Ford EDIS and MegaJolt Lite Junior ignition

Hot-rodding the Nash/Rambler 195.6 overhead valve six

Feb 2012


If you aren't familiar with EDIS, you will need to research that yourself; a good place to start is the MegaSquirt page. It's a great system, and has been adapted to lots of orphan cars. It puts out as much spark power as many aftermarket ignition systems, for a fraction of the cost. And for old or odd cars, you get to ditch worn out, hard to replace and frustrating distributors. The MegaJolt Lite Junior controller is designed specifically to manage it for enthusiast installations. RTFM.

The ignition on this engine is just plain primitive. Almost no spark advance, no spark accuracy, and in 2012, no OEM-quality parts left; no one reproduces it, there are no good cores left, they're all worn out junk and terrible by design. (I did manage some improvements to the thing though.)

The Ford EDIS ignition is a bit of overkill, but they are very reliable. The hardest part is the damned trigger wheel mounting, but a simple adapter lets you bolt one onto the harmonic balancer. I don't run any accessories (power steering, etc), just water pump and alternator, so mine's free.

Bought a "kit" of EDIS parts from Boost Engineering if you are less lazy than me you can scrounge 'em from a junkyard or pick from eBay. I got a coil pack, 36-1 wheel, sensor, wires, all in one batch. Clean and apparently tested good.

EDIS needs a computer to manage it. (By itself it will run the car though, in "limp home" mode, fixed at 10 degrees advance; very nice if your computer dies! Relaibility matters.) That is what the MegaJolt Lite Junior (what a name...) is for. Nice little box, buy as a kit or built from various vendors. Single biggest expense of the project. See AutoSports Labs for details and installation instructions.

Trigger wheel and sensor install

The hardest part of the job on this engine is mounting the sensor and 36-1 wheel. When I rebuilt the engine, i fabbed up a nice clean stiff mount for the sensor and welded it onto the timing chain cover. It would not be all that hard to work up another solution with the engine in-car, probably from below. it's a little tight in there, as you can see from the photos (the one on the right was taken from below).

The mount for the sensor (pickup) is simple and easy, with the timing cover off the engine:

here's the 36-1 ring to harmonic damper adapter. Ignore my shitty welding.


I chopped the mounting ears off the provided bracket, welded up new ones to mount it to the inner fender and accept the EDIS6 module, to create an ignition module I could wire up on my bench.

The end result was a neat package with four wires to pierce the firewall, and a short cable that runs to the pickup.

Pay a lot of attention to grounds and shielding! The signal from the pickup is weak, and susceptible to noise picked up in the wiring. The MJLJ documentation is fairly clear here, but I as always go for double-overkill. All of the grounds -- coil ground, EDIS module ground, and MJLJ module ground -- were short heavy wires, twisted and soldered, with crimped lugs soldered, and screwed to the inner fender in one spot. A #14 wire was run from there through the firewall for the MJLJ box -- do not ground it "conveniently" under the dash. I extended the pickup shielding up to the EDIS module connector using aluminum foil -- it looks rough in the picture above (I wrapped #22 uninsulated wire around it for an electrical connection, then 3M electrical tape for mechanical strength). Overkill for sure, but I've mounted the module very close to the spark coils, there's a lot of energy in the air there. Easy to do now and avoids trouble on the road!

The wires that came from Boost Engineering are crazy long, and loop all over the place. I've ordered a set of replacement MSD spark plug boots from Summit, I'll shorten them up, that will tidy up this mess. The distributor hole plug was simple, just a flat piece of cold-rolled stock cut to fit and silicone sealed.

The sensor to 36-1 wheel gap worked out to be .045".

Drilled a 9/16" hole in the firewall for the harness, mounted the MJLJ box under the glove box. Wired up everything but left the SAW signal (MJLJ control signal to the EDIS controller) disconnected so that EDIS would run in stand-alone "limp home" mode. Much easier to test one thing at a time.

Firing order

The firing order for all Rambler and AMC sixes is 1-5-3-6-2-4, clockwise crank rotation, counterclockwise distributor rotation. The EDIS coil pack is marked, but it's for the wrong engine; ignore it. The proper wire order for all AMC sixes (1950 - 1989) is below, but working it out from first principles is easy.

Inline sixes have three pairs of cylinders. Each pair of pistons is 120 degrees apart on the crank, and is in the same physical position in the cylinder, but each in the pair is two strokes apart through an Otto Cycle; eg. one on Compression while one on Exhaust.

It's easy to determine which cylinders fire when: for an overall order of 1-5-3-6-2-4, the pairs are: 1 & 6, 5 & 2, 3 & 4. Look at the top of the distributor cap: note that starting with #1, 6 is directly opposite 1. Next is 5; 2 is opposite 5. Next is 3; 4 is opposite 3. Those pairs are the ones with the pistons at the same position in the cylinder.

EDIS takes advantage of this physical pairing. By firing both cylinders in a pair simultaneously, it doesn't need to know camshaft position, only crank position; when the pair approach TDC one of them is in compression stroke, so sparking both lights it off without knowing which. The other spark is "wasted". This vastly simplifies the ignition, and triples the time to saturate a spark coil, each coil is smaller, lighter, runs cooler. Win win win.

Wasted-spark lets you re-think what firing order even means. Because it fires each cylinder twice in an Otto Cycle -- compression and exhaust -- the firing order is really just three pairs, not six individual cylinders. EDIS 6 coil packs are marked A, B, C. EDIS fires them in this order: A, C, B.

AMC six EDIS firing order
cyls... coil
1 + 6 A
5 + 2 C
3 + 4 B

Six cylinders, but only three engine revolutions, a coil firing every 120 degrees. It doesn't matter which terminal of the coil you wire a cylinder of a pair to! As long as cylinders 1 and 6 go to coil A, etc, it works. Electrically it doesn't matter, but under the hood it means you can attach plug wires so that they're neat and don't cross.

Initial checkout

Once wired up as a module and installed, there are four wires that go through the firewall (+12V switched, ground for the MJLJ, and PIP and SAW in a shielded cable), and the sensor cable. That's it! The engine started and idled first time. If it backfires, you may have the firing order wrong, or the trigger wheel positioned badly (the MJLJ installations are pretty clear). If it doesn't fire at all, you may have the sensor wired backwards (swap the wires) or the sensor is too far from the trigger wheel.

Configuring the MegaJolt Lite Junior

It goes as the MegaJolt documentation says. I am very comfortable with computers, but I deeply want to keep computers out of my old cars... but it sure is nice to have precision ignition timing! And tuning the MegaJolt is easy, there's just not that many variables or complexity.

Laptop, Keyspan serial/USB adapter, and the MegaJolt configurator program downloaded from their site. Easy enough. I created a spark map by the seat of my pants -- well not really i know how my distributor is curved from spending so much time with it (and taking notes...)

The first thing I did was to change lobal stuff (number of cylinders, normally aspirated), and the axes of the map -- the default is 500 to 9500 rpm (YEAH RIGHT!) and load 10 - 230 (1 atmosphere boost). I changed rpm to 500 - 5000 in 500 increments and "load" 10 - 100 (103KpA is atmospheric pressure, eg. zero vacuum).

Second I verified that the sensor and 36-1 wheel are aligned right -- i entered 0 in the lower two high-load spark map boxes, uploaded it, and with a timing light looked at the timing marks. My system was off 2 degrees. There's "timing mark" global setting. Nice!

Creating a custom timing map

Now to make the initial map. I started with what I know as good starting points for the four corners of the map. These are numbers you will be familiar with if you're comfortable tuning your engine now.

The first thing to set is the load=100 horizontal row of values -- this is pure mechanical advance, eg. a distributor with the hose pulled off. Mine, 500 rpm is 10 degrees advance (static setting), 2500-5000 rpm is full advance (22 degrees, what my hacked distro did). I simply interpolated values by eye, 1000 - 2000. Somewhere, I actually measured this with a hand vacuum pump and a timing light, I'll pull out my notes sometime and look at it, but i recall it being more or less linear.

The far "corner" of the map -- high rpm, low load (high vacuum) -- that's total mechanical advance (static 10 + 22) plus full vacuum advance (11) or 44, 2500-5000 rpm gets that value.(That's a hypothetical, it's unlikely you'll run it > 2500 rpm at high vacuum; in fact I entered lower numbers 3000+).

The other "corner" -- idle, high vacuum -- where a distributor you're always fiddling with "ported vacuum" vs. manifold vacuum you get to actually set it directly, no more compromises. Often, full vacuum is too much, and no vacuum too little. Initial experiments determined that 18 degrees total advance "seems nice" (utterly unscientific, but a starting point).

From there I simply eyeball-interpolated the center of the table; the low-load runs from 18 (500 rpm) to 33 (2500-5000 rpm).

Here's the current spark map, in tabular and 3D map form:


So far I've made one pass at tuning, it took a couple of rough iterations, about 45 minutes, to get it so it unscientifically "felt right", and all of the numbers are in line with what I had with the distributor. I pushed spark advance too high in the RPM/load band where 65 - 70 mph cruise is, got it to ping, set it back, so now I'm certain I'm in the right area for final tuning. No idea how I'll do that! -- any one have a chassis dyno handy?