AC-CDI Yamaha


How Yamaha’s CDI WORKS?

8 wires Zoom

(Thanks to Mike and Gottfried who provide me with a “naked” CDI)



In the cover of alternator is a dual pickup coil exciting by two magnets fixed to the flywheel.
Each time a magnet pass in front of a sensor, this one provide one positive pulse followed by a negative one when the magnet leave the sensor.

Yamaha, Kawasaki Honda, Suzuki
pickup animation pickup animation

As my cdi ONLY detect the positive signal, if the positive shape is the second one like Honda and Suzuki, the timing will be wrong…

The tension of the pickup’s signal is proportional to the engine speed

The first sensor to be excited supplies pulses on the white/red wire with 36° advance.
A few millisecond later, the second sensor of the dual pickup to be excited supplies pulses on the white/green wire with 12° advance.
This two sensors produce alternatives pulses. (goes from ±2v while kick-starting to ±10v at low RPM until ±20v at high RPM)
The green wire is the common point of these two sensors.

While kickstarting and when pickups are disconnected from the CDI, the pickup provide ±2volts :

At starting of the engine, the pulses of the 1st pickup (White/Red at 36°) are blocked.
Only the pulses of the 2nd pickup (White/Green at 12°) are used to obtain 12° advance and not risk a kick back.

At idle and low revs, pulses from the 1st sensor at 36° are so delayed that they don’t have time to trigger the SCR.
Only the 12° is then used.

At mid revs, pulses from the 12° pickup loads a resistor + condensator cell to obtain a Frequency to Tension conversion. (the more the rpm, the more the tension.)
This resulting tension is used by a monoflop for delaying the pulses coming from the 1st pickup at 36°.
The more the revs raises, the less is the delay and the sooner is the spark until it comes before the spark triggered by the 12° pickup.

At high RPM then the SCR is triggerd 2 times (first by the 36° then after by the 12°).

at 1000RPM:

at 4000RPM:

The tension of 400Volts produced by the alternator loads a condensator through two transistors to supply the power of the CDI.


(Thanks to Gottfried for this draft)




8 thoughts on “AC-CDI Yamaha”

  1. Really helpful, thanks for the hard work in creating the descriptive schematics and explanation.

    What does SCR stand for?

  2. Interesting!
    i have a 1994 xtz 660 with a sick ignition. At this date, no spark at all. My “CDi” has 8 connectors. Is it possible to adapt one of the described units to my machine? I don`t know the advance values for my machine. Anyone done this adaption?

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