Onto a new learning curve…CD ignition

[fleetwin]

Yes, having the ability to check ignition outputs is a must, just don’t want you to overlook the simple stuff, or get confused/frustrated by meter settings/values, etc.

[johnyrude200]

I’m always all ears to advice from experienced mechanics, much appreciated!!!!! As the saying goes, ‘fixin it ain’t the problem, it’s figuring out what’s wrong that’s the challenge….’

[frankr]

At the risk of repeating what has already been written here and elsewhere, I am going to attempt to do a "Mag CD 101" class. The system is very simple once you have a basic knowledge of it.

1. Terminology: Yes, it can get confusing. When a magnet passes by a coil of wire, a voltage is generated in the coil. Harkening way back into the 1920’s or so, that alone was called a "magneto". Of course, we have come to associate the word with points & condensers etc. But in the strict sense of the word, a magneto is a coil and magnet arrangement. That is the "Mag" part of Mag CD". The coil(s) are the two big coils under the flywheel (3 on a 3-cylinder) and are called "charge" coils because they charge the capacitor in the power pack. The rest of the coils are for charging the battery, if equipped.

2. OK, we have voltage (electricity). That is fed through a couple of wires to the power pack. It is AC electricity as it comes from the charge coils. Within the sealed power pack is a rectifier, which changes that AC to DC. The DC is then stored in a "Capacitor", until it is time to fire a spark. That is the "C" of "CD" ignition.

3. Also under the flywheel is another voltage generator called a sensor, trigger, or whatever somebody wants to call it. Whatever you call it, when it is time for a cylinder to fire, a magnet passes by the sensor, and a voltage is generated. That generated voltage can be either + or – in polarity, depending on which end of the magnet passes by first, North or South.

4. There are other goodies in the power pack, called SCR’s. Those are nothing but electronic switches, similar to transistors but not exactly. There are one for each cylinder. In a two-cylinder outboard, there are 2 SCR’s. One is turned "on" by a + voltage from the sensor, the other is turned on by a – voltage from the sensor (see #3). Whichever is turned on at the moment, it acts as an "on" switch, connecting the capacitor to the appropriate coil. That voltage that was previously stored in the capacitor rushes out through the coil, which causes a spark to occur at the plug lead. That is the "D" part of "CD" ignition.

Once you digest what you have just learned, it is fairly easy to put the knowledge to work. If you have spark on one cylinder but not the other, you know the charge coils are supplying electricity to the power pack, and the capacitor within the pack is storing it. Also, the sensor is ok because it is supplying either a + or – signal. It simply cannot do one without the other. So, what else can possibly be wrong? Ah—the SCR for the cylinder that isn’t firing. Or the coil that isn’t firing. Coils are easy to swap to check for that. If it is the SCR, well since you can’t get at it, the whole power pack gets replaced.

No fire on any cylinder? It is unlikely that all SCR’s are bad at the same time. Possibly there is no voltage from the sensor. Or possibly there is no or inadequate voltage from the charge coils. Or perhaps the kill circuit is shorted. It is possible, but unlikely that both spark coils are bad at the same time. Now is where the DVA meter or adapter comes into it’s glory. You can measure those sensor and charge voltages and capacitor discharge voltages with it. Otherwise, there is the expensive trial and error method.

[johnyrude200]

See Frank, this is all I need to get moving. Of course, there is more to it than that as part of the learning process, but you take 10 minutes of your time to explain this just made a huge difference. Thank you.

The logic behind how stuff works, that is what I’m looking for (the theory and principles), the nitty gritty details past that will all come along the way.

As I said on the other boards…I’m fine with throwing in a contribution to the Frank, Bigbird (eh-um, t2stroke), fleetwin, Gary, and et al retirement fund(s) for all this GREAT tech support.

And anyone who complains the AOMCI membership isn’t worth it must not be utilizing the knowledge base made available by folks on here!!!!!!

[johnyrude200]

Question. If you see weak spark on one cylinder and none on the other, is it possible that there are 2 faults; 1 at the powerpack (or that ignition coil which has no spark) and the 2nd being a weak charge coil?

If there is a weak charge coil, will the powerpack just shoot off a weak spark to the ignition coils (assuming there are no ground issues)?

I ask this because seeing a working CD ignition, there is bright blue spark jumping in most cases 3/4 to 1 inch. In weak systems, I’m seeing it still jump that far, but it’s barely blue or even orange, or just a smaller gap.

For what it’s worth, the old magneto point & condenser system, completely rebuilt with new wires, boots, etc, I’ve been seeing them go 5/8" with bright blue spark, maybe even 3/4 inch. Pretty darn powerful for things that were ‘obsoleted’ pretty much by the mid 70’s (with exception of the small motors up through the early 90’s…was interested to recently work on a 2hp, 1 cylinder OMC from the early 90’s with a points/condensers system still).

[fleetwin]

Unfortunately, ANYTHING is possible. Generally speaking, the sensor and charge coil are simple/reliable parts that rarely fail. Their failure mode is usually caused by be being hit by the flywheel magnets due to a sloppy mag plate or loose hardware flopping around under the flywheel. Flywheel magnets are rarely a problem, except on larger engines with glued in magnets.
CD ignition spark should be able to jump a 1/2" gap, 3/4-1" might be stretching it a bit.
Again, my best advice is to start with the simple stuff. Don’t do the "it looks it" check on grounds, use a good analog ohm meter that shows low resistance readings clearly. Just because a ground "looks bad", doesn’t mean it is, the same goes for a ground connection that "looks good". It is always nice to find an actual problem/cause by checking the grounds before ripping them apart to clean them. Those pesky amphenol connections can cause problems as well. Bent pins, pins/sockets partially pushed out, corrosion all can create nightmares. Be sure to eliminate that stop circuit completely as well. Those push on primary leads can cause problems in salt water as well. Be sure to pull the connections, clean the terminals, crimp them down stlightly for a good tight fit, and apply a little grease to the primary terminal on the coils. Chafed/melted leads, along with sloppy repairs are another big problem.
Once you have exhausted the simple stuff, you will need the proper test equipment.
You will have to spend the bucks on the amphenol tools, now way around that. Stevens makes some nice terminal extenders for the coils and amphenol terminals to make resistance and voltage tests easier.
You will need one of the peak reading voltmeters as well, your coil tester will need an "amplifier" from stevens to check the CD coils. There is a little adapter, called the PL-88 that is needed to check pack output on newer engines also.
Finally, polarity and voltages have changed over the years, so improper meter set up prior to testing can lead to confusion and headaches. Use the proper OMC manual to avoid confusion. And, it is best to check every component during repairs to avoid repeat pack failures due to marginal parts/grounds/faulty stop circuits.

[rjoynt]

I will pass on a tidbit about Steven’s. I have bought items for them before, and being a frugal type, asked them if they had any repaired units at a discount. They always did; saved a few bucks. Very nice folks.

[legendre]

quote FrankR:

Gotta tell you though, you could actually cobble up your own DVA Adapter. I did. Very crude though, but it shows how little is actually in the real deal.

I can’t image there’s a whole lot needed.. it’s just a peak hold circuit. A fast, high-voltage diode, a small value low-leakage cap..? What else is in there?

Why can’t a standard DVM with peak-hold feature be used?

ETA: I just found a discussion on another site. Apparently, I was correct – nothing more than a cap & diode in there. For some reason, the builder also used a 1M resistor in parallel with the cap – I have no idea why. All that would do is cause the "held" reading to fall off to zero more quickly. He also used a 2.2uF @ 450V electrolytic cap – I’d have tried more like 0.22uF – 1.0uF first (in a low-loss film type) before venturing into electrolytic territory.

[johnyrude200]

Well, this might as well be hieroglyphs to me…. someday, it won’t be 🙂

[legendre]

quote johnyrude200:

Well, this might as well be hieroglyphs to me…. someday, it won’t be 🙂

Diodes are one-way electrical valves, used to convert AC to DC, and capacitors are electrical storage tanks. Caps only store DC, they cannot store AC. Magnetos / CDI ignitions all produce AC, at their heart.

So you connect the diode-capacitor circuit across the leads from the flywheel coils to the power pack, the same leads you’d measure with one of the fancy DVA testers, and give the motor a few spins.

When the flywheel magnet & coil produce an AC pulse, it’s converted to DC by the diode and stored in the capacitor as a DC voltage – and there it remains. You then read that voltage stored in the cap (with your meter), as it is approximately equal to the highest level of the AC pulse.

Did that make sense?

BTW – no capacitor is perfect, nor is any diode a "leak-proof" valve. So the voltage stored on the cap will start to bleed-off, right away.. best to measure it ASAP. And believe it or not, just +measuring+ it will cause it to bleed off faster – that’s because the meter itself must tap some electrical energy just to generate a reading! Even in a digital meter, where there’s no meter movement to drive, the meter still sucks down the voltage..

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