How noisy are your ignition coils? Tachometer low pass filtering.....

Gerald Morris

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Greetings My Wise Moparians!

While my Taylor coil still produces enough spark to run Gertrude, last August's engine fire indubitably compromised that coil. I replaced it this past weekend with an NOS Delco coil made for Mopars, and VROOM!, spark MUCH improved! Alas, All yet needs be made Well....

I ran my old Stewart Warner tachometer unfiltered for over 3 years off that Taylor Vertex coil, with stock Mopar points, condenser et al, and only very recently began to see malfunction on that tach. It ALWAYS has malfunctioned with a loose connection inside, likely a result of a physical shock, which until recently, a gentle finger tap would set aright, giving good, consistent data. After severe rains since the New Year, this venerable instrument began to decay. It still measures SOMETHING from the coil, which increases or decreases as engine revolutions do, but doesn't accurately convey data. I have an Actron meter which I customarily use to tune the engine, with a reasonably reliable tachometer.

I purchased a shiny (PLASTIC! DAMN THEM!) Jegs tach for about $70 which clearly uses far more digital methods of measuring ignition pulses than the old Stewart Warner, which was made circa 1975. I attached the leads, saw the stepper motor inside run up and down to self calibrate the tach the first time, as per instructions, then started the engine. For the first minute, I saw my customary idle around 7-- rpm, THEN THE NOISE CAME.....!!! The needle commenced jumping all over the place, so I immediately turned the engine off, turned the ignition on to permit calibration again, restarted, again to some VERY noisy signal.

This came as no surprise, alas, as even my Actron meter now jumps some to noise off this Delco coil. Some questions have arisen to me at this point, which I seek your illumination for:

Shall I place a big old fashioned condenser on this coil? Such were used for AM radios, and this MAY help....

Are modern coils made with some remedial noise filtration from their outputs?

Would aged askarel permit noise more than modern equivalents? The Taylor coil also is so insulated, but it is nearly 50 yrs newer, as far as manufacture date goes.

Could my ignition be afflicted with some subtle short circuit yet? I HAVE looked, using continuity checks and such as my crude instruments allow.

Should I replace my ballast resistor? I can, and likely will, but suspect this won't solve the problem.

Is my veneralbe SW tach in fact still accurate, but merely AVERAGING noise pulse into the proper ignition pulse, thus showing higher readings than reality? Knowing a bit about analogue electronics, this seems probable, but I need more data. Old capacitors oft die in humid air....

Would a $32 tach filter be my best, most cost effective investment? I see one from Classical Instruments which looks acceptable. Have any of YOU used it?

Feel free to respond to these, and other queries which may NOT have yet occurred to me. I enjoy good electrical puzzles, except when my life or property might be adversely affected by them.
 

70bigblockdodge

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I don't know about noise, but I have had a recent failure of a coil with the spark jumping out of the tower and grounding to both the POS and Neg terminals. I have seen this before on high mile/old coils or hit and cracked on top. This was a newer coil, never damaged. One thing that is better with modern tech is plastics, or so I thought, apparently they only use good plastics in the packaging which you need a razor sharp knife to open, everything else is plastic shopping bags that will not hold a bag of cotton balls without failure.
 

Gerald Morris

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I don't know about noise, but I have had a recent failure of a coil with the spark jumping out of the tower and grounding to both the POS and Neg terminals. I have seen this before on high mile/old coils or hit and cracked on top. This was a newer coil, never damaged. One thing that is better with modern tech is plastics, or so I thought, apparently they only use good plastics in the packaging which you need a razor sharp knife to open, everything else is plastic shopping bags that will not hold a bag of cotton balls without failure.

My entire motive in purchasing that NOS coil comes from my regard for modern quality control, or the severe lack of it.

I've tightened my alternator ground, which eliminated SOME of the noise, enough to use the new, cheap, but U.S.A. made Jegs tach. I also ordered some caps, resistors and a small punchboard to solder them together on, and will make a filter. Outlay for that ran just over $6. Now, having removed the old tach, I found that the power stud has broken loose inside the tach, which very likely contributes to its malfunction. IFF I can open it sans damage, I can repair that problem, then see if I want to use it again.

THIS is why I prefer ALL older consumer goods; they can often be repaired. This shiny new Jegs tachometer clearly is sealed shut, extremely light in construction and digital in function. It seems fairly close to the readings from my Actron meter, so that bodes well enough.

I'm looking into inductive pickup tachometers for both work instrumentation, and future driving display.

I also plan to replace the crappy old thermo-electric gas tank gauge power supply with a nice solid state one. THAT WILL eliminate considerable electrical noise from my dashboard in general.

I also will try today running an isolated ground back to the battery for the tach. I have a bonding jumper under my hood, connecting the big negative ground lug across the intake manifold to the coil, then on to the firewall and presumably, the dash, but since that jumper picks up all sorts of noise, and I observed the Jegs tach running smoother when I first tested it from in front of the car, attached under the hood direct to the battery and coil, I think an isolated ground likely will do it good.

If one needs spend more to attach an instrument than the instrument cost, then one may reconsider the purchase economy.
 

Gerald Morris

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Final Entry:

I made up a $6 filter, all passive from a 5.6k 1/2 W resistor going in, a .15uF cap going to ground from that, which also is series connected to a 10k resistor. The filtered output then has a little 100 pF resistor going to ground off the output end of the 10k resistor. Here's the schematic:

attachment.jpg

I STILL will clean up the wiring, but I suspect that old Delco coil IS noisy, so the filter is a Good Idea.

BTW, this filter works damned well.
 
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