Rotax Aircraft Engine Electrical System Troubleshooting.!


It might be useful before we start our troubleshooting endeavours if we consider what our overall aircraft's electrical system actually consists of and break it down a little. With the exception of the Rotax 912iS and Rotax 914 all Rotax aircraft engine electrical systems at their most basic level are comprised of three essentially separate circuits.

  • 1. The electric starter circuit.
  • 2. The ignition system circuit.
  • 3. The power supply circuit (and ancilliaries).

The electric starter circuit (if your Rotax engine has one - many Rotax 2-stroke engine installations do not) comprises of main aircraft battery, starter switch, starter solenoid, starter motor and associated cabling.

The ignition system creates a spark at the spark plug which is essential for your engine to run. All Rotax aircraft engine ignition systems are self-powering e.g. they do not rely on the main aircraft battery to run. They derive their power from an ignition system "charging" coil (or sometimes referred to, somewhat confusingly, as the "generator coil") in the magneto generator. The ignition system is only able to generate enough power to power itself once the engine is turned over to a minimum of 300RPM in the case of Rotax 2-Stroke engines and 220RPM in the case of the Rotax 912 / Rotax 914 series.

The power supply system is the generator component of the system which may be used to charge your aircraft's main battery, power your strobe system, instrumentation, radio, GPS, heated clothing, landing lights, and whatever else you wish to connect to the power supply. The power supply circuit only generates useful power once your engine is running and how many watts of power the generator is capable of delivering will vary with RPM. It's total power output will be significantly less at cruise RPM than is it at WOT (wide open throttle). The output of the generator is usually connected to the aircraft battery supply via a regulator / rectifier (sometimes with a smoothing capacitor). The battery is usually connected to the main power supply bus via an isolator switch (ON/OFF switch) and all electrical items that require power are connected via a fuse to the bus. The fuse for each circuit is extremely important as this is the circuit's overload protection. The prime purpose of the fuse is to disconnect the circuit from the bus in the event of a fault developing. This is not to protect the to protect the cable (and you) from the pssibility of an electrical fire and for this reason it is very important to include a fuse / circuit breaker in every circuit.

There is some inter-dependence between the three main circuits particularly where a starter motor is fitted e.g. the ignition system cannot brought into play if you cannot crank the engine over using the electric starter and without a power supply circuit you would not be able to use the battery indefinitely without a power supply to keep it charged, and without a well charged battery the starter motor is useless etc. Beyond that they can be considered separately from the point of view of troubleshooting.

Your aircraft's main battery can be thought of as a persistent power reservoir. "Persistant" meaning that after the enigne has stopped rotating and all output from the magneto generator has ceased the battery is there to provide enough power to bring your engine back to life (via the starter motor) and resume generating power. This is one of it's primary role's. The main battery also has an important role to play in smoothing out the power generated by the power supply circuit and acting as a load buffer for the power system. To enable the battery to perform this role of power reserve it must continually be kept topped up by a healthy power supply circuit when the engine is running and topped up by a battery conditioner or mini solar panel etc. when the aircraft is stored in it's hanger.

Before you start to troubleshoot the electrical system you need to determine whether you believe you have a problem with the electric starter, the ignition system or the power supply and focus your attention initially on that part of the system. It is quite possible for one to be working without the other and you don't want to disturb the ignition system components unnecessarily if you have a power supply problem and vice versa. If you think that more than one of these circuits has issues start tackling them one at a time in the order they are listed above.

Identifying which Electrical System you have.

Next you will need to identify which electrical system you have fitted to your engine and this in turn is most easily done by recognising which ignition system is fitted. This is because the ignition system components e.g. ignition coils or "Ducati E-boxes" or SMD's (whichever your engine has fitted) are the most readily accessible and easy to distinguish between without removing the flywheel. If you're not sure the ignition coils or "Ducati E-boxes" or SMD's whichever it is that you have is most easily found by following the spark plug cables back from the plug caps to the other end of the spark plug cable. There you will find ignition coils or "Ducati E-boxes" or SMD's.

Rotax 912 / Rotax 914

There have been two significantly different systems fitted to the 912 / 914 series of engine. The first of these, fitted to very early Rotax 912UL's, had some of it's components housed in a largish aluminium box. They will not be dealt with here as there are very few of these Rotax 912UL's still in circulation due to age and lower hour TBO. The replacement system was a Ducati DCDI based system which will hitherto be referred to as the 912 Ducati DCDI to distinguish it from the Ducati DCDI system fitted to the later generation Rotax 2-stroke aircraft engines. There have been some revisions / updates to this system over the years but not so significant that it would make a great deal of difference to the way you might go about troubleshooting a 912 ignition system.

Rotax 2-Stroke's

There were three significantly different ignition systems that Rotax fitted to its more popular 2-stroke aircraft engines. In historic order of introduction into production they are the ...

  • 1. Bosch SCP2 - Points / contact breaker based Ignition system.
  • 2. Ducati SCDI - Single circuit Capacitor Discharge Ignition system.
  • 3. Ducati DCDI - Dual circuit Capacitor Discharge Ignition system.

See table below for a guide as to which systems were commonly fitted to which popular 2-stroke Rotax aircraft engines ...

   447   462   503   532   582 
Bosch SCP2  Yes   Yes   Yes   Yes   No 
Ducati SCDI  Yes   No   No   No   No 
Ducati DCDI  No   No   Yes   No   Yes 

There are several differences between the various different igniton systems but the easiest way to identify the them at a glance and without removing the flywheel to examine the stator plate is by having a look at the ignition coils or equivalent. Please see some images below listed by ignition system to help you identify which Rotax system you have if you're not sure.

Rotax Bosch SCP2 Points based ignition system

Bosch SCP2 Points-based system

Here you can see the Bosch igniton coil mounted on a very early Rotax 447. Note there is one coil per cylinder and each coil has one spark plug cable emerging from it. If you have one of these coils fitted then you have the Bosch SCP2 Points-based system fitted.

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Rotax Bosch SCP2 Points based ignition coil

Here you can see another image of the same Bosch ingniton coil on the bench with spark plug cable plugged in the HT output.

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Rotax Aircraft Engines Ducati SCDI Ignition Coil

Ducati SCDI - Single circuit CDI

We're sorry we don't have any images at the moment. Image to be supplied in due course. The Ducati SCDI system was only fitted to the Rotax 447 of the more common Rotax 2-stokes so you can deduce that you have Ducati SCDI if your 447's ignition coil doesn't look like the Bosch ignition coil (above).

Rotax Ducati DCDI Ignition Coil mounted on a Rotax 582

Ducati DCDI - Dual circuit CDI

Here you can see a Ducati DCDI igniton coil mounted on a Rotax 582 (mod 90). Note there are two spark plug cables emerging from it. One from the bottom of the box and one from the top of the box. Each cable serves a different cylinder. If you have one of these coils fitted then you have the Ducati DCDI - Dual circuit CDI system fitted.

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Rotax 912 Ducati DCDI SMD's

912 Ducati DCDI - Dual circuit CDI

Here you can see a pair of 912 Ducati DCDI igniton modules or SMD's (Surface Mount Devices) as Rotax refers to them. Note they are mounted "piggy back" on top of the ignition coils.

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Rotax 912 Ducati DCDI SMD's

Here you can see a more familiar side view of a pair of 912 Ducati DCDI igniton modules or SMD's.

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Rotax 912 Ducati DCDI SMD's

Here you can see another view of a pair of 912 Ducati DCDI igniton modules or SMD's and coils and connectors.

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Proceeding to the correct part of the system.

Now that you have hopefully correctly identified which electric systyem your engine has fitted you can use the links below to focus on troubleshooting the right part of your aircraft engines electrical system ...

Troublehooting the electric starter circuit ...

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Troublehooting the ignition system circuit...

Read More

Troublehooting the power supply circuit ...

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