Re: Onto a new learning curve…CD ignition

[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.