Let's talk about grounds. But first, a story to put into context why understanding what grounds are, and how they affect aircraft systems, to understand why this is so important.
I have a 08 CTLS in my shop that I've been fighting with for a while now. Dynon D-120 EMS has an issue where both EGTs, both CHTs, and the oil temp fluctuate rapidly. However, it would never happen on the ground for me to test for. If I can't test for it, I can't fix it unless you just want me to throw parts at the problem, and even then it's not a guarantee. Still though, I did the typical thing, checked grounds. I put an amp on the wire to ground on the EMS connector and checked for voltage drop to the battery. Hmmmmm, got a little over a tenth of a volt. I disconnected grounds, and noticed something. Something that didn't click before. The grounding block inside the aircraft mushroom is freaking anodized aluminium. What.
I mean really. Anodized aluminum must be sanded at the contact points to get a good connection. Removed the bolt that goes through the firewall, and nope. Not sanded. Really???
So, I take every ring terminal, bolt, and grounding connection from the EMS to the battery and clean them up with a wire brush on a Dremel. I then apply a little DC-4 to prevent oxidation in the future (including to the sanded aluminium block). Clamp everything down, and now the resistance has gone down to about a third of an ohm, and it translated into about 10 millivolts. A lot better! Still would prefer less, but that's acceptable enough. Reassemble, start up aircraft.... everything seems fine. Released to service.
It comes right back with the problem again after about 20 minutes in flight. Alright you know what? Let's go fly so I can see what exactly the issue is. So I grab my little USB data logger, plug it into the cigarette lighter to monitor the power bus, and off we go Sure enough about 20-30 minutes, the whole panel starts going crazy. Both EGTs, both CHTs, and Oil Temp all fluctuating wildly. Come back, fully expecting it to be a regulator issue. Download the data from the logger, and it's relatively stable.
I've never seen anything like this. What I figure is something is overheating inside the EMS unit. I need to determine though.
I make a diagnostic harness. 111 solder joints, 37 crimps, and 148 shrink tubes. Basically it's a male to female connector with a third female connector pigtailed off it, so that I can insert pins with my meter and get live readouts. I do a ground run and start probing the readouts, everything looks good... wait no it doesn't. The panel starts going crazy. I hook into various pins and can't make heads or tails, everything is going wild. I call up Dynon, and tell them all I did, and they say "Alright... that's everything we would have suggested, send it in."
Before I do though, I have another EMS unit I could hook up real quick just to see what happens. After a few minutes, it too goes wild. Welp, not the EMS.
Oh. GREAT. What in the hell could be so wrong that it causes all kinds of problems across multiple pins? I even checked voltage drop to battery through my diagnostic harness, it's swinging from as little as a few millivolts to as high as a decivolt. Quick check of resistance on the harness... 0 ohms. So there's something real screwy going on.
So I cut loose every piece of tape on the installed harness from firewall to EMS, and start checking for shorts or something to explain what is going on. Nothing. Nothing at all.
Alright. I'm not going to go tearing apart every single harness in this aircraft yet. Let's see if there's some weird ground oddity. So I pull all the connections from that grounding block and hit the master.
The EMS still turns on.
I take a closer look at that grounding pin. HUH? There's a wire splice a little ways back. Where's this going?
I follow the little 22 gauge. It goes to that grounding block. Mental note: replace that with 14 guage later. 22 is not large enough for a trouble free ground.
The other wire is... a 14 gauge? Where's this going? Through the firewall. So I find the wire on the other side.... it's going through the fire sleeve. OH GOD DON'T TELL ME.
It was bolted to the engine intake on the right side.
WHO DOES THIS S***?
So what's wrong with this? Lets talk about grounding.
Grounding doesn't just provide a return path for power. It also serves as the electronic reference of 0 volts. This is CRITICAL. So critical, that the National Electric Code dedicates a HUGE ARTICLE in the code just to grounding. It's probably the LARGEST article in the whole codebook. It's CRITICAL. (mind you, their grounding is a little different because they dedicate a wire to just grounding, while we combine our ground/return into one for weight concerns)
Everything electrical is primarily driven by the difference in electrical potential. There's a few oddballs like inductors and radio, but let's not go there. So when you have a 12 volt system, if ground is not effectively 0 volts, then you don't actually have a 12 volt system as far as your systems are concerned.
When you do grounding, it's incredibly critical that you bond systems in a way that:
Provides the least resistance to ground while under load. Just tossing your voltmeter on it isn't enough. Resistance drops more voltage as more amperage flows, which causes your power differential to go down. Those milli-amps your multi-meter are pumping won't detect a dirty ground, you need to put a heavy load on it and do a voltage drop check. That's also why grounds should be HUGE; much larger than what is typically called for for normal current carrying.
The grounds should be branching, with no loops. Loops can pick up stray electromagnetic fields, converting them to voltage and current that go round and round the loop, and screw with the potentials and voltage drop. This is called a ground loop, and creates a lot of interference.
Do NOT provide alternate grounding paths without sizing the wire to the maximum potential current they could experience. THIS IS A FIRE HAZARD. It's also easy to do without realizing what you are doing. If you follow rule 2, you won't have this problem.
Do not provide ground paths that could back feed to systems and generate interference.
This little extra wire violated rules 2, 3, and 4. Basically, by attaching a wire to the ground block inside the aircraft, and a junction to the engine block, an alternate ground path has been provided, that in extremely unlikely circumstances, can provide a new path to the battery and burn up the 22 gauge wire.
In addition: if any grounds get dirty, it would cause a huge amount of interference on the EMS. Remember: electricity follows all paths, but favors the one of least resistance. As the ground gets dirty, more and more power will be shunted through that alternate ground. This will raise the potential on that ground via voltage drop. Now we know the alternators on our aircraft are not a smooth output. They fluctuate badly, by design (most are designed this way). That means every single on-off pulse is a voltage spike to EMS ground.
I cleaned this up, and no more fluctuating gauges yet, as well as a hazard eliminated.
Now, I know that there are those that advocate adding an extra wire from the engine to the battery. I've said before that I don't like it, but because the sizing is supposed to be large enough to handle the current it may experience, and both effectively terminate at pretty much the same point, I find it acceptable. My personal preference is that I would rather people increase the wire size of the original engine ground and terminate it on a terminal block as close to the battery as reasonable, and branch out from there. That's how it should be. If you stick to branching grounds without creating loops, rules 3 and 4 are not needed.