CDI Box and Audio Interference

[LucaScali]

So I had an “episode” with a replacement CDI box which required me to temporarily reinstall the Permatune. One happy side effect was that the loud, variable whine coming from the head unit and speakers all but disappeared with the Permatune in place. I hadn’t realized that the faulty CDI box had been interfering with the stereo, and now I’m concerned that the interference will return with installation of the new Bosch box. Is it common for these Bosch boxes to cause noise, or was it probably some defect in that particular box? If this is indeed a typical issue, what steps are usually taken to eliminate CDI interference? Please tell me I don’t have to choose between reliability and my audio system.

[Lorenfb]

"Is it common for these Bosch boxes to cause noise"

You need to question your source for your CDI unit,
since mostly likely you didn't purchase it from Pelican Parts!

[LucaScali]

Great, is that a no? I'm not asking about the reliability of the source, I'm asking if some defect in that particular box could have caused the interference even as it performed reasonably well otherwise. With the Permatune installed, everything is quiet, but I intend to replace it with a rebuilt Bosch. What can I expect in terms of audio interference?

[HawgRyder]

A filter...placed as near to the CDI box as possible would probably reduce or get rid of the noise.
A proper filter would be a "T" type ....which is the power line connects to a coil (inductor) with a capacitor leading to ground at each end....and then the out of the filter would power the CDI box.
This should be available from car parts suppliers...like PEP boys...or Canadian Tire here in Canada...and is usually in the audio section.
Choose one that thru-puts at least 20 AMPS....maybe more...so it runs cool.
Bob

[wwest]

I think my '78 came with a filter "box", mounted very near the CDI and external regulator.

[LucaScali]

Thanks, guys. So am I correct to assume that I will need some kind of filter with the Bosch, even one that has been correctly rebuilt?

[RoninLB]

I needed a noise filter with my MSD along with resistor spark plugs

[HawgRyder]

The problem is the high frequency part of the CDI.
The pulses get back into the wiring of the car...causing "hum"...or"clicking".
The idea is to isolate the CDI from the rest of the harness and keep the noise inside the CDI box.
Remember...grounding of the CDI box is VERY important!
Because the box is mounted on a plate which is then bolted to the side of the engine compartment...it has several places where the ground could be marginal.
Bob

[LucaScali]

That makes sense, Bob--I got both hum and occasional clicks. Why weren't the original stereos impacted by the high frequency?

Curious, what is it about the design of the Permatune that moderates this effect? The Permatune is basically silent.

[tirwin]

LucaScali,

Out of curiosity, what kind of plug wires are you using? Original ones with the ground wires or something else?

Reason I ask is that I've heard that is a reason the factory used the braided plug wires with grounds. Don't know if it's true or urban legend. But your setup would be an interesting data point.

[LucaScali]

I can't say for sure, tirwin ; they were installed by the PO shortly before I bought the car. What I do know is that they are not original.

Can anyone speak to the the Permantune design?

[LucaScali]

So I installed a refurbished CDI box courtesy of Specialized ECU Repair along with a new MSD High Vibration coil. The noise is back. Can Bob or anyone else suggest a specific filter and provide some details about installation?

[RoninLB]

i used MSD 8830

[RoninLB]

it's the round red unit

it ain't cheap

[HawgRyder]

Try to get a filter made for a BIG amplifier from the local auto store.
Tell them it has to pass 15 - 20 AMPS.
The real secret to success is to get the filter as close to the CDI unit as possible.
If one end of the filter is right at the plug going in to the CDI...you will eliminate any stray "RF" noise from getting into the harness.
Also try using one on the back of the alternator to keep the brush noise from interfering in the harness....this one will have to pass the total current that the alt is capable of (55 AMPS for the early ones)...so the coil in that filter will be fairly big and heavy (make sure it is supported...not just hanging on the wires.
Bob

[LucaScali]

Someone recommended the filter below. Would something like that do the trick?

Monster Performance Car - Monster Silencer™ In-Line Noise Reduction Module

[Porboynz]

I must have been just plain lucky, I ran a stock CDI with standard plug leads and a Kenwood head unit. No special filters, but the CDI is earthed as is the fuse/relay panel and no noise on the radio or cassette. (Well, there is/was plenty of noise, but that was the RS style muffler, thats a good noise, right?) Maybe your head unit is a little susceptible to RF noise? The CDI box should not radiate noise but those plug wires might. Of course that does not explain why the Permatune is silent.

[wwest]

Is the CDI putting noise on the 12 volt system or is the high voltage "sparks" radiating the noise. Run the CDI separately on a small 12 V motorcycle battery and see.

IMMHO it is much more common for the alterator/regulator to be putting NOISE on the 12V system due to poor grounding or poor front to back, battery......alternator/regulator, electrical connections.

[HawgRyder]

That "monster silencer" is just a Ferrite core around the wire.
It will lower the RF interference in an audio circuit, but does nothing to get rid of spikes in a DC circuit (well...maybe just a bit).
A DC filter is a coil (big wire...#14..maybe #12 or #10) with a capacitor to ground at each end of the coil.
The power passes through the coil and the spikes are buffered so that they are drastically reduced.
We used several of these in series for some military applications when the equipment was sensitive to outside noise (some Radar sets would show false echoes if not shielded).
What ever you do...remember...all capacitors MUST go to a GOOD ground!!
If the grounding system is suspect...clean everything religiously.
My old saying is: If you are NOT willing to lick it with your tongue...it's not clean enough!! (you don't really lick it...just think about how clean it needs to be).
Bob

[HawgRyder]

BTW...I have seen filters where the capacitor to ground at the ends is really several in parallel.
The different sizes of caps each pull down the spikes at different frequencies.
So...if you have high freq noise....you use small caps like .01uF and for heavier or lower noise like the alt..perhaps .47uF or even bigger...like 1.0uF (these are for noise).
For ripple (waves in the DC) you would use BIG caps...10 - 50uF.
Go to Radio Shack or similar and ask the old guy there what he used to use....or better still...find a Ham Operator and ask him/her.
Bob