Thomas wrote:
APR mod was flashing but the airplane didn’t couple
Not sure what you mean here, but I assume you know that if the CDI is more than half scale off-track, it remains in HDG mode, until the track is intercepted, whilst APR (or NAV) light is flashing. Once the selected track is intercepted, AP switches to NAV/APR mode, and the light will come on steady.
I’m sure you know this 
I also assume when you say “it didn’t couple”, you mean that the AP flew through the track, without intercepting?
Hello Peter,
I had studied your article about upgrade Sandel and I hope you can answer my question. Sometimes I could observe my AP didn’t capture the NAV-SIGNALS ( VOR, GPS and ILS) but this malfunction is now permanent. For an expert verygood to find out the fault. So sole source for NAV-SIGNALS is my GNS 530. I can see all NAV-SIGNALS on my SN 3308. As usual I fly with HDG BUG tointercept the
VOR-Radial or GPS-Track the KC 191 is flashing in NAV-MODE or APR-MODE but my TB 20 don’t couple.
That was the reason to pull my KC 191 and send to my shop ( Straubing). The result on the bench was no error.
I got the KC 191 back and put it in with recommendation to check the wires
( NAV-Deviation from KC 191 contacts U/17 to GNS 530 contacts 21/22). I did it.
The contacts are fine and the wires to the units have zero Ohm. A test on the ground and a flight the fault is still exact the same.
My question is now, is it right that only these 2 wires are responsible for the proper capture?
Thanks in advance Thomas
Maybe @wigglyamp might have an idea here?
A number of signals need to be present for the autopilot to capture and track.
I suggest checking the 3308 IM wiring diagrams for a KAP150 (in the back of the IM) and checking the wires. It is a lot more than 2 wires.
The “proper way” to wire up an autopilot is to use the EHSI’s FCS (autopilot) outputs. But often it was not done that way and the EHSI was bypassed so the autopilot was picking up signals direct from the radios and the GPS. If that is the case, then the EHSI showing the correct indications woul not tell you anything. And if the EHSI was installed as the simplest possible “KI525 replacement” (for which the wiring is provided by Sandel, too) then this is how the autopilot works; it is not driven from the EHSI.
This is the simple approach, done in most TB20s I would think
Can you get the work pack for your EHSI installation Thomas? You have a right to a copy of it. It should contain the wiring diagram used.
Hi Peter,
it’s very nice of you to give me further information.
Firstly I have no chance to get the correct diagram. It was 12 years ago. I assume my installation is a bypass between SN 3308 and KC 191.
If I understand your diagram right the switch ( GPS / NAV) is symbolic and must be in the GNS 530 normally. That could make sense for me and explain that the NAV-SIGNALS be missing. These Signals come from the GNS contacts 22/21 and go to KC 191. In conclusion is the switch intergral part of GNS so the error must be in the GNS. Tomorrow I am going to measure the voltage.
Contacts at the KC 191 should be 150mv.
I hope I have understood it right.
Thomas
With a KLN94 that switch is real, because that is a GPS only and “navaid stuff” is done with the KX radios. That switch switches many signals so is implemented with an avionics relay like this
which contains several 4-pole telecom-type relays and some circuitry.
With a GNS box the switch is in the software, because a GNS contains both a GPS and the navaid radio (VOR/ILS). That incidentally confuses the hell out of many users… the “VLOC” or whatever stuff (I never had a need to know, so I don’t 
).
You should try to trace the installer and get the work pack. It will save somebody a lot of work one day. One of the biggest things avionics shops hate is somebody turning up with a plane full of weird unknown wiring. They have to quote on the assumption it will be that (because it usually is) but if they pitch it too high they don’t get the job.
Pre-GT TBs are normally wired up to the wiring diagrams in the ATP TB MM CD (for GT models, Socata never published most of the wiring to ATP, but it “exists in the wild”) but you still need to mess about to establish which of several optional circuits were actually wired.
So get that work pack if you can.
The autopilot doesn’t couple, HDG BUG is OK
In memory of the wires form GNS to KC 191 everything was ok, zero Ohm. In accordance to the diagram I have checked the voltage today.
Signal-ground to KC 191 PIN 17 and PIN U = 0V ( proper would be 150mV).
I suppose the switch for NAV-SIGNALS is in the BOX GNS 530. NO voltage is coming out.
Can it be in that case that my GNS 530 this malfunction caused( no capture of Track, VOR an ILS) ?
Thanks in advance
Thomas
After nearly 2 years I can say the temporarily malfunction is finally eliminated. It was the analog output of GNS 530.
A new mainboard and everything is fine.
Thanks for supporting
Thomas
Interesting. Well done for narrowing this down.
Intermittent faults are a real bastard, especially in the certified avionics business where if a box works on the bench to the test spec in the MM (tested in the 145 facility) it is tagged as “serviceable” and then the next poor bugger gets the same issue, possibly outside the (much shorter on used items) warranty. I have had this myself and one has to sometimes really force people to replace such an item and not just keep sending it back as “serviceable”.
Really makes me think about this.
Th difference is that I could not engage or arm NAV or APPR most of the time. When I was able to, the AP would follow the source.
Flipping the GIA63 unit (1 and 2), reset and reconfiguring, gave me a low cost solution.
Happy to see one more AP troubleshot here :-)
We have a KAP150 autopilot (KC191 computer, KG258 Attitude Indicator, KI525A HSI and KAS 2978 altitude pre-selector) installed in a TB20 that refuses to engage. This started as an intermittent problem but now happens all the time. The KC191 passes the self-test at startup – trim light flashes 4 times, 6 beeps then AP light flashes 12-13 times. We replaced the autopilot disconnect switch as this is a known problem and had the KC191 bench-tested at the Bendix-King Service Centre in France and it was OK.
What else could be causing this problem and is there anywhere in the UK we could take the aircraft for diagnosis ?
Thanks !
