On Chinese scooters with a GY6 engine (139QMB 50cc, 152QMI 125cc, 157QMJ 150cc) there are two types of CDI that look the same and whose connectors often fit each other: the AC CDI and the DC CDI. Fit the wrong type and you get no spark.
AC or DC: where does the power come from?
The difference lies in the power supply, not in what the CDI does: both receive a pulse from the pickup and drive the coil.
- AC-CDI — the power supply comes as AC voltage from an ignition or charging coil in the stator, under the flywheel. The spark comes from the engine itself: with a flat battery you still get a spark as long as you kick.
- DC-CDI — the power supply is the 12 V from the battery via the ignition switch; the unit contains a converter that turns it into the charging voltage. So the ignition depends on the charging circuit: with a flat battery or a faulty voltage regulator there is no spark.
The connections on a GY6-CDI
Common on GY6 is a connector block with six pins of which five are used, often as a 4-pin plus a 2-pin section. What you'll almost always find:
- Power supply — from the ignition coil (AC) or +12 V via the ignition switch (DC).
- Pulse generator — the signal from the pickup at the flywheel, which determines the ignition timing.
- Earth — the return to the frame and engine block.
- Output to the coil — the low-voltage side; the coil turns it into the high voltage.
- Shut-off (kill) — the ignition switch and kill switch connect this wire to earth.
- Limiting — sometimes an extra wire or small connector for the limiter or remote control.
Pin layout and wire colours differ per manufacturer, even between scooters that look the same. Never go by colour: photograph your connector before you disconnect it and check the pin layout in the wiring diagram for your own model.
Why the other type does not work
An AC CDI has no inverter: 12 V is too little to charge its capacitor, so no spark. Conversely, the 12V input of a DC CDI receives the high AC voltage from the ignition coil — also no spark, and that can damage the unit. Yet the connector fits: that's the pitfall.
How to determine which type you have
- Read the markings. On many units AC or DC is printed on the housing, next to the item number.
- Follow the power wire. If it runs to the connector of the engine block, it's an AC system; if it comes out of the wiring loom at the ignition switch or fuse, it's DC.
- Measure with a multimeter. Connector unplugged, ignition on, engine off. Black probe on bare earth, red probe in the supply pin on the wiring loom side, meter set to DC voltage: around 12 V is DC, 0 V indicates AC. You can only measure AC voltage while starting: range at least 200 V, and don't touch the contacts.
- Check the earth. The resistance between the earth pin and bare frame should be practically 0 ohm; even a few ohms from corrosion is enough to cause an intermittent spark.
- Measure the pulse generator. There should be a measurable resistance across the two pickup wires; infinite (open circuit) or 0 ohm (short circuit) is faulty, and the exact value is in the workshop manual for your model.
- Choose the same type. AC for AC, DC for DC, same number of poles, same limiting.
Restricted or unrestricted
On many Chinese scooters the speed limiter is in the CDI itself: there are versions for 25, 35 and 45 km/h, plus adjustable and 'fast' units. A light moped is approved for 25 km/h, a moped for 45 km/h; that design speed belongs to your registration and insurance, so choose the limiter that suits your scooter. Also pay attention to the wheel size (10 or 12 inch): the wheel circumference determines at what rpm that speed is reached.
The classic mistakes
- Bought the wrong type — the plug fits, the unit stays dead. Establish whether you need AC or DC before you order.
- Poor earth — a corroded earth point gives the same symptoms as a faulty CDI.
- Pickup or coil as a 'CDI fault' — no pulse means no spark. First test the pickup and coil using the 'no spark' troubleshooter, and start at the spark plug.
- A quick CDI as a repair — it won't fix a running problem.
The right parts straight away
Browse the CDIs and ECUs, the coils and the complete ignitions with stator, flywheel and pickup; with a DC system the voltage regulator belongs in the same check. Filter by vehicle brand China or look up your model in the technical drawings of the China GY6. What fits together is listed in GY6 parts; if it won't start, work through 'scooter won't start'. More in scooter maintenance and the FAQ.
