January 27, 2020

Redo of the Connector for Autopilot Servo

In my last post I detailed my first attempt to build up the d-sub connector for the Garmin autopilot; I was using their G3X video series on YouTube for instruction. Surprisingly, the official G3X install manual recommends a process for wiring the connectors that’s quite different, but it’s much more in line with what I’ve seen in production aircraft wiring. So, I took a few steps back to redo the roll servo connector.

The first difference is in how the shield drains are attached to the shielded braid on the M27500 wire. Per the Garmin install manual, the shields should be exposed in a window by removing a portion of the Tefzel jacket, approx. 4.5″ from the end of the wire. I used a sharp razar knife to create a window, no more than 0.35″ wide. This is an improvement over the technique in the Garmin YouTube video, which instructs you to expose the braid at the end of the wire; this can lead to chafing of the inner connectors.

Creating a window to attach a shield drain

The second change is using a flat braid of wire for the shield drain, rather than a piece of insulated M22759 wire. The rationale here is that the bare braid is easier to see during “Instructions for Continued Airworthiness” (ICA) inspections. The flat braid is wrapped around the window and then soldered. The braid is similar to solder wick, so it was extremely easy to solder to the shield. In the case of a daisy-chained connection like the CAN node connection to the servo, each of the M27500 wires gets its own shield braid but shares a common ring terminal on the back shell, rather than pig-tailed arrangement that was shown in Garmin video.

The shield drain braid solders on quite easily
Sliding heat shrink tubing over the soldered shield drain

The next change is in how the spliced conductors are soldered together. With the 22-gauge wire used, I found it very difficult to form the 2 wires-to-1 wire splice needed for the CAN connection. You’re supposed to twist the wires together to form a mechanical connection and then solder it, but I found them hard to twist together — maybe my fingers are too big! A little trick that helps is to wrap the joint with a single strand of wire as shown in the next picture, and then solder the joint.

Wrapping the joint with a single strand of wire
CAN node connection ready

One suggestion I did take from the Garmin video (not mentioned in the manual) is to wrap the bundle with fusion tape, in the area where the strain relief is attached. I used plumbing friction tape during my first build, but this fusion tape is nicer.

Wrapping the wires with fusion tape

I’m not a fan of closed-barrel ring terminals. Maybe I’ve never found good ones, but my experience is that they easily fail, allowing the wire to pull out. The shield drains are supposed to be connected to the backshell with ring terminals like shown below. Instead, I bought a bunch of open-barrel ring terminals, and then used an appropriate crimper that curls the barrel tabs into the wire and bites into it, forming a very strong connection.

I hate these ring tongue terminals!
I am using this style of terminal instead
Ring terminal crimped onto the shield drain

After crimping the ring terminals onto the braided shield drains, I attached them to the backshell using hardware as recommended in the Garmin manual: MS51957-42 pan head screws, flat and split ring washers.

Complete connector with shield drains attached
View showing more of the autopilot connector and harness

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