CDI unit bad wiring

I was given a yamaha cv80 and the whole thing was in very bad shape. I started rebuilding and have gotten things almost to a running point but, the wiring was shot it had been rewired at least once before. The wire harness was almost destroyed, So I started to rebuild I have checked the magneto and it seems ok, the coil was damaged so I put a new one but the cdi was blown. I have attempted to rebuild it too, I cut the black box off and used an exacto knife to remove the thick black rubber off of the circuit board. I am strapped for cash so I am doing this the hard way. My problem is and I have researched magneto systems a lot I have books on motorcycle and moped repiar but I still have no spark. Now the cdi in this moped is not like the one in the circuit diagrams, it uses a Thyristor to kink up a certain amount of voltage and then the discharge coil signals to the thyristor to release it's voltage. This then goes to the coil causing a spark. Or so at least from I can figure out. I had a bad diode on the board and a bad Thyristor and replaced them but still no spark. I could use some very technical help on this. I was even thinking about maybe building a new cdi unit from scratch using the standard diode, capacitor, and zener diode circuit. Thanks for all the help in advance.

Re: CDI unit bad wiring

A thyristor is also known as a silicon controlled rectifier (i.e. SCR). Basically it acts as a regular diode, except that it only allows forward bias current to flow when voltage exists at the gate. This flow continues until the current flow drops to zero - regardless of gate status.

While I am having a hard time with the circuit, (biggest problem is that we don't know what leads are the cathode, anode and gate on the thyristor, and we don't know where or what the colored wires are.) I can tell you this - as an electronic tech, parts are so cheap that it pays to replace anything that could possibly fail. i.e. replace all the diodes, all the resistors and resolder everything.

While I've never seen a working thyristor ignition, I see how it could definitely work. A pulse from a second magneto coil or second winding on a single coil turns on the thyristor, and allows current to flow into the condensor. Once it charges the condensor and the magnet has passed the main magneto coil the thyristor turns off, the cap fires into the ignition coil, and you get a spark.

How/Where is your condensor? Thats missing in your pic. Unless I'm confused about how it should work that is.

Re: CDI unit bad wiring

OK, here is a nice link. It describes CDI, and even has a nice schematic of how it should be connected, and several variations.

Re: CDI unit bad wiring

John Joedicke /

Here is a site with a wiring diagram for a CV80

Re: CDI unit bad wiring

Re: CDI unit bad wiring

I have an updated picture of the cdi with the poles on the SCR. I am also wondering where the condensor is but from what I understand about thyristor vs: triacs is that it kinks up energy and may not ned a capcitor but I am not sure. I also have a picture of the whole wiring diagram from the coil but it doesn't show some ground wires, I have figured out where they go, "I think". Let me know what you think of these. Thanks in advance again.

Re: CDI unit bad wiring

I think the b/r goes to the b/w and the w/r goes to the g/w amd the black goes to coil, and g/r goes to ground


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