Capacitive Discharge Ignition - Page 1
Forum section: electronics / projects

author: RyanG
author_rank: Regular Contributor
timestamp: July 11, 2018, 07:30:38 pm
content: I've been working on a boat motor from 1969, which uses a points ignition system with capacitive discharge. I am trying to understand exactly how it works in the motor that I am working on because it seems to be slightly different than most of what I have found online. I have attached the wiring diagram for the motor I am working on. The CDI box that I am trying to figure out and eventually hope to be able to make so that I don't have to pay around $300 if it goes again is the gray box on the right side of the picture.The wires are:Gray - Tachometer (not used in my situation)Black/White - Goes to ground through the pointsPurple - 12v from batteryBlue - ~200v output to ignition coilThere is a ground wire that is not in that picture.I have had to replace the CDI box twice in the last 3 years on this motor. I have read that if the stator's output goes too high or if the battery voltage drops too low then the CDI box can be damaged. The battery is known good, but I do not know the output of the stator at the suggested 3500 RPM. I did buy a "regulated rectifier" this time, which is supposed to keep the voltage coming off of the stator within the limit. I get the basic concept of how the box works. It just steps the voltage up into a capacitor and then discharges it through the ignition coil, but I am trying to fully understand the best way to do it. I've tried to disassemble the old one, but it's potted. Any help would be great.

author: james_s
author_rank: Super Contributor
timestamp: July 11, 2018, 08:31:30 pm
content: I actually have the 114HP version of that same motor, it's a really nice outboard, I love the sound. I've heard of issues with the CDI box however I haven't had mine fail, knock on wood.For the potting have you tried heat? I've reverse engineered a number of potted modules by using a heat gun to heat the epoxy and then digging it out. I've thought it might be interesting to engineer a modern replacement, surely solid state electronics have improved a bit since 1969.

author: Gregg
author_rank: Super Contributor
timestamp: July 11, 2018, 08:40:15 pm
content: Have you looked at the output of the rectifier when it is running to see if major spikes are present as well as possible over voltage at higher RPMs? I don't see anything resembling a voltage regulator and a lot of the older methods of regulating voltage with a permanent magnet alternator just dumped the excess voltage to ground via a simple circuit using a SCR. It could be the regulator and rectifier are in the same module.

author: RyanG
author_rank: Regular Contributor
timestamp: July 12, 2018, 02:53:08 am
content: I haven't looked at the output of the rectifier at this point because it hasn't been running, and I thought it was going to be okay last time I replaced the CDI box. The CDI box that just broke was not an original. I replaced it with another of the same, but made sure to buy it with a full warranty this time. I did buy a rectifier from the same company that is supposedly regulated, so that should take care of the overvoltage problem. I have made decent progress on the potting material. I cut some holes into it, filled it with paint thinner, and lit it on fire. I know that sounds really damaging but it wasn't.

author: Gregg
author_rank: Super Contributor
timestamp: July 12, 2018, 04:02:58 am
content: Quote from: RyanG on July 12, 2018, 02:53:08 amWe are all awaiting the video

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2018, 05:36:07 am
content: A heat gun is much better and more controlled than setting it on fire. You heat it up until it softens, pick away at the epoxy until it starts to cool and then heat it again. Do it somewhere well ventilated because it can really stink.