How To: Read a Wiring Diagram

Wiring diagram courtesy John Hogan 

If you’re interested in engineering, you may very well need to learn how to read wiring diagrams, which are basically a visual representation of how the wires in an electrical system are connected and work together. If that sounds overwhelming, we’re here to give you a head start. In this guide, Aviation Technician John Hogan walks us through the steps he uses to read this example diagram to install a Garmin GFC 500 autopilot system in a plane.

Before You Start: Check Manual Currency
First and foremost, you should always go into the Garmin databases and make sure that [you’re] using the most current [version]. It'll have a date next to it when you look it up. It depends on what's getting updated, but the manuals will also have a record of revisions at the beginning stating what got changed. They also have [a] little bar on the left margin of the pages. If you see a bar, that means it's part of the most recent version. So for example, when this one was updated in July, it says “Updated the trim switch drawing in figure A-7.”

Step# 1: Determine Relevant Manual Pages
I'm looking at a wiring diagram from Garmin for installing their GFC 500 autopilot system. It sounds complicated, but the autopilot system itself is relatively simple. It's several pages. But, step one would be determining which pages you actually need, because Garmin will provide anything that could involve you playing with this system. You don't need all of those things. For example, the first two pages here are page 1 for a 14 volts Aircraft Power and Ground, and then page 2 is for a 28 volts Aircraft Power and Ground. Your plane isn't both, so you only need one of these. So let's say you're doing a 28 volts aircraft. You'll need that page. You won't need the 14 volts page. 

You have an audio and lighting bus [page]. You'll need that because you'll need audio in some fashion wired to your audio panel from your autopilot, because there's certain warnings and alerts that come through your headset, over the speaker. Then there's [the] switches page. Do you have a go around switch? Do you have an autopilot disconnect switch? Do you have a trim switch? These are all switches that can or may not be wired into the controls of the plane. Then you have what's called “CAN Bus / Servo Strapping.” This is how you connect the autopilot servos [motors] to the controller. So you have a couple little motors that control how the plane tilts one way or the other, or how it tilts forward and back. And you can put in multiple servos to have it control all kinds of things.

You could have up to five servos in certain planes. Then there's more detailed drawings of the switches, if you want those. Then after that, there's an interconnect for a GI 275. Now, maybe you have that in your plane, maybe you don't. GI 275 is a digital Garmin indicator that can display anything, really. It's a multifunction display: it can drive your autopilot, show engine instrument status, show heading course, and a whole bunch of different things. You can cycle through pages on it. Do you have a GI 275 or not? Maybe you want to connect it as shown in this picture. Maybe not. Then there's 14 volts, 3 servos [and] 14 volts, 2 servos. It's different from a 28 volts plate, and 3 servos is different from 2 servos. Then after that, it'll show you the connectors, and what each individual pin in the connector goes to. So first, what do you need? We need a 28 volts interconnect diagram. And let's say we do have a GI 275. 

Step #2: Evaluate Your Needs
So step two would be [to] look at the diagram. I highlight all the things that I need on that diagram, because the diagram will show the connectors and a whole bunch of different things coming off of it, and you don't need all of it. Garmin provides the pinouts [diagram for electrical connections] and connectors for everything. Usually what ends up happening is I have maybe a 15 pin connector that I only have 7 pins in because I don't use the other pins. So here it shows pin 7 goes to power and pin 15 goes to ground. We need both of those. And then it shows the servos for a roll servo and a pitch servo [motors that control different plane movements]. Ground is 9 and power is 10. The controller ground is 15 and power is 7. I would highlight all of those because I'll need all of those. 

Then we'll go to the audio and lighting bus. It shows the connector for the controller. For the autopilot, it says audio out high is pin 13, audio out low is pin 14. Attached to those, it shows a number of different options for audio. If you have a Garmin GMA 350, you'd use these two pins. If you have a Garmin GMA 347, you use these two pins. It even has options for non-Garmin units. If you had a Bendix King KMA 28, you use these two options, and it shows a direct line from the autopilot system you're wiring to a whole bunch of different options. 

Then you highlight which ones you have in the plane. So say we have a GMA 340 in the plane. My audio out high would go from pin 13 in the autopilot to pin 31 in the GMA. Then you just follow the list of this goes to this, and this goes to this. For the lighting bus, it's in number 11. Then that goes to, it just says dimmer bus, which is basically how the lights are controlled in the aircraft. It's usually attached to a little knob that you can turn up and down to dim or brighten the lights. After that, we have different switches to check. Do we have a go around switch? We'll highlight that and say, yes, we do. Do we have an autopilot disconnect switch? You should, if you have an autopilot connected. We'll highlight that. Then you follow those pins. 10 from the autopilot pin, 12 from the autopilot goes to this switch, and then this switch. 

Step #3: Read The Notes
Every wiring diagram has a notes section and in the wiring diagram itself there will be a little triangle, like an asterisk. If you're reading something, it's a little triangle with a number in it. You look at that number. For example, on our audio wiring diagram, there's a little note next to the wires that go from the audio out high and low to the panel that says 2. So, we go down to the notes, go to 2, and it says “ground shield of audio wire at one end only to prevent ground fluid”. A lot of the wires that get run for these systems are shielded wire, which means it has an overbraid over it that needs to be grounded into the aircraft. 

When you are flying through the air, obviously you're not attached to the ground anywhere. So, you have to have a way of grounding static electricity and completing loops and everything. [If] you don't have that grounded, you can create a feedback loop and it creates a whole mess of problems. So, that's just a little note there saying, “Make sure you do this.” It also has a note that says when you're connecting it to the audio panel, [it] should go [in] this pin or this pin. The note says it's acceptable to use other inputs as long as they are a certain type of input. If your inputs are taken already, you can use other ones as long as they're the same kind. [They’re] good notes to have.

Step #4: Check Your In Outs
There's also references to other diagrams too. Next to the wiring diagrams, there’s a drawing of what the connector looks like. And then each pin will have a designated description. So, if I'm looking at my autopilot connector, it is a 15 pin connector. That means I have 15 pins that can be used. It shows pin 1. What type is it? Is it in or out or both? Pin 1 is an out, and it's a certain type of communication channel. In this case, it's RS-232, which is a serial data port: it's a data transmission line. Pin 5, for example, is out. That is a sonalert [a type of alarm], so that's out. Audio in 7 is in, because that's power. Then you can look at all [the others, and] if you don't want to use [something], you don't use that pin. Check your connector pin outs to confirm you are using everything that you need.

Step #5: Get Building!
As far as reading the wiring diagram, I would say that's it for reading it. Step five would then be [to] build it. 

Thank you so much, John! For any engineers (or engineers-to-be) reading, we hope this helps you understand a crucial, everyday task in Avionics.