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Electrical noise suppression

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jakelandry

10+ Year Contributor
976
157
Oct 13, 2009
Minden, Louisiana
Not sure how many electrical gurus are left around the forums but I figured I would post up for advice. I've spent north of 100 hrs planning out a mil spec wiring harness for my car and I have finally obtained everything needed to start. The harness will be concentrically twisted with shielded wires on the crank, cam, and knock sensors. My question is, would it be overkill to build a separate harness from the mpi to coils? I plan to relocate the power transistor inside the car to clean the engine bay and keep it cooler so the Post ignitor wires, which will be 14ga, would run the entire length of the engine harness. The fact that they are a bigger wire gauge than the rest means they will be at the center of the concentric twisted bundle along with all the critical shielded cables. I do not have the best understanding of how our coil system works but I assume there is high amperage between the ignitor and coils and it seems foolish to run those wires right through the middle of the sensor signal wires if not needed. Am I just being overly cautious thinking it is smarter to run them as an independent harness? I have about $800 in materials and an invaluable amount of time invested in this project and I would hate to make a new harness just to introduce electrical gremlins.
 
My question is, would it be overkill to build a separate harness from the mpi to coils? ...... Am I just being overly cautious thinking it is smarter to run them as an independent harness?

It's an interesting thought. While I am also not an expert on how our coil systems work, my first thought is: these coil charging events are DC. There isn't a lot of concern for emissions, both radiated or conducted, when dealing with DC (obviously excluding switching DC supplies etc). The frequency with which these coil charging events are happening is exceedingly slow in electrical world. 2 coil charge events per rotation (2 firings per coil per 2 engine revolutions, 2 coils * 1 firing per revolution). At 8k rpm that is 266hz (16k charge events per minute / 60). Hardly what I would consider a high freqncy electrical noise source. Also, the current flow is unidirectional as the coil primaries will always be at a lower or equal potential to the charging source. No AC weirdness here.

I see no issues running the ignitor wiring in your bundle but I will give a few suggestions that I would do to ensure reliable operation:

1. Twisted pair is great, shielded twisted pair is better. But make sure your cable has a shield drain that you attach to a ground on your chassis or battery negative. If it doesn't have a drain I wouldn't bother buying the shielded type. Also, always make sure your shield drain is attached at one end only for every cable run. Attaching both ends of the shield can actually cause more issues by creating a ground loop.
http://www.almorpowercables.com/blog/purpose-of-drain-wire-in-shielded-cables

2. Utilize a ferrite bead on your ignitor cable. A simple clamp on is fine and easier. A solid ring with a few turns of the cable is great too. While these are normally used for RF EMI suppression, they certainly won't hurt even at the sub 300hz region we are dealing with here.
https://en.m.wikipedia.org/wiki/Ferrite_bead

Like you, I might be over thinking much of this too but if you are taking the time and spending the money I'd make it as right as possible.

In the end, with this being DC and low frequency, I think you'll be fine no matter what approach you take. But definitely something to consider before moving forward.
 
Interesting point regarding the frequency. Sadly upon further research, it doesn’t look like I am going to find a black and white answer on this. Thank you for your contribution.
 
Interesting point regarding the frequency. Sadly upon further research, it doesn’t look like I am going to find a black and white answer on this. Thank you for your contribution.

Unfortunately the world of EMI isn't black and white, it's a spectrum. (frequency joke intended)... I think if you use good shielded wire and install everything with best practice techniques you will be fine. Lots of people making lots of cars run and they never consider anything to this level of detail. I am sure it will work out fine.
 
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