach button after the glideslope comes in and is around the one bar high indication.

Thinking the HSI was the issue, we pulled it yesterday to check it and installed a loaner. The loaner is doing the same thing.

My 530 was not WAAS so I don't know how it would have behaved with that.

I also do not know what the test movements are, or should be with the 540/HSI combo. I will start watching to see what it is doing.

Does anyone know what the HSI SHOULD be doing during the test when starting everything up ?

Doing some research on the HSI, there are some common GS issues. One is a capacitor issue. The other is possible issues when the unregulated voltage is out of parameter (it uses regulated and unregulated circuits)
It's possible that even the loaner could have the capacitor issue or of course a voltage issue should make both HSI's behave the same. Good reading: https://miscsolutions.wordpress.com/2011/06/05/what-you-probably-didnt-know-about-the-bendix-king-ki-525a-hsi/

In my Mooney I really need a heads up before any decent starts...to slow enough so that I can drop the gear or it is difficult to do. And of course a heads up to press the approach button instead of "NOW" would be great. As it stands, a WAAS approach in IMC using the glide slope may not be the best choice.

I THINK the GS indicator works smooth and normal on an ILS. I may check that today. If it does then I may be really lost in what's going on.

I have my IFD540 connected to both the kcs55 HSI and a KFC200 autopilot which is similar to the 150 and it functions normally. What you describe is abnormal. When the IFD is first powered up and displaying the welcome statement screen it is activating the output channels and testing the CDI, glide slope and flags just like a G product. This behavior is described in the pilot guide.

Before you spend a lot of money on a replace and test cycle, do yourself a favor and find an avionics shop that has the King autopilot test machine. It is a KTS somethingorother. It will allow the Bench test and calibration of the autopilot, HSI, and FD. In addition it can be used while flying the airplane to fine tune or analyze an autopilot problem. If your shop doesn't have one, find one that does. It was by far the best money I've spent and resolved a number of issues (unfortunately none like you are describing).

Thanks all !