Howdy folks,

I have been thinking about what slantvaliant and others were writing about a few months ago in a thread about radio suppression capacitor on the car firewalls. I was relating that to the Pertronix capacitor on coil thread a few weeks ago because I am doing a Pertronix install for the fun of it. Now, unlike cars, the 60's trucks had the suppression cap mounted and grounded on the coil bracket and a lead attached to the coil positive. The question on the pertronix thread was wondering whether a capacitor was supposed to be on the (+) or (-) terminal of the coil. While the coil voltages were discussed by mopar_nocar, the terminal question was never answered. I have been doing some rewiring preparatory to switching to a GM SI12 alternator, and have about decided the capacitor could just as well be anywhere in the switched 12v line from the ballast resistor as on the coil (+ ) terminal....and still not interfer with a Pertronix unit....am I right?

Cheers!
rock
'64d100

No, that's where the radio noise suppression cap was mounted on the passenger cars, too. The cap on the dash was specifically to damp out spikes created by the instrument cluster voltage limiter, so the temp and fuel gauges wouldn't 'dance' as much as they would without the cap.
Radio noise suppression caps always go on the coil (+) line.
Correct, the cap doesn't know or care whether it's at one end of the wire or t'other.

I once ran out of room to mount an (ignition) condenser inside the distributor, so I installed a coil bracket with a condenser tab and threaded hole, mounted the condenser there and put its wire on the coil primary (-) terminal. It worked fine.

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Too many people who were born on third base actually believe they've hit a triple.

The cap itself doesn't care, but for maximum effectiveness when using a cap in this application (noise supression) you always want it as close to the load as possible, in this case the load being the coil. Will it work at the ballast? Sure. Will you notice a practical difference? Maybe not, probably not without a scope. But just as a rule of thumb, as close to the load as possible.

Purely in theory, you're right. But in practice, you will not see the difference even on a scope.

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Too many people who were born on third base actually believe they've hit a triple.

In small current applications like this, perhaps not. But on a larger scale, like high power audio amps (multi kilowatt), its there.

Mopar put one cap by the coil, one cap under the dash by the regulator. Why not one bigger one in the center, woulda saved cash right?

As I was saying, theory vs. practice...
Separate circuits that need to stay separate need to have separate caps. That's pretty basic theory.

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Too many people who were born on third base actually believe they've hit a triple.

They are on the same circuit aren't they, power on with key on? I need to dig up schematics to verify 100%.... But regardless wether they are behind a different fuse or not they all tie back to the same battery.

And about the audio amp example - thats not theory it has been shown in practice. I don't have any documented examples off hand though. In high speed electronics (networking, computers, etc) decoupling caps are always as close to the load as possible and bulk capacitance surrounding them. Open up a computer case, you will likely see bulk caps around your cpu socket.

Sorry, no intent of being annoying I'm just continuing this discussion for sake of conversation.

Go study the wiring diagrams, please. The cap on the ICVR is on a nominal 5v circuit. The cap on the feed side of the ignition primary circuit is on a circuit that varies from 6 to 12v. In order to isolate those circuits from one another to avoid frying the gauges, there'd need to be a diode. In order to avoid feeding the ignition primary via the gauge circuit and roasting the ICVR and the gauges themselves, there'd need to be another diode. Not only would these diodes nuke the cost savings from using one cap instead of two, they'd also effectively prevent the caps from doing their job for either circuit.
Maybe it has, but I don't buy it. The "Monster Cable" people are always babbling about how it's been "shown" that their ridiculously overpriced cable works better than plain wire, even though in actual fact it does not. I think probably this demonstration is a product of the brain-damaged types who feel the need to inflict their grossly overloud car stereos upon the world.
Sure, but that's because in high-speed computing, the time it takes electrons to travel the length of a wire is longer than the critical intervals in the computing processes. That time is very, very, very nearly instantaneous, but the critical time interval is just a skoshe closer to instantaneous, so the cap has to be over here instead of over there.

There is no such computing going on under the hood of any car. You could mount your radio noise suppression cap on the *rear bumper* and run a wire from it three times around the whole car and then connect the other end to the coil (+) and still not see any practical difference in the effectiveness of the radio noise suppression.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Hmph my fsm doesn't even show the radio supression cap or the dash regulator cap. Unles its on other pages besides the engine and instrument panel wiring. Point me to a section? I believe the dash regulator cap was on the +12 side not the +5 side.

I knew that comparison was going to come back and bite me. In my mind I was thinking of a ballastless ignition system, +12v from key on directly to coil (or aftermarket ignition box) and then to the gauges. I haven't ran a ballast in so long I almost forgot our cars originally came with one

Like I said earlier in practice you may not see it in case of our cars, but if you can't see it on your scope then we are getting into the "how fast is your scope" region and thats a whole other ballgame.

I'm not going to be able to articulate my words well enough to have any significant meaning so I'm staying out of this.

Dan and Pierre,

Thanks!. You guys always inform me so willingly. I had also noticed that emstivl seems to be a pretty good auto ignition circuit guy too. I am dangerous because I am not completely unschooled. I had studied 10 years worth of wiring diagrams and was amazed that the only truly RIGHT ON diagram for my '64 truck was in the '64 truck service manual from CANADA mopar! I have been steadily acquiring these old CN books because they are a blend of FSM and "here is what you as Mr. Service are to do to this Mopar when it comes in". My dangerousness comes in because I was schooled in radio tube electronics and try to apply fundamentals like LC circuits to the ignition circuit on or vehicles.....and keep forgetting the fundamentals you both articulated...as in the electron really doesn't care where in a straight path through which it can pass, if omething is inserted offering no block to that passage! In other words I confuse myself trying to apply radio circuits and house wiring to cars and keep forgetting I don't have the same knowledge of why parts in car electric circuit were put there as I do those of radios and houses.

I was about to get a scope out to the truck to look at the waveforms of the voltage regulator on/off and the field collapse at the coil, for instance, and then moved to thinking of what kind of diode system I could put inside the voltage regulator to upgrade it, then realized guys on this forum and at Chrysler had already thought about it all. (Remember I'm the guy who measured throwout bearing pressure against clutch pressure plate fingers as a function of how far in or out the pivot ball was moved in the bell!). Sorry to ramble, rainy Saturday and your excellent discussion got me going. I never found a diagram with the noise caps, either, Pierre but will keep mine at the coil, even though I never had a radio in the vehicle.
rock
'64d100