Function and diagnostics on the SEM ignition on the Rotax 123

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.

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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.

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.

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Bibliography

[1] Rotax, Reparaturhandbuch Type 123. aprilia, 1995.
[2] Thierry, “AC-CDI KTM SEM,” Transmic CDI, Jul. 03, 2016. https://transmic.net/2016/07/03/ac-cdi-ktm-sem/ (accessed May 21, 2020).