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Let's talk about grounds. Again.


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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.







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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

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Corey, amusing and educational.  I don't see on your diagram where the ground from the battery connects to ground buss, engine, etc.  I just see the grounding symbols that are in space and these don't show a return to the battery.  Do I assume these return to the battery?

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Everywhere that uses the ground symbol:

Image result


means its connected to a common ground (be it a ground plane, a terminal bus, or a string of jumper wires). We don't have to always use this symbol, we can also draw the grounds out. I didn't want to distract people with the irrelevant grounds, which is why I did not draw them, instead using that symbol, and drew the single important ground from a "-" connection on the EMS.

The battery is at the bottom. I drew a box around it to help the reader understand that is the unit.

Battery symbol:

Image result for battery symbol

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I understand the symbols. I do see that the 22 gage wire is woefully insufficient, should it be asked to carry a major ground load.  What I was asking about is what is the main path from the battery to the ground buss shown on your diagram?   On my CTSW, I believe that the ground runs from my battery "-" terminal direct to the starter solenoid attaching bolt.  Then, I believe the bolt which connects the solenoid to the firewall is used to connect to the ground running to the ground buss behind the instrument panel.  And, as I recall, this same bolt is used to attach the ground running to the engine.  For simplicity, your diagram shows the ground points but you didn't show where these go.  I is a simple matter for me to just go back and look at my plane (for the hundred plus time) to try to get the grounding main circuit logged into my brain.  The wiring diagram that came with my CTSW shows this but I find it easier to manually trace wires.  Good info on the ground buss.  If this is anodized, like yours is, I will take care of this.  It doesn't hurt to go into this area occasionally and wiggle wires and make sure screws are tight.  I'll also check the main ground(s) running from my battery to their next attaching points.

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My illustration was just focused on the extra ground and where it was wired.

Depending on the model year, the engine ground will go from the base of the alternator case, to the engine mount. A new wire from the engine mount will go to the starter solenoid case. From there, to the battery.

There is also a wire that goes to the bolt from the solenoid case to the grounding block mounted on the firewall, that everything inside attaches to.

One other thing i do recommend is tho move all the grounds on the starter case to one side. The starter case should not be used as a ground conductor. Terminal rings should either be grounded to a terminal block designed for it, or placed on a single post against one another.

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Retracting my earlier statement about the amperage flowing across the engine ground. I have remembered that rotax's alternator uses two wires on the charging circuit to the regulator. That means the regulator's ground is the critical component, not the engine ground, and it is usually directly jumpered to the ground bus in the aircraft.

Engine ground is still needed for the sensors on it though, and the ground loop problem still applies, generating interference. Especially since there are ignition events going on, those will generate a LOT of interference in a ground loop in the right circumstances due to the extremely high RFI. Unsure if they will affect this particular configuration though.

It's still possible to put a lot of amperage across that 22 gauge, if the ground between the bus and the starter is broken, the new path is: ground bus -> ems ground connections -> engine -> back to the starter -> battery. HIGHLY unlikely for the full brunt though because this wire isn't subject to vibration. However, it's still an extra path to ground, and there will still be some power flow across it, dropping voltage.

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What a weird issue this aircraft has. Even after cleaning up all the grounds, replacing the regulator (at owner's insistence), and fixing a ton of connections, it still won't replicate the erratic instrumentation until takeoff.

Going to have to tear through the electrical system even more.

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24 minutes ago, Anticept said:

What a weird issue this aircraft has. Even after cleaning up all the grounds, replacing the regulator (at owner's insistence), and fixing a ton of connections, it still won't replicate the erratic instrumentation until takeoff.

Going to have to tear through the electrical system even more.

See if you can fool it into acting up on the ground by adjusting the barometric pressure until you are as high AGL as you were when reported false readings.

In my CT I have 2 systems that detect altitude and react lectrickly (Sony & Cher lyric)

  1. EFIS dissengages AP if not greater than 500 AGL
  2. Transponder changes mode and begins counting flight time when off the ground ( I think )

Even if there is no functionality there could be a loop running that now essentially says 'when altitude > x create ground issue'



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It's both CHTs, both EGTs, and Oil Temperature that all fluctuate when the problem starts happening. I grabbed the EMS from my own aircraft as a super quick test on the ground and the occasional EGT flicker happened with it.

What I can't replicate is all 5 instruments flickering at once to a great degree. Today it did it on the takeoff roll and aborted the test flight. But I just can't get it to happen in the shop or in front.

Pulled the generator breaker in flight when this was happening the first time, and it had no effect at all on the erratic readings. Pulled every breaker on the ground except the EMS and it had no effect on them either when there were a few flickers. But again, I can't seem to get them to all happen at once unless we're in flight or rolling out.

I'll be tearing more into the electrical system at a later date and checking every single connection for shorts and pumping some serious amperage across each wire to check voltage drop. There's so much freaking shrink wrap and tape everywhere though, that this is going to suck.

Why couldn't flight design do it like a normal company and zip tie? sheesh

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  • 2 months later...

I had been sidetracked by other projects but finally finished this one for the most part.

Fixing that ground didn't solve the problem. That wire still needed to be fixed for sure though, avionics grounds shouldn't be going to the engine and the airframe at the same time. Though it did decrease the severity of the fluctuations.

Ultimately, what seems to have fixed it was that I turned my attention to the positive feeds. They too were corroded and nasty out in the engine bay.

The oil temp still fluctuates a little due to the beacon, and I believe the positive feed wire to the ammeter shunt, which is where the EMS also picks up its power, is too small. I suspect it's dropping a little voltage every time the beacon pulses. Should be easy enough to test for by putting an o-scope across the wire and monitoring the voltage drop. I'll need to make sure the EMS power is isolated too at a later date and make sure someone didn't get the bright idea of pig-tailing off it.

There were some other connections I cleaned just to be sure as well.

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Hi Corey,

this reminds me to the same experience i had with a 2007 CTSW. Your story is exactly the same I had with the D-120. Even the breakoutboxes are similar. I fighted for years against the grounding ghost. Everytime I thought I fixed it, the symptoms came back. Sometimes after months.  I have a video of the situation here: EMS D120 Fluctuating values

After all, I learned that there is a Achilles heel with the Dynon sensor system. It is the 5V stabilized circuit. It is one 5V power source, which supports all sensors. If one of the sensors or the sensor cables do a shortcut, the 5V goes down somehow and this leads to fluctuating displays of all values.

At the very end, I found a defective cable (the oil-pressure-sensor-cable) which was internally broken and created a short. Since that repair. The problems are all gone.

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That is very possible and I remember seeing this a long time ago, but we has fluctuating EGTs as well this time. EGT circuits are self-exciting, but I suppose the amplifiers could be fed by that same 5v circuit inside the unit. I will consider checking a bit harder for a break in a sensor wire if it crops up again.

I also have a megger but I don't use it without disconnecting the avionics. 1000v will busy through just about any silicon barrier.

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  • 2 months later...


Problem was finally solved. It wasn't the inputs, I had installed a bypass from battery to breaker as a test, and the EMS fluctuations didn't stop.

I tested the ground again, and it was yet again dropping voltage. That means every time someone turned on or used a high power device, like a radio transmission, it would raise the voltage potential of the ground bus, which affected instrumentation.

The cause was the through bolt used on the ground bus on the inside right side of the firewall. It carries the current from the grounding bus aluminum backplate inside, to grounding conductors connected on the engine side. I previously cleaned that bolt, but it's made of cheap steel and developed a layer of oxidation again.

My solution was to take an 8 gauge wire with ring terminals, and run it from the inside under the bolt head so it directly touches the ground bus, to the outside tail of the bolt up against the other ground conductors.

This reduced the ground resistance from 1.8 ohms to .02 ohms (my meter starts getting inaccurate with that low of a reading, it was probably lower than that). Voltage drop registered at around 2 volts before with high load, to 0.05 volts to 0.08 volts. Wild EMS indications appear to have stopped.

I can reduce the ground resistance even further, and I will a bit later, but I first I want to make sure this truly solved the issue. What I intend to do is bypass that aluminum grounding backplate used by the ground bus, and install large gauge jumper wires directly from the ground terminal blocks mounted on that backplate, to the wire I installed (using ring terminals, not splicing). That aluminum block accounts for a bit of voltage drop as well, but it is very insignificant compared to what that bolt was dropping.

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Problem was significantly better, but not completely resolved. So I opened things up to work on some more.

As I was removing the bolts on the terminal blocks outlined in red in my photo, I noticed they were incredibly loose. I remeasured the resistance and noted the resistance dropped to near zero when I fiddled with them.

So, I removed the coating under where one of the bolts would cinch down on the terminal block to the backplate, and tightened up all 4 bolts.

Resistance is still too low for my meter to read anymore. Voltage drop under load tops out at 25 millivolts, the lowest it has ever been. It's so low that I might just be seeing electrical noise at that point.

Roger: there is an extra wire run from the battery to the engine that I haven't touched yet. I'll see how things go, I really don't want to be a solution in search of a problem at this time.

I'm also installing the bigger capacitor. Hope this finally resolves the issue.





For the first time with this airplane, all indications are rock solid stable. It was that backing plate and bolt that caused an unacceptable voltage drop, and thus erratic readings.



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