Avionics Wiring Harness: A Tutorial

[alexM]

I just want to bump this thread because it is so useful. I just ordered all the tools and a boat load of wire using the links Eric provided.

I'm also here to ask a follow on question. When I bought my brand M avionics I got their wiring harnesses. All (most anyway) of my sub D connectors are already done. In come cases there is enough wire that I can connect the various contraptions together directly but in others I will need to butt splice wires and put heat shrink over the top of them. I did plenty of that in my previous installations so I'm comfortable with the work.

After placing my orders I realized that I didn't see butt connectors mentioned. A google search produced the following hit:
wow that's a long link
Are these up to the task or do I need to go somewhere else? They look identical to the ones I used but I don't know crap when it comes to this stuff.

I also just realized that I'll have a bunch of terminals to crimp and I didn't see that tool on the list. The one I borrowed when I was doing supervised work was a fancy ratcheting tool that wouldn't let go until you had crimped the terminal sufficiently. I'd like something like that if you have any suggestions.

[MikeFairbanks]

I spent 12 years in the military as an avionics tech... went through aviation soldering and connector repair school, and for the last 25 years have worked in the satellite communications industry, where I had to get my NASA soldering certification.

First I want to respond to the comments that didn't like the exposed wire on the crimped pins. When I went to school for this if you brought up work that had the insulation right up against the pin you failed. I was taught that there should be a slight gap there... if you can push the insulation all the way up against the pin or cup, then you can't really be certain that the wire completely fills the pin or cup.

Second, regarding butt splices. I like the hermetic sealing splices with the solder ring in them. What you have to be careful of with these though is that there is not enough solder in that ring for bare wires... you need to tin your wires before using them. The ring is just to bond the tinned wires together. If you use them with bare wires it will probably stick them together, but the electrical connection will be questionable, and occasionally you'll be able to pull them apart. We used these on F/A-18's in the military, so if you use them right there is nothing wrong with them.

http://tinyurl.com/23uuv8s6 here's an example

[bumsteer]

Hi Alex

I purchased the ratcheting crimper I'm using from Aircraft Tool Supply. The crimper itself is P/N E300-002. It was in a kit with a bunch of terminals that I purchased at Oshkosh. It came with jaws for crimping ring/spade etc. terminals.You can then purchase jaws for BNC and open barrel connectors. You can probably find it elsewhere also. Works great.

Rick

[jiott]

I got my crimpers, wire stripper, etc. from SteinAir.

[Eric Page]

The problem with @alexM's linked butt splices is that they provide no insulation support, so the wires are permitted to bend where they exit the crimp. Since the goal of a crimped connection is to crush the wire and connector into a homogenous, gas-tight mass, you essentially turn a stranded wire into a solid one. If that solid conductor is allowed to bend, it may crack and break. Hence the need for insulation support -- to prevent the wire from bending at the crimp.

There are a few ways to skin this cat:

1. The gold standard: Use TE Connectivity AMP PIDG butt splice terminals, which include a metal insulation support crimp. Note that there are numerous kinds of insulated butt splices out there. The ones with brightly colored opaque insulation (available at hardware and auto parts stores; like this) are not suitable for aircraft use, as their insulation crimp is accomplished only by the plastic covering. With age and heat cycles, the plastic will suffer from... well... plastic deformation: they'll eventually let go of the wire insulation. The PIDG's metal insulation support crimp won't let go. You can get PIDG butt splices from B&C Specialty Products, SteinAir or any decent electronic component supplier. Both B&C and SteinAir (frame | die) sell crimp tooling at reasonable prices (which will also crimp PIDG ring and Fast-On terminals). This is the quickest and easiest solution: strip wires, make two crimps, done.

2. The butt crimps that @alexM linked above can be used, along with two layers of adhesive-lined heat shrink. Cut one piece (smaller diameter) about an inch longer than the gap between insulation ends on the wires, and the second piece (larger diameter) about an inch longer than that. Center the first piece over the crimp, shrink it, then center the second piece over the first and repeat. You're using two layers because hot glue doesn't make a chemical bond and tends to let go when it's flexed, especially when it's cold.

3. Use adhesive-lined solder sleeves. I have no personal experience with these but they seem to work, and one of the big names in connectors and heat shrink products (TE Connectivity Raychem) make some of them. As you'll see in this Kitplanes article comparing different brands, the installation process can be difficult to get right. SteinAir sells the RayChem version in three different sizes, as well as sleeves with a pigtail lead-out wire and a torch for installation. That link also has a video from Stein showing how to install them. The other brands tested by Kitplanes are all over Amazon and eBay.

[Eric Page]

What you have to be careful of with these though is that there is not enough solder in that ring for bare wires... you need to tin your wires before using them. The ring is just to bond the tinned wires together. If you use them with bare wires it will probably stick them together, but the electrical connection will be questionable, and occasionally you'll be able to pull them apart. We used these on F/A-18's in the military, so if you use them right there is nothing wrong with them.

Be very careful here. The amount of solder in these sleeves is engineered to provide an electrical and mechanical connection without providing enough solder to wick up the wire and stiffen it beyond the glue ring. If you tin the wires before inserting then into the splice, there's a very good chance that you'll wick enough solder up the wire (inside the insulation) to defeat the purpose of the insulation support. This can result in a wire that breaks inside its insulation, making a difficult-to-troubleshoot intermittent fault.

[Eric Page]

Here's a tutorial by Bob Nuckoll's showing how to do another wire splicing technique, the soldered lap splice. This obviously carries similar risk to tinning wires before using solder sleeves, so good technique and minimum solder is called for.

This tutorial shows how to terminate multiple wires into a single PIDG terminal or splice.

[Eric Page]

All (most anyway) of my sub D connectors are already done. In come cases there is enough wire that I can connect the various contraptions together directly but in others I will need to butt splice wires and put heat shrink over the top of them.

Another technique to consider, if you just need to extend MGL-supplied wires that are pre-terminated with D-Sub pins or sockets, is to crimp the mating pins or sockets to the extension wires, mate the terminals and cover with adhesive-lined heat shrink. Offset each connection by an inch or so to keep bundle diameter to a minimum.

Links for D-Sub pins and sockets are at item 3, in post #3, above. The links to eBay are dead; here are new ones to search for pins and sockets.

[Danilo]

Thanks Eric for this post. Gonna be wiring a g3x panel on my build and this is super helpful!

[tdldedcd]

Eric, thanks for all the posts you have done in regards to wiring. It has been extremely helpful to myself and many others. I have another one for you. I have 2 Ray Allen control sticks with trims switches. I need to bring them together with the panel mounted trim switch and then back to the GA28. What is the best way to bring these wires together? Do I need to utilize a mixer?

Thanks again for all your help.