Manual Electric Trim Switch - KAP 150 AP - Need a New One

[dwanzor]

Recently took my new to me J model into Bevan Avionics in Wichita, KS for a transponder upgrade and asked them to troubleshoot a minor autopilot issue. 

Then ended up swapping my KAP computer. That fixed the original issue, however, a new issue has popped up which they have traced back to the manual electric trim switch on the yoke. Some malfunction within this switch is causing the autopilot to disconnect in flight at different intervals. 

They would like to replace this switch. A new part from Honeywell is $3k+ and is not available until at least October 2025. I see a couple on Ebay for around $300, but before rolling the dice on an Ebay part, I thought I would check here to see if anyone had one due to a recent autopilot upgrade. My understanding is that this trim switch will usually get replaced during an autopilot upgrade. 

I have the KAP 150 autopilot, but I think the Bendix King trim switch could be somewhat more universal than that. It is a split switch, left side is smaller. 

If anyone has one, or can point me to a good source, I would appreciate it. 

[mike_elliott]

I have one from a J that was pulled when AP was changed out a number of  years ago.  I also have the elusive vaccum switch for the KI256. DM me if interested

Edit, just checked it with a multimeter, works ok.

[IvanP]

If you cannot find replacement for reasonable price, the switch can be rebuilt. There are 3 microswitches tied together in the contraption. These can be sourced from Mouser and other places relatively cheap. The wiring is somewhat confusing, but with a bit of patience and some soldering skill, you can do it yourself. I did it couple of years ago, but cannot recall the price of the individual switches. At that time, new switch was listed at Textron for about $1,200 if I recall correctly. 

The images are of switch installed in Bravo with KFC 150 A/P, but I believe that the yoke trim switch is identical to that used by KAP.

[carusoam]

6 hours ago, dwanzor said:

Recently took my new to me J model into Bevan Avionics in Wichita, KS for a transponder upgrade and asked them to troubleshoot a minor autopilot issue. 

Then ended up swapping my KAP computer. That fixed the original issue, however, a new issue has popped up which they have traced back to the manual electric trim switch on the yoke. Some malfunction within this switch is causing the autopilot to disconnect in flight at different intervals. 

They would like to replace this switch. A new part from Honeywell is $3k+ and is not available until at least October 2025. I see a couple on Ebay for around $300, but before rolling the dice on an Ebay part, I thought I would check here to see if anyone had one due to a recent autopilot upgrade. My understanding is that this trim switch will usually get replaced during an autopilot upgrade. 

I have the KAP 150 autopilot, but I think the Bendix King trim switch could be somewhat more universal than that. It is a split switch, left side is smaller. 

If anyone has one, or can point me to a good source, I would appreciate it. 

good news…

find the threads regarding the microswitches that get actuated by the physical switch…

swapping out the microswitches is a much lower cost.

 

PP thoughts only, from an ancient memory… hoping this memory is still intact.  

 

best regards,

-a-

[milotron]

On 1/29/2025 at 10:38 AM, IvanP said:

If you cannot find replacement for reasonable price, the switch can be rebuilt. There are 3 microswitches tied together in the contraption. These can be sourced from Mouser and other places relatively cheap. The wiring is somewhat confusing, but with a bit of patience and some soldering skill, you can do it yourself. I did it couple of years ago, but cannot recall the price of the individual switches. At that time, new switch was listed at Textron for about $1,200 if I recall correctly. 

The images are of switch installed in Bravo with KFC 150 A/P, but I believe that the yoke trim switch is identical to that used by KAP.

@IvanP Any chance you have images of the inside of the switch assembly? I am having an issue where the wider switch half is a bit floppy. It still seems to switch fine, but feels like a spring has broken. Or does the switch just rely on the microswitches for returning to center?  

 

Thanks 

[IvanP]

3 hours ago, milotron said:

@IvanP Any chance you have images of the inside of the switch assembly? I am having an issue where the wider switch half is a bit floppy. It still seems to switch fine, but feels like a spring has broken. Or does the switch just rely on the microswitches for returning to center?  

 

Thanks 

Seems like the return spring is tired or broken. Unfortunately, I did not take pics of the process. I will try to describe it as best as I can. There are two springs (one for each half of the switch) in the top bezel of the assembly that do most of the work of returning the control levers into neutral positions. These are just simple steel wires that may be tired after decades of work. Each switch also has a flat spring/actuator, but these are not very strong. Once you have removed the switch assembly from the yoke and disconnected teh wires, the disassembly porcess is relatively simple:

Remove the two philips screws on  the right side of the assembly and that will enable you to take out the microswitches. Be careful not to damage or lose the copper spacers on each side of the assembly - they will probably fall out with the switches once you take the screws out.  Then you will need to carefully push out the rolled pins in the top bezel of the switch assembly. I used my watch repair tools fo this. This will release the wire springs from the assembly and you will be able to replace them or at least bend them back to desired shape. 

Even though the switches may still be OK and only the springs may be tired, replacing the michroswitches at the same time may be a good idea so you do not have to take it apart again. They are cheap. I recall that getting the assembly out was somewhat challenging for me - too many wires in very small space. 

Hope this will help you.

Edited June 2 by IvanP

[milotron]

That was actually quite helpful, thanks @IvanP.  Seems that I need to pull the switch assembly and see if that spring is serviceable in any way. 

[milotron]

As a follow up, I found this bad boy. So very close to being correct, but the wiring would need to be modified. ~$500USD, but no stock anywhere.

U2-451.pdf

[Fritz1]

the BK switch is in stock for about $2,600 the three microswitches are 1SX74-T , about $30 a pop and easy to replace, just take two screws out, most likely the tiny wire return springs are tired, too need to be reformed or replaced, I found a new switch, the switch in my Bravo is tired, left toggle for clutch does not reset any more by itself, triggers trim fail warning, not sure if left toggle is worn itself or just spring tired, will take apart after I have installed my new switch, think great care has to be taken that the two small toggle return springs do not jump out when the pins are extracted from the bezel