Size is not always an indicator of DC vs AC but it is often that AC is smaller.
The easiest way to check if a CDI is AC vs DC is that on a DC CDI the power pin is usually wired to the key switch on a DC CDI. If you attach a meter or test light to the power pin on a DC CDI and turn the key on the DC CDI power pin will light up because it’s being fed power and if you turn the key off the power pin should once more be unpowered. On an AC CDI the power pin will never light up until the motor is running as the CDI is pulling power directly from the motors wiring harness.
I did not have a look at the bottom wiring on my 6 Speed CDI yet but my understanding is all CDI have two connectors one being a four pin and the other being a two pin connector.
If you unplug the CDI and look at the connectors head on and with them at the top my understanding is on the four pin both bottom pins are ground. The top left is signal wire and top right goes to the coil.
On the two pin connector the top pin is wired to the kill switch which will ground out this pin telling the CDI to kill the engine and the bottom pin is the power pin.
AC or DC it should be the same connector wiring setup except for where that power pin connects to. So the ultimate test is to see if that power pin energizes with key switch or not.
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