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Power supply splice question AEM Smart Coils

EFI Wiring Fundamentals

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I have a question regarding splicing power and grounds for coils.

I have made a harness for 4 x AEM smart coils using Tefzel M22759/16 wire

I have run a single 16AWG from the relay for a length of around 700mm then splice it into 4 x 18AWG wires to the coils

For the grounds I have run a single 16AWG from the star earth on the engine (same point I have earthed the ECU) for a length of about 500mm then spliced it to 4 x 18AWG for high current earth and 8 x 20AWG for sensor ground and rotor housing ground.

I suspect that the coils may be putting out a weak spark as it fouls plugs very easily.

I have tested 14v at the coil plugs while engine is running (but not current).

Have I undersized the power and ground or over spliced the ground?

Could this cause a restriction of current and make the coils under perform?

I have attached a freehand diagram of how I have made the harness

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I think your wiring is fine. My only question is are you sure there are 3 grounds per coil? Perhaps one of those is really an ouput (feedback signal) and should be left open.

If it's fouling plugs, the first thing I would check would be the plug temperature range. Can a hotter plug be fitted? I bet that improves the fouling situation.

If you really think it's a weak spark (which could be caused by a fouled plug, not necessarily causing the fouled plug), then I would first check the ECU setup for coil charge time (sometimes referred to as dwell time). This usually need to vary with voltage, and many race engines either do not have an alternator, or do not provide sufficient voltage at idle that the coil charge time needs to be much longer to get a reasonable spark.

Your wire sizing should be fine, what sort of RPM are you revving to?

Pin 2 of those smart coils should head to the ECU sensor ground, as its the reference level the coil compares the trigger input level to, to decide whether it should be dwelling or not. As long as you've not got any large ground offsets though, it'll be fine the way it is.

It’s a Microtech so all the grounds are joined together in the loom in one point and the power ground branches off this joint (all from ecu manufacturer).

I have this power ground going to the block where I have grounded the coils.

It’s a brand new engine so it’s only getting a run in tune. On the dyno the plugs and mixtures are perfect, it’s only idle that gives trouble.

The car was built for big HP on E85 but at the end of the build the customer asked to put the car on petrol for cruising duties and only run E85 at the track.

its a 13B Bridgeport with 4 x 2400cc injectors, I think it’s just too much injector for idle on petrol.

I have a pair of 1050cc injectors to fit into the primary’s to see how it goes.

A birdgeported Rotary with huge injectors. That's an important detail to mention!

Rotaries are notorious for plug fouling unless you can get the Idle RPM high enough. I usually just hear the "braaappp, braaaappp, braaaappp" as the guys on the grid work to keep the plugs clear. I would try to get the engine to run super lean at idle to avoid the problem. Perhaps your 1050 cc injectors can help, good luck!

The smart coils from AEM are just rebranded IGN-1A coils. From my own experiences on a 3 rotor we're developing they actually are somewhat sensitive to how they are grounded when maximizing their potential. Here is what fixed my woes regarding fouling plugs on a rotary (side port though)

Pin A - Signal

Pin B - 'Logic' ground - Zac is correct, this is essential

Pin C - 'Drain' ground (connect this to the specific rotor housing that the coil is firing, I use the knock sensor mount point)

Pin D - Power ground

Pin E - Vbat

Separating the grounds (for me) was important when trying to run 4+ ms of dwell at redline. Hope this helps!

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