912 CDI (SMD) Failures

Hi Angus

Sorry you have problems with your used engine.  First rebuild suggests it was not an overhaul.  In an overhaul the complete ignition system is tested on a bench tester if it is done to Rotax standards.  To my knowledge Lockwood Aviation in Florida has one and also Rotech Motor in Canada have one.  I would recommend you contact one of them and have it completely checked over.  Outside of there in Europe I know that Franz Germany has one also.  Verify that they can do a bench test if you are dealing with anyone else.  

Take your engine serial number and submit it to the forum so they can tell you the year it was made.  In addition look for the part numbers on the modules and the individual serial numbers of the modules that are on the side of each of them.  This will give a clue if they were changed at some point.  Rotax can supply a full build list of what serial numbers were used originally if you make a request, again submit to the forum in a new thread would be best.  That way you can find out what was used in the "rebuild" of the engine or if someone has changed parts over the years.  

There are literally thousands of Rotax 9 series in service within the USA.  While failures do occur I would not call it common.  Other parts of the system may be the issue and you might consider a full check of the coils, high tension wires, spark plug connectors and even spark plugs before you invest in expensive SMD boxes.  The stator would have no bearing on an SMD failing.  

Cheers

Here in the UK CDI module (commonly referred to as SMD modules) do seem to fail with some regularity. I have had 2 failed modules myself and I know of many other owners who have had units fail. There are many forum threads about the various failure modes but they broadly fall into one of 3 camps: fail to start the engine, fail to run the engine, fail to switch off. I have personally suffered from the first and last.

Your options are: replace with a Rotax part, have yours repaired by someone like Carmo Electronics or replace them with a pair of Ignitech modules. Most owners over here are now opting for the latter. They are mounted away from the engine where it is cooler and they cost considerably less than the Rotax parts. They now have approval from the various certifying bodies in the UK and so it is a simple exercise to make this change.

I have a relatively detailed knowledge of the inner workings of the Rotax ignition modules and I doubt that your problems with the damaged coil have caused a problem with the module.

Having looked inside a number of failed units, the root cause of the module failures is a mixture of poor design, poor manufacture and a harsh environment.

Hi Kevin

"Here in the UK CDI module (commonly referred to as SMD modules) do seem to fail with some regularity."

I hope this is limited to the UK (a Pommy affliction). I have about 600 hrs on a year 2000, Rotax 912, ULS, (total hrs 920) without so much as a hiccup. This engine would be pre Soft Start - so the old system.

I have recently acquired a brand new, almost complete, kit aircraft, fitted with the latest Rotax, 912 ULS (0 hrs) - despite never having been started, is now out of warranty. So a tad disconcerted by the above comment.

"Having looked inside a number of failed units, the root cause of the module failures is a mixture of poor design, poor manufacture and a harsh environment."

I am both intrigued & disconcerted by the above - I would guess "harsh environment." is the heat from the engine & vibration? but what of the "mixture of poor design, poor manufacture" ? Please expand.

I know that they have had enough failures in Australia that Mark Kyle has started manufacturing an upgraded version of the module which he sells down under.

The design has some under-rated transistors  that seem to account for some failures and I have seen some failed solder joints. There is insufficient heat sinking on some of the components and this is made worse by the location of the modules on top of the engine. The main capacitor is also a weakness and they sometimes fail.

Our new club aircraft has already suffered ignition problems and at the next service they are planning to replace the modules with the Ignitech ones.

Ironically some of the older modules seem more reliable. 

It would be interesting to know the failure rate, I might see if I can conduct a small survey on the Eurostar owners group. 

About a year ago I repaired two 966-727 soft start modules (2010 date code). The complaint was “failure to start when cold.” Both had been sent to Lockwood for evaluation, one was returned tagged “Dead,” the other “Intermittent.”

First some background on the module operation. The ignition module is designed to NOT turn on below 150 RPM and MUST turn on above 220 RPM. Without getting into a lot of technical detail, the module determines RPM by looking at the stator power waveform. In a given cycle, the pulse is short and wide at low RPMs and tall and narrow at high RPMs. The height of the pulse, or voltage is used to disable or enable the module. Bench testing showed the “Dead” module needed a voltage equivalent to over 2000 RPM before it would turn on. The module tagged Intermittent turned on at voltage estimated at just above 220 RPM, maybe a better description for this one is “Marginal, out of Spec.”

The circuit monitors the “RPM voltage” and fires a small gate sensitive SCR to turn on the module, then allowing the main capacitor to charge. The SCRs gate current is measured in micro Amps (uA), that’s millionths of an amp, it’s very sensitive. The SCR was damaged during removal and replaced. Later examination found the SCR to be within spec. Based on a small sampling of only two units, my current theory of this failure mode is; “Internal Parasitic Losses to the SCR gate that cleared with the removal of the SCR and the removal of the potting compound local to the SCR.” Both modules functioned within spec after re-potting.

The actual failure rate is higher than what’s known. For the cold start/low RPM failure mode you need BOTH modules to fail. As long as you have one good module you will start just fine, completely unaware the other module failed. Because it’s a low RPM failure issue, the modules will operate just fine once started.

Now if it’s a 5% failure rate with this failure mode, then the probability of both failing on the same plane is one in 400 (a conservative estimate based on reports.)

Here’s another way to look at it, 400 aircraft have 800 modules, 40 of these modules have failed,  one person knows it, 38 people don’t.