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Shielding of ignition wiring harness

EFI Wiring Fundamentals

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Good day to everyone,

I recently converted my Honda B series engine from a distributor to Coil-on-Plug ignition. I have built the harness personally and the car is running properly. during testing, I didn't notice anything but since I daily drive the car I noticed today that if I have the radio on I can hear the firing of the coils through the stereo speakers and I would like to add some sort of shielding on the harness.

The harness structure is like this:

1 12V cable splitting into 4 for the coils

1 GND wire splitting into 4 for the coils connected to the engine body

1 speedo cable from the ECU to the tacho

4 trigger cables running from the ECU to the coils

I was thinking of running an extra wire through all the harness and grounding it also in the engine body. I have attached pics of the schematic of the harness and the harness it shelf.

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Guyz anyone?

G'day Kindinos. Sorry for the late reply, I was away last week, but am back on board as of this morning.

Couple of questions to help narrow in, where do your coils get their 12V supply from, and where have you got them grounded to?

The original 12V supply to the coil that was in the distributor should be fine to use, but you'll want to make sure you ground the coils to the cylinder head, as this will help keep the coil secondary winding current out of any other parts of the system.

Good day to you Zac. If you see the Untitled.png is the schematic of the harness. The +12V is taken from the factory harness in order to be able to cut the supply when I switch off the car and to ground the coils I used one of the 3 bolts of the distributor. I have to keep the distributor on the car for Crank/Cam signals!

Hmmm, interesting. Using the original power supply, and ground the coils to the head as you've done should eliminate you having any problems.

Try temporarily running another (decently large) wire from the ground point on your distributor to the battery negative directly, and see if it helps.

Have a read of this too:


Similar problem, but our noise was on the engine speed and position trigger signals to the ECU, not in the stereo.

Is it the OEM, or an aftermarket stereo, and where is it powered / grounded?

Sorry for all the questions!

No problem about the questions!

The stereo is aftermarket and the power/ground is the ones on the factory harness! I've read the article and that reminds me of something I noticed on a log when I went to the dyno to make a power check on the car! During the ramp run at about 3500 rpm there was a spike going to 7000 rpm and back normally to 3500 rpm, the car didn't stumble or anything that's why I noticed it later in the log, it was instant! I think I have the log I'll try to upload it later! Back then the car was running with the distributor! Right now I think I have more issues than I think!

Try running that external wire and post up if it makes any difference :-).

I did run the wire as you suggested Zac and started the engine with the speakers of the radio on a high volume level to make sure that I will hear 100% if there is interference and there was none! In order to double check, I took out the wire and checked again and no interference at all! When I noticed the interference I was listening to the radio when I switched the source to the cd the interference stopped! Is it possible for the radio to pick-up the frequencies from the coils?

E.g. I also uploaded the log file from the dyno I was talking about!

Thanks in advance

Attached Files

That's really good info George! We can be be sure now that the interference isn't in any of the power supply wires.

This will sound a bit weird, but can you temporarily disconnect the tacho output wire from the ECU to the gauge cluster and see if the radio interference dissappears? I'm wondering if the way the ECU is driving the tacho is the source of the noise, and not the coils themselves.

Does it have resistor spark plugs? Suppressor fitted on coil power supply?

Those are bloody good questions! I had assumed the suppressor would still be in place as the original wiring was re-used, but you're right, its completely possible it's been removed.

I did the test with the tacho cable and all the interference were gone!

The spark plugs I'm using are resistor spark plugs according to NGK.

I don't know anything about the suppressor you are talking.I didn't touch the factory harness, in order to keep everything looking as stock as possible, I made the harness to with a plug from an old distributor I had so If I like to switch back to the distributor to be fairly easy! another plus to this is that if a coil goes bad I add the leads on the distributor change the plug and off we go again! (I have a very bad experience with Volkswagens before so better safe than sorry). Is it possible for the suppressor to be inside the distributor?

By the way Zac your course on EFI wiring was very informing keep up the good work!

Well, that's good we've gotten to the cause of the interference. The level of noise radiated from a wire is majorly determined by the speed at which the signal on it changes, often called the 'speed of the edges'. The ECU will be sending a square wave along the tacho wire, which might be radiating just enough noise to be picked up by the stereo antenna wiring.

What ECU have you got in the car? I'm wondering if you can make it output a tacho signal while the engine isn't running to just 100% confirm this is the source of the interference?

I have a P30 honda ECU with a Moates Demon II running Neptune and a Honda Rulez COP conversion Board! I can post some pics from the ECU if you like!

If indeed we manage to replicate the signal would be possible to filter it or is a shielding issue? If it is to filter the signal does that mean there would be also a lag on the tacho? If it is to shield the cable where I should ground the shield? Does a loop in the harness amplify the event and what is considered a loop?

any update for this?

In original form, what generates the signal to the tachometer, the ECU, or the distributor?

From what I understand from the schematics given to me when I converted from distributor to C.O.P the distributor gives the signal to the tacho in factory form

Attached Files

Ahhh, I see, the COP module (the PCB) sends out a tach signal via the COP harness, which then connects to the OEM honda harness at the original distributor plug?

You could try changing this to a shielded wire, ground the shield at the ECU end. You could also try a filter on the tacho signal to try and slow down the edges and limit radiated noise. Its not something I've done before though, so I couldn't offer much advice on it unfortunately.

Ok thanks, Zac I'll try with a shielded wire one last question on the matter the ground on the ECU end should be on sensor ground correct?

In practice I've never found a difference, and ecu manufacturers have different recommendations, but I ground them to the power ground of the ECU.

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