Electrical Ground Potential

flickroll · #1 02-08-2009, 05:13 PM

I have a Dynon EMS D120, and everything seems to work well during inital check out. The only issue I have is the fuel pressure reads 0.4 psi with no pressure on the fuel system. Dynon sent me a replacement sender, and it reads the same, so it's not a sender issue. I reread the installation manual, and it has the following regarding grounds:

"Because of the current drawn by the EMS-D120, even very small resistances between battery ground and instrument ground can cause voltage differences which adversely affect engine sensor readings. An easy way to test the quality of the instrument?s ground is to measure voltage between the ground pin at the EMS-D120 and the ground lead at your aircraft?s battery. With the EMS-D120 powered on, connect one lead of your voltmeter to a free ground lead coming from the EMS-D120. Connect the other lead of your voltmeter to the ground terminal of your battery. The voltage between these two points should measure very close to 0 mV (within 5 mV). If it does not, you must improve the ground connection between the ground of your battery and that of your avionics bus."

If I check the voltage between my battery ground and the avionics ground I get 3 mv. However, if I check voltage between a Dynon free ground lead (the one used for fuel pressure or the one used for carb temp yields the same result) and the battery ground I get about 45 mv. My ground system is like this:

All of my instrument grounds go to electrically isolated ground blocks in the instrument panel area. The ground blocks are tied back to the battery ground via a #8 cable. And again, the voltage between the ground blocks and the battery ground is 3 mv. It's the return to the Dynon that has the higher reading.

Any ideas? Is this OK? Or could it be the cause of my high fuel pressure reading?

Thanks

agough277 · #2 02-08-2009, 05:27 PM

Check the grounding of the transducer. Use no sealing compound in the transducer threads. Check by using a ground lead from the transducer to battery ground and then to the dynon ground.

flickroll · #3 02-08-2009, 05:37 PM

Good idea, that's one thing I did not do. Tomorrow I'll ground the transducer straight to the battery ground and see what I get.

jthocker · #4 02-08-2009, 05:43 PM

Jim
I was having an erratic oil temp reading on my D180. It would go up about 20 degree's when I would turn on a landing light. I was pretty sure it was being caused by a defective ground cable, the one that goes from the engine sump to the firewall. This cable felt like the crimp or swedge(sp?) at the sump was loose. I think it may be one of the service bulletin ones, so I swapped it out with an extra cable that was laying around. This time I sanded the firewall, sanded the backing angle on the backside of the firewall, and sanded the paint off the engine case at the dedicated ground point just aft of the dipstick tube. Then I replaced the existing P10 cable with a P26 cable(you have extra parts after 6+ airplanes).

I test flew the plane today and here is a picture of the results.

It is very difficult to see from this pic of Mike Hoettings RV8(Johan Kars RV8 builder GIB, photo by Alex Thocker), but my oil temp reading was now rock steady with or without the landing lights. I also noticed that I had fuel pressure on the ground also but did not notice if the new ground cable cleared this anomaly up.

I'll check that out tomorrow.
Regards,

flickroll · #5 02-08-2009, 05:52 PM

Jon I'll be interested to know if you have the same fuel pressure issue that I have (~ 0.5 psi with no fuel pressure on the system). I'm gonna call Dynon again tomorrow. When I called the first time I did not get a tech support guy, the person I was talked to was manning the TS line and did not normally provide assistance. I really feel my grounding system is good, but am also perplexed why I have such a high voltage (45 mv) from a Dynon free ground lead to the battery ground. The fact that I have just 3 mv between my ground blocks and the battery ground leads me to believe my ground system is OK.

jthocker · #6 02-08-2009, 06:17 PM

Jim
After all these years and all these RV's, I guess I've grown accustomed to fluctuating or odd fuel pressure readings. Maybe its the fact we have digital capabilities now instead of a needle, and these aren't expensive senders, and our eyes are "glued" to the gauges expecting trouble from our "experimentals", that we are seeing a problem that really isn't a problem.
I'll let you know if I still see "pressure" with no pressure.
Regards,

flickroll · #7 02-09-2009, 11:07 AM

Today at lunch I grounded the fuel pressure sender directly to the airframe, thereby not using one of the Dynon 'free ground' wires. When I did that the fuel pressure fluctuated between 0.0 - 0.1 psi. So now I'm scratching my head why I get a higher reading when I use a Dynon 'free ground'. Could the length of the ground wire - 6 feet or so - have enough resistence in it to cause the fuel pressure sender to read high? It makes no sense to me why I'd have different readings since everything grounds to the same place. Thanks

jthocker · #8 02-09-2009, 03:15 PM

Jim
I just got back from the hangar. My fuel press. reads .3 psi with no press on the system. The planes twin reads 0.0 psi. Both are wired identically.
My engine/ batt/firewall gnd are as per Vans firewall drawing.(gnded at firewall).
My Dynon and all interior gnds. are per Vans electrical schematic, which has gnd studs on a gear tower cross brace.

I intend to live with my .3 psi anomaly. Good luck with yours.

agough277 · #9 02-09-2009, 06:35 PM

Jim: Remove your transducer and clean the threads good. Also make sure the manifold has a clean attachment to the firewall.

jthocker · #10 02-09-2009, 06:56 PM

The Dynon fuel pressure transducer has 2 tabs, one for the sensor, one to ground. The oil pressure transducer is the one that grounds thru the threads and manifold.