Before you start
Electrics, CDI and ECU
- Name the ignition before you unplug it.
- Check the battery and the earths before a black box.
- There is no immobiliser bypass on this page.
How many steps: 7
Guide 07 · All generations
Electrics, CDI and ECU
SEM on the 123, Nippondenso on the 122, and the checks you can do without an invented wiring diagram.
Step 1 of 7
Name the ignition before you unplug anything.
A 123 uses SEM. A 122 uses a Nippondenso CDI. Do not fit a SEM stator to a 122. Resistances are on the ignition specs page. CDI wire colours are not published.
Step 2 of 7
Measure battery voltage, key off and key on.
You'll need: A multimeter.
The brief gives no minimum. A weak battery can look like a CDI fault. The pass voltage is To verify.
Step 3 of 7
Clean the engine earth you can see.
A dirty earth makes every sensor lie. Find the other earths in the manual. Do not add random wires.
Step 4 of 7
Check latches and green corrosion in the plugs.
Step 5 of 7
Check for a spark with the plug earthed.
You'll need: Insulated pliers, and a plug you can spare.
A spark in air is not spark under compression.
Hold the cap with insulated pliers. It does prove the coil is not open-circuit. Coil-cap resistance is To verify.
Step 6 of 7
On an RSV-style bike, include the immobiliser.
There is no immobiliser bypass on this page.
Also check the side stand, the kill switch and the CDI. Coded keys are a dealer or specialist job when the manual's procedure is exhausted.
Step 7 of 7
Repair a wire only once you know what it is.
Do not twist and tape a join.
Solder and heat-shrink. Do not add scotch-locks where the loom flexes. A colour from memory is To verify. There is no wiring diagram in this edition.
Finished!
This guide is done. Your place stays on this device.
Tools
- A multimeter that can read volts, ohms and continuity
- Back-probes or pins that do not open the connector latches
- Dielectric grease, after the joint is proved clean
- A wiring diagram for this VIN. This site does not draw one.
- The immobiliser notes for a PY, if that is what you have. No bypass is described here.
Parts
- SEM stator, 123 only, and already scarce
- Nippondenso CDI on the 122
- Coil. The ‘coil damp’ in the brief is a topic, not a resistor value.
- Immobiliser parts only through a dealer or the manual. No override is listed.
Slots for these parts, with empty OEM numbers and empty prices, are in theparts directory.
| Item | Figure | Status |
|---|---|---|
| 123 ignition | SEM, including a stator the brief says is hard to find. | Sourced Research brief |
| 122 ignition | Nippondenso CDI. An environmental retard around 5–6,000 rpm is described. Removing it varies by unit. | Sourced Research brief. The bridge itself is To verify per CDI. |
| HT coil, Type 122 | Primary 0.11–0.21 Ω. Secondary 4.3–8.1 kΩ. Source coil 0.1–1.0 Ω. Pick-up 190–300 Ω. Timing is not published. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| HT coil, Type 123 | Primary 0.5 Ω. Secondary 1.9–2.4 kΩ. Source coil 1.58–1.94 kΩ. Pick-up 170 Ω. Static timing 19° / 1.84 mm BTDC. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Charging | Regulated output 13.5–15.0 V at 6,000 rpm. Type 122 unregulated output 53 V AC at 6,000 rpm. Stator 0.1–1.0 Ω on a 122 and 0.2–0.4 Ω on a 123. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| Battery, fuses, bulbs | 12 V, 9 Ah. Specific gravity 1.26 at 20°C. Charge at 0.9 A for 10 hours. Ignition fuse 7.5 A. Bulb wattages are on the electrics specs page. | Sourced Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
| CDI wire colours | Not published. Do not invent a bridge. | Not published Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) |
There are two ignitions in this record, and they are not interchangeable black boxes. This page names them, says what owners already report, and stops where a pinout would have to be invented. A redrawn diagram is future work. It is not in this edition.
SEM and Nippondenso
The Rotax 123 uses SEM ignition. The stator is the part the brief says is hard to find. If you are buying a 123 project, treat the stator as a scarce part. No price is recorded here.
The Rotax 122 uses a Nippondenso CDI through the bee-tail and the RSV-style bikes. The brief describes an “environmental” retard around 5–6,000 rpm. The derestriction steps say that retard can be removed, and they also say the detail varies by unit, so verify it per model. This guide does not publish a wire colour or a solder bridge. A wrong bridge on a CDI is a dead ignition, and the colours are To verify on the unit in your hand against a manual for that part number.
What you can check without a diagram
These checks are general workshop practice. They do not depend on a colour code.
- Battery voltage, key off and key on. The brief does not give a minimum. A battery that falls over when the headlight is on will pretend to be a CDI fault. The pass voltage is To verify.
- Earths. A dirty engine earth makes every sensor lie. Clean the earth you can see, and find the others from the manual rather than adding random wires.
- Connector latches and green corrosion, especially under the tank and at the tail, where water sits.
- Spark, with the plug body earthed to the head, using a plug you are prepared to replace. Hold the cap with insulated pliers. A visible spark is not proof of spark under compression. It does prove the coil is not open-circuit.
- Continuity of the one wire you have identified from the manual, not of “the green one” from memory.
The brief lists “coil damp” among the topics for this guide. That is likely the suppression on the coil or the cap. The resistance is To verify. Do not replace a cap with a random resistor plug cap and expect the same CDI to be happy.
Immobiliser
On the RSV-style generation, owners report trouble with the factory immobiliser and alarm. The brief records the complaint and not the cure. There is no bypass on this page. If a PY cranks and does not start, or does not crank, include the immobiliser in the list with the side stand, the kill switch and the CDI. Transponder keys and coded units are a dealer or a specialist job when the manual’s procedure is exhausted.
Loom repairs
Solder and heat-shrink a broken wire once you know what it is. Do not twist and tape it. Do not add scotch-locks in the steering head, where the loom flexes. If you do not know what the wire is, leave it and get the diagram. A wiring diagram redrawn for this site is one of the later downloads in the plan. Until it exists, the official diagram is the source, and any colour called out from memory is To verify.
Common mistakes
- Fitting a SEM stator to a 122 because both are RS125 ignitions.
- Bridging a CDI from a forum drawing. The brief says the detail varies by unit.
- Cutting the immobiliser out of a 2006 loom.
- Probing the back of a connector until the seal is split, then wondering why it fails in the wet.
Notes from readers
No reviewed note is attached to this page yet.Be the first to send one.
Spot a mistake? Tell usIt is reviewed before anything is published.
Sources
- Differences between Rotax 122 and 123 (opens in a new tab) — Hartrusion
- Modellübersicht RS 125 2T (opens in a new tab) — Hartrusion
- Derestriction guide (opens in a new tab) — The RS 125 blog
- Haynes manual 4298 (Aprilia RS50 and RS125 1993-2006) (opens in a new tab) — Haynes
Research brief dated 24 October 2026. Where this page says “the brief”, that is the sourced master plan behind the first edition.