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01-22-2017, 09:07 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rv7charlie
Dan, This thread seems to be mixing MegaJolt>EDIS and EDIS-only setups. In your last post, I assume that you're speaking of programming your MegaJolt, right?
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Yes.
When I say Megajolt, I'm referring to the aftermarket Megajolt/E box produced by Autosport Labs: https://www.autosportlabs.com/produc...jolt-ignition/
When I say EDIS module, I mean the Ford Motorcraft OEM part, which comes in 4 cyl, 6 cyl, and 8 cyl versions. As noted, the EDIS module (combined with a coil and pickup) will function just fine as a fixed timing ignition, without the Megajolt box. The MJ is only needed for variable timing.
Megajolt boxes and Ford EDIS modules:

__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 12-05-2018 at 08:42 AM.
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01-24-2017, 10:55 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Sorry Tom, missed your post. My bad.
Quote:
Originally Posted by Tom Martin
If the shared positive connection at the master solenoid fails, you have that 3.8ah left to get you on the ground....
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Should have both batteries and two ignitions...unless the solenoid end of that positive cable breaks off and shorts to the airframe, discharging the big battery (and probably melting some aluminum). That particular failure mode is a possibility with any airplane, so we go with robust terminals and good wire fixation. I make mine from welding cable and lugs, because I like the flexibility.
Quote:
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.... but it something should happened to the main ground connection, or the main battery itself, you could potentially lose both systems.
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That's a real risk, although again, we're talking about robust components. The EDIS/Megajolt grounds run to the adjacent single point fast-on panel inside the firewall, which is linked to the rear batteries with a #2 welding cable. Baring terminal breakage (again HD welding lugs) the disconect risk is one of nuts or bolts unwinding, one at the single point ground, one at the main battery negative terminal. Not too concerned with the 5/16 stud up front. I might drill the heads of the battery bolts and wire 'em. Right now they're 6mm with split washers.
Quote:
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I think it would be possible to ground that second battery to separate ground, eliminating a single point ground failure? This might cause a ground loop problem for your radios but at that point you are just trying to get on the ground.
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One possibility might be locating the little battery in the front, near the modules, for a easy direct connection. Or I could run an individual ground wire back through the airframe for each ignition. Here I've elected to rely on the big cable, which raises the classic "single point of possible failure" question; how robust must it be to consider it safe enough?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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01-24-2017, 01:59 PM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Hi Dan,
On the single ground issue, perhaps he meant that as drawn, the battery negatives are tied together and then a single wire goes from a negative terminal to ground. If each battery has its negative tied to the chassis, then losing one ground leaves the other battery still active. That might be how it's actually wired, but it's not drawn that way in the image.
FWIW...
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01-24-2017, 05:17 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rv7charlie
On the single ground issue, perhaps he meant that as drawn, the battery negatives are tied together and then a single wire goes from a negative terminal to ground. If each battery has its negative tied to the chassis, then losing one ground leaves the other battery still active. That might be how it's actually wired, but it's not drawn that way in the image.FWIW...
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No airframe grounds. It's a single point ground system, nice and quiet. Both ignitions ground to a standard "forest of tabs" ground point on the firewall, which is linked to the batteries via a #2 cable. Thus the classic debate...is a single high quality cable, with bolted connections at the ends (the red dots), robust enough to assume a very low probability of failure?
If you think not, the simple cure is an additional ground wire from IGN2 directly to the little battery's negative terminal. You won't get any argument from me. Might even talk me into it.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 12-05-2018 at 08:43 AM.
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01-24-2017, 05:35 PM
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Super Moderator
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Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,626
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Quote:
Originally Posted by DanH
This may help (or maybe not).
No airframe grounds. It's a single point ground system, nice and quiet. Both ignitions ground to a standard "forest of tabs" ground point on the firewall, which is linked to the batteries via a #2 cable. Thus the classic debate...is a single high quality cable, with bolted connections at the ends (the red dots), robust enough to assume a very low probability of failure?
If you think not, the simple cure is an additional ground wire from IGN2 directly to the little battery's negative terminal. You won't get any argument from me. Might even talk me into it.

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Dan, I would highly encourage you to think of adding a second ground. And on top of that, check the battery connections regularly. It always seems like the corrosion starts on the negative terminal.
Vic
__________________
 Vic Syracuse
Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
Kitplanes "Unairworthy" monthly feature
EAA Sport Aviation "Checkpoints" column
EAA Homebuilt Council Chair/member EAA BOD
Author "Pre-Buy Guide for Amateur-Built Aircraft"
www.Baselegaviation.com
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01-24-2017, 06:01 PM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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When I said 'airframe', I was referring to your forest of tabs location. Surely the forest of tabs is bonded to the airframe. What I was trying to describe is moving the 'white' battery negative from the 'green' battery negative terminal to the forest of tabs. If both batteries are in the rear fuselage, and the forest of tabs is at the firewall, I can see the difficulty in implementing that change.
I'm *not* saying what you've got in the drawing is unreliable. Just that if the negative connection to the airframe is near the batteries, it would be simple to make both battery negatives connect directly to airframe ground, and would achieve operational redundancy. If the 'white' battery doesn't participate in starting, perhaps running a smaller gauge negative wire to the forest of tabs would be feasible to get that redundancy.
Something that comes up all the time on the Aeroelectric List is not designing for reliability (which should be a given), but for redundancy. If you think about it, a modern battery is extremely reliable. If maintained properly, it's probably the most 'reliable' component in the plane. So why are there two?
Charlie
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01-24-2017, 06:57 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by rv7charlie
If you think about it, a modern battery is extremely reliable. If maintained properly, it's probably the most 'reliable' component in the plane. So why are there two?
Charlie
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Batteries have been known to take an unexpected dump, especially some lithium ones. With a battery dependent ignition system, you lose a single battery, the engine stops. That's a good reason for a second one in my view. I have personal experience in this regard...
We never recommend anyone running two EIs in an aircraft to use a single battery.
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01-24-2017, 06:57 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by vic syracuse
...check the battery connections regularly. It always seems like the corrosion starts on the negative terminal.
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Now there's a good point. Corrosion could be a problem.
Lord knows, I've seen a few corroded battery terminals in my day. Never on an Odyssey however. Vic, anybody, you're seen 'em do it?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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01-25-2017, 12:16 AM
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Join Date: Oct 2007
Location: Salmon Arm, BC
Posts: 933
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Quote:
Originally Posted by rv6ejguy
We never recommend anyone running two EIs in an aircraft to use a single battery.
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Even with two alternators? I have a Plane Power and an SD8.
Quote:
Originally Posted by DanH
Lord knows, I've seen a few corroded battery terminals in my day. Never on an Odyssey however. Vic, anybody, you're seen 'em do it?
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Yep, I have cleaned the negative terminal of corrosion (green) on my Odyssey PC680.
__________________
Mark Olson
1987 RV-4 Sold
2003 Super Decathlon - Sold
F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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01-25-2017, 06:00 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by SMO
Even with two alternators? I have a Plane Power and an SD8.
Yep, I have cleaned the negative terminal of corrosion (green) on my Odyssey PC680.
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Two alternators should give you adequate redundancy too.
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