grounding

prkaye · #1 05-01-2015, 03:42 PM

I've had some minor issues and behaviours that suggest I might have a grounding problem (fluctuating sensor values, EFIS generating an audible hum on my radio). I have two forests of tabs on each side of the firewall, with brass bolts between them. I have braided cables from the negative of the battery to one of these bolts, and another braided cable from the bolt to the engine.
A local builder (of a composite aircraft) suggested I run dedicated ground wires for my avionics directly to the engine, instead of grounding these through the forest of tabs.
Thoughts?

gasman · #2 05-01-2015, 04:15 PM

Quote:

Originally Posted by prkaye

I've had some minor issues and behaviours that suggest I might have a grounding problem (fluctuating sensor values, EFIS generating an audible hum on my radio). I have two forests of tabs on each side of the firewall, with brass bolts between them. I have braided cables from the negative of the battery to one of these bolts, and another braided cable from the bolt to the engine.
A local builder (of a composite aircraft) suggested I run dedicated ground wires for my avionics directly to the engine, instead of grounding these through the forest of tabs.
Thoughts?

Yes, do this for your EMS. Good grounds are important here.

dynonsupport · #3 05-01-2015, 04:22 PM

Phil,
We wouldn't generally recommend grounding the avionics to the engine. You can create a really good ground between the engine and battery, and then a good battery-electrical ground, and be successful. Running 20+ grounds through the firewall isn't really a robust solution, and the ground you care about is really the battery ground as the center of the ground world, not the engine.

The first thing to isolate is where your ground problem really is. If the sensor issues and headset noise are only there with the alternator spinning, and get worse with load, then it's engine to battery. If you have the issue even with the engine off, then it's somewhere else.

A jumper cable between things like the case of your intercom and various other grounds in the plane can do wonders in finding the ground issue.

prkaye · #4 05-01-2015, 04:43 PM

Thanks Dynon Support. The electrical noise I hear is audible when the Dynon EFIS is on, regardless of whether or not the engine is on.

Quote:

A jumper cable between things like the case of your intercom and various other grounds in the plane can do wonders in finding the ground issue.

Could you explain this diagnostic procedure? So I hook a jumper from the case of my intercom to my main ground forest of tabs... what am I looking for?

dynonsupport · #5 05-01-2015, 05:49 PM

Ahh, if you have EFIS noise all the time, then your EFIS ground is probably iffy. The noise you hear is the EFIS drawing different amounts of power, which through a iffy ground causes varying voltage, which the intercom can "hear" because it assumes the ground is stable.

Chances are about 90% that if you touch a jumper cable to the EFIS case and the other end to the intercom case, the noise will go away.

Is this a non-metal instrument panel by chance?

prkaye · #6 05-02-2015, 05:40 AM

Quote:

Originally Posted by dynonsupport

Ahh, if you have EFIS noise all the time, then your EFIS ground is probably iffy....
Is this a non-metal instrument panel by chance?

It's a metal panel. So I should re-do the spade connection on the ground wire coming out of the D-180 wiring harness?

lr172 · #7 05-02-2015, 10:08 AM

I would also make sure your panel is well grounded. Most of these instruments tie their case ground to the ground plane and problems here can cause noise. I believe Dynon recommends this in their installation instructions as well.

Good luck

Larry

prkaye · #8 05-02-2015, 01:19 PM

Quote:

I would also make sure your panel is well grounded. Most of these instruments tie their case ground

Yes, last year in my attempts to fix some of this I ran a wire directly from the dynon case (through the screw at the back that secures it to the mounting tray) to my ground forest of tabs. But I didn't check the actual ground wire connection from the wiring harness. I guess that should have been the first thing I checked! I'll also run a dedicated ground wire from the case of my intercom back to my ground forest of tabs.

Norman CYYJ · #9 05-02-2015, 05:34 PM

DO NOT run grounds to the engine!

If for some reason the main starter ground breaks or comes loose you WILL melt the small ground wires which in turn can cause all kinds of expensive problems.
I have seen this happen many times over the years mainly on certified airplanes with improper wiring done on them.

wirejock · #10 05-02-2015, 06:21 PM

Quote:

Originally Posted by Norman CYYJ

DO NOT run grounds to the engine!

If for some reason the main starter ground breaks or comes loose you WILL melt the small ground wires which in turn can cause all kinds of expensive problems.
I have seen this happen many times over the years mainly on certified airplanes with improper wiring done on them.

Norman
Not to hijack but I have a question it sounds like you can answer.
Would it be advisable to insulate the FOT from the air frame so the only path for equipment ground is through the ground lug to the battery?
Some FOTs are solid brass plates.
If it were making contact to air frame, any ground such as the starter could find a path through the FOT. Or am I over thinking?
Seems like the ground from the engine and air frame should terminate at the same battery ground lug on the FOT. Technically all should be at the same potential but, a loose or dirty connection results in a new path of least resistance and you don't want that path to be the equipment. If the FOT is insulated from air frame then a bad connection should cause a starter to fail. Just wondering. I'll be planning electrical soon.