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A diagnostic tale of trim gone wrong

KJBSouth

Well Known Member
The dream weaver lost its ability to trim left roll. (Stick grip hat switch; Ray Allen 2-10a trim servo; VPX power supply and Garmin GSA28 roll servo) Trim to the right worked just fine. The autopilot had pushed it to the right during the last flight. The fault was found on the ground when I attempted to center the trim.

Day one: I started diagnosing at the wrong spot in the electronic chain. I pulled the seats and floor pans, and removed the Ray Allen only to find that its motor ran well in either direction depending on voltage polarity. I believed the hat switch was working OK as the small trim active light was appearing on the G3X PFD when the switch was pushed either right or left.

Day two: I finally resorted to studying the excellent wiring diagram Stein had supplied with the G3X system. The hat switch grounds a pin on the VPX box which then connects voltage and ground to the Garmin GSA28 sutopilot servo located in the right wing.

The GSA28 is a smart servo. When the autopilot is on, the servo generates it own trim instructions depending on the aileron induced torque at the servo arm. When the autopilot is off, any hat switch/VPS trim signals are passed directly on to the Ray Allen.

Finally enlightened I pulled the wing cover plate and unplugged the DB15 connector from the Garmin servo. I refreshed my memory of connector pin layout and checked for voltage and polarity on the two incoming signal wires. All checked OK. Since there is only one well secured wire pair directly from the Garmin servo to the cockpit and on to the Ray Allen, the fault must be with the Garmin servo.

At this point I brought Stein Air back into the picture. The Garmin servo is beyond warranty. Chris and Nick have arranged an exchange with Garmin at less than half the cost of a new servo, delivered. It should arrive in two days.

I plan on being back in the air this weekend.

Lesson forgotten and relearned: Thoroughly understand the operation of any system before tearing into it for a repair!
 
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you should be able to jumper pins 11 to 13, and 12 to 14 at the GSA28 servo Dsub cable connector to verify the trim system, less the GSA28 servo. That will prove that the servo is at fault.
 
you should be able to jumper pins 11 to 13, and 12 to 14 at the GSA28 servo Dsub cable connector to verify the trim system, less the GSA28 servo. That will prove that the servo is at fault.

It might be easier to install the adapter that comes with the install kit...

18.5.2.5 GSA 28 Removal Adapter The GSA 28 connector kit is shipped with a GSA 28 removal adapter (011-03158-00). This part is not intended to be installed with the GSA 28, but is used to replace the GSA 28 when the harness connector is un-plugged. The removal adapter contains an internal 120 Ω resistor between pins 2 and 3 for CAN termination. It also contains shorts between pins 11 & 13 and 12 & 14 to pass through power for trim
motors. The intention of this component is to allow operation of the CAN bus and trim motors when the servos are not plugged into the harness. It is recommended that a removal adapter is kept with each servo installation, in case the GSA 28 needs to be removed without losing functionality of the CAN bus and trim motors.
 
PS: you can also look at the servo page in the setup menu and see if the servo is getting trim commands.
 
It might be easier to install the adapter that comes with the install kit...

18.5.2.5 GSA 28 Removal Adapter The GSA 28 connector kit is shipped with a GSA 28 removal adapter (011-03158-00). This part is not intended to be installed with the GSA 28, but is used to replace the GSA 28 when the harness connector is un-plugged. The removal adapter contains an internal 120 Ω resistor between pins 2 and 3 for CAN termination. It also contains shorts between pins 11 & 13 and 12 & 14 to pass through power for trim
motors. The intention of this component is to allow operation of the CAN bus and trim motors when the servos are not plugged into the harness. It is recommended that a removal adapter is kept with each servo installation, in case the GSA 28 needs to be removed without losing functionality of the CAN bus and trim motors.

I'd like g3xexpert to chime in on this incase someone finds this thread later looking for info. 120 ohm resistors are normally used high speed CAN bus and require 2 resistors in parallel from the can high to can low lines. If garmin is running resistors in each device, they're using a low speed fault tolerant design. This is a defined standard, but requires 2 resistors per device, one on the can high and one on the can low side. Removing the autopilot servo and installing a 120 ohm resistor across the data lines is likely not the correct solution if you can't find your garmin supplied plug.

I would be really curious to see which design they're using and if they're using the high speed design, I'd like to see how they're accommodating installations with 0,1, 2, or 3 devices with terminations resistors in them.
 
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Pretty much all the info you are asking about can be found in the G3X install manual.

Below is the CAN bus troubleshooting document they published as well which describes how to test/check things out if you need it.

The termination resister (120 ohm) is only used at the 'ends' of the bus (which then yields 60 ohms) and is generally located at the pitch and roll servo's, but maybe the GMU11 if you extended one of the legs out to it.

The termination resister is included internally in most devices and it is placed in the circuit only if you install the appropriate jumper in the connector.

In your case it didn't really matter about the CAN bus because you were trying to test the trim system I think?
 

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That helps. I haven't dug into the install manual yet since I haven't got to that point of the build. I didn't realize it was just a jumper to activate/deactivate the termination resistor.
 
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>Lesson forgotten and relearned: Thoroughly understand the operation of any system before tearing into it for a repair!

I worked with an overly rambunctious A&P once who would jump right into a project and then in short order, be in over his head and we would have to bail him out. So I told him one day...."Jim, I'm going to give you best possible advice....when you walk up to an airplane project put your hands in your pockets, and _Leave Them There_ for the next five minutes!:D
 
All's well

Garmin expedited a new servo. It's been installed and the linkage aligned. Trim works fine now. I'm sending the old servo back to Garmin today.

Also, I've dug into the box of left over parts and found the "removal tool". I never before knew it's purpose. Thanks to all for your advice.
 
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