Yes you read the title right... I believe i have the reason why the 84 CDI dies! Mine just started doing the spark then no spark bit today, i found some posts saying i should smack it around a bit, i did, and what the heck it works again!! I ran it around a bit more and tapped on it a few times and the engine promptly died, i tapped on it some more and started it back up. Well this must be an intermittent connection i thought. I went through all the wiring, connector on the wiring harness, i even removed the pins from the shell and tightened them up a bit and reinserted, nothing worked except for tapping that stupid CDI box!!
I had a spare so i put that one on it and took the bad one in the house and called it an afternoon. After a couple hours of looking at it on the bench it was just taunting me to tear it apart as the good electronics guru that i am, so i did!
I looked at it closely, and with my electronics experience i knew right away what i had, a plastic box with the electronics set inside and potting compound poured in. So i carefully cut around the bottom edge to remove the bottom end of the box. The first thing i noticed was the corrosion on the component leads, its all thru hole old school electronics, not surprising for its age.
I flipped it over and sure enough the connector is two pieces and the o-ring can be removed, meaning they were trying to seal it up with this o-ring, but the problem is the female connector that connects to it is NOT sealed on the rear of it where the wires enter, and the wires are not in sealed insets so the water can migrate along the wires into the connector and then that o-ring does nothing but trap the water! Exactly the opposite of what you want. Eventually the water works down through the box and contaminates the circuit board connections. The sides of the box were also very loosely holding onto the potting compound, putting a small screwdriver in on the inside edge of the box side showed it was loose against the potting, meaning water could migrate in around the edges as well.
I then cut down the side of the plastic box and pealed it off to inspect whats on the other side of that board. Not much for components in there, a couple diodes, a capacitor, a triac, and one unknown component that i think is a resistor but i can't see clearly enough to fully identify a value, it could also be a axial capacitor thats next to the one diode. I checked the diodes and they are reading as they should, the cap won't test out with my capacitance meter in circuit and without being able to remove the potting i couldn't do anything more. It looks to me like its a very basic circuit, the inputs like the reverse and neutral switch along with the kill switch ground out the base lead of the triac, but not knowing much about how a CDI unit should function (yet, i am researching it so i can i figure out the circuit in this CDI box better for further diagnosis and duplication also if necessary) thats the best analogy i can make so far.
So i tackled what i could see! I cleaned the board of corrosion with a stiff brush and some contact cleaner, i will also use that to clean out the connector later but didn't get a shot of that. Then i took it to my rework station and desoldered all the component leads, applying fresh flux to clean out as much of the old solder as i could, then i soldered them again, then i desoldered them one more time, and soldered them again, this is how i do it to try and remove as much of the old solder as possible without access to both sides of the board, works every time reflowing it multiple times. If you do this just remember thats lead based solder with the age of these units, so use a fume unit to pull them away from breathing them in, but you don't have to be paranoid we are talking a very small exposure here.
I took it back out, wrapped in electrical tape to prevent shorts, and connected it up. Fired up on first pull with the recoil, and also worked fine with the electric start as well of course. I tapped and tapped and tapped, no problems the engine kept running perfectly, and i also noticed that with the spare on earlier it was cutting out on hard bumps. So this must be the problem, water gets in, causing a connection problem, and cleaning and resoldering fixes that up ok. I even shut off the engine and heated the CDI unit with a hot air gun, from enough distance as to not melt anything but heat it up, i got it to about 200 degrees fahrenheit according to my infrared thermometer, flipped on the kill switch and tapped the start button, fired right up and ran great! It doesn't seem to be shock or temp sensitive anymore.
I have a number of ideas for sealing this back up, among them is repotting in a slightly larger box, pouring potting into the bottom of the box first then filling it to totally encapsulate the circuit board as they should have. The problem is the way it was made, the circuit board is a tight fit to the inside of the box, so there is no potting compound under the circuit board or around the edges, so any moisture that does get in will slowly get to the board anyway by wicking through the board edges. You could probably smear the whole works with RTV and get a better seal then it had. If you go that route be warned, you need to put a couple layers of tape over the board and around it first. Some RTV has chemicals in, acetic acid, that quickly corrodes and eats away soft metals like copper. The better 100% silicone type sealants won't cause this problem. The other reason for the tape either way is it give you a release layer that would allow you to access the bottom of the board again if ever needed.
My other thought here was to make a custom CDI, removing the stock connector and soldering on a pigtail and a weatherpack connector, and replacing the harness connector with a weatherpack unit as well. But i think that the fact that its held up for 28 years, at least to the best of my knowledge as the previous owner stated he never replaced it and he only owned it for three months after buying it off the original since new owner, it should hold up another 20 years sealed up better after repairing!
Another thing you will have to do, and i plan on doing, is sealing up the mating connectors rear end where the wires enter. I figure a little RTV will help, but ultimately i will just make a rubber shroud to go over the back end and use some glue lined heatshrink tubing to seal it up.
Keep Wheelin!
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Originally Posted by kb0nly
