Although the ignition of the Rotax 123 is officially called “magneto ignition” in the repair manuals, it is actually a classic capacitor discharge ignition, a CDI (Capacitor Discharge Ignition).

The ignition system includes the components

  • Flywheel SEM Type K 101 799
  • Stator: SEM, Type WG, Alternating Current
  • Ignition Coil SEM TM14-04 101 888 00

Such an ignition system was also used on the KTM LC4 620, here however with ignition coil TM14-05 101 890 00.

The ignition system is a finished, closed unit which, except for the ignition coil, does not require any electronics outside the engine. The ignition is switched off by connecting the orange shutdown wire to ground, otherwise it can run independently without external power supply. A too weak battery is therefore never a reason for a missing ignition spark on the Rotax 123, at most the resulting low starter speed can cause an issue.

If the ignition spark is missing in the older Aprilia models with Rotax 123 engine, a defect in the SEM magneto ignition can be a cause. Unfortunately it happens that parts of the ignition system fail out of the blue.

A useful hint when looking for the cause of a missing ignition spark: First check whether you have pressed the emergency stop by mistake. You can also measure this: Disconnect the orange wire from the ignition coil to the wiring harness and measure between the plug on the wiring harness to ground: With the ignition on and the emergency stop not engaged, there must be no connection between this plug and ground.

Measurements on the Ignition Coil

The ignition coil contains beside an ignition coil some more electronic compontents, this is cast and unfortunately therefore almost impossible to check without destroying the ignition coil.

The following resistances must be measurable at the ignition coil [1]:

  • Resistance between red (charge generator) and orange (shutdown wire): Less than 0.5 Ω
  • Resistance of the secondary winding of the ignition coil between black and the connection of the ignition cable: (2.2 ± 0.4) kΩ

Measurements on the Stator

The SEM stator in the Rotax 123 consists of an alternator with 12 individual coils, a set of four so-called ignition generator coils or charging coils and one firing impulse generator coil.

These coils deliver a very high voltage during operation and have a high number of turns, so the ohmic coil resistance can be measured here with a multimeter.

Schematic Diagram of the winding of the SEM stator (as seen on [2])

The following resistances can be checked [1]:

  • Resistance of the firing impulse generator winding between green and black: (170 ± 2) Ω
  • Resistance between the two yellow wires: 0.3 Ω (generator winding)
  • There must be no measurable connection between yellow and black.

The ignition generator winding is connected to ground on one side with a rectifier diode [2], which is why a normal measurement of the resistance is not very meaningful and the shown value depends on the measuring device used. Between red and black resistances between 500 Ω and 2 kΩ or completely different values can be measured. In no case a direct connection to ground should be measurable.


[1] Rotax, Reparaturhandbuch Type 123. aprilia, 1995.
[2] Thierry, “AC-CDI KTM SEM,” Transmic CDI, Jul. 03, 2016. (accessed May 21, 2020).

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  1. Good morning, I have a question: what program did you use to draw the ignition system – it looks extremely realistic
    Regards, Bogdan

  2. Good day. I am repairing a KTM and I would like to know what resistance the CDI coil should have. So the resistance is forced between the red and black wires on the stator. Alternatively, what voltage should it generate correctly. I think it should be up to 200-300V. And in my case it is only 20-30V. Thank you. Peter.

  3. Should the red or green wire give me a clean rpm signal to a 96-98 digital tacho? Red gives me a signal almost 2x what it should be. Green wire currently gives me no signal. But I may need to retry it.

  4. Hi i got on my ktm250 88year that ignition.
    Bike starts normal and work normal on low revs.
    Not want to go on higer turns of engine and is cutting
    Do you think is problem ignition?

    1. got the same problem on my ktm which shares the same parts here. after testing my impulse coil reading black to green on stator was showing just 148 ohms instead of 170 +- 2 ohms so mines over 20 ohms out. exactly the same symptoms as you are describing. hope this helps Luke

      1. Hi.. I can confirm exactly the same problem here on a 1993 300 exc that seems to also have this ignition, I may be missing something obvious but other manuals seem to suggest 165 ohms with a tolerance of 20 ohms between black and green making both ours at 148 ohms and mine 150 ohms just in spec, that said we all have the same symptoms.. I’ve seen a direct replacement upgraded stator but didn’t want to risk the cost if it wasn’t the cause of the problem.. can I ask if youve been able to fix yours as yet with a new stator or anything else..? Many thanks..

  5. Thanks. Víctor but I check the rectifier in 6000rpm the voltage go up to 14.5 15 volts. But the battery dead. Maybe it’s the battery?

  6. Hello thanks for this blog help a lot. I have aprilia af1 125 with rotax 123 and have a little problem. The engine start OK and all works OK but in 10min take a little hot the motor stop and no start the battery empty. I check the stator and coil but you write. Resistance of the secondary winding of the ignition coil between black and the connection of the ignition cable: (2.2 ± 0.4) kΩ but my coil have 7 koms. The spark plug make light but engine no start. I load battery and start again. It’s posible is a coil problem? Thanks

    1. Hey, generally the empty battery should have no effect on the spark plug. The generator and the ignition are two separated circuits. 7 kOhm is too much but this could be just a connection problem, maybe the connector is corroded. Some damaged coils I have here have no connection / infinite resistance between black and the ignition cable connection. You should check why the battery is getting drained, maybe the rectifier is damaged.

    2. take off your spark plug boot. the boot adds between 5 k ohms and 10 k ohms. sounds like you got the correct reading