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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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 Posted: Thu Jan 06, 2011 6:16 am Post subject: trouble shooting an ignition issue |
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Hi All,
This was posted by a very knowledgeable friend of mine on another forum and is good advise for those wanting to trouble shoot an ignition issue.
First of all I just have to say that I love ignition problems, why? Because most of the time they can be quickly diagnosed with some very basic systematic troubleshooting, I have to admit that I have spent some time over the years pulling my hair over ignition issues before I developed a full proof method for diagnosing faults.
General things to consider...
1) If one cylinder was not firing at all then when you do your mag drop at 4000rpm you can expect to see a 400-600rpm drop
2) If two cylinders were not firing at all then when you do your drop at 4000rpm you can expect a 900-1100rpm drop
Step 1) If you are getting a high mag drop (over 200rpm) you can pretty much rule out it being an issue with your ignition switch ground, in most cases ignition switch issues are never severe enough to cause more than a slight irratic running, 98% of the time ignition switch issues lead to a complete failure of one whole system. To clarify it's going to be one or the other.
Step 2) Try switching the black boxes, why? Because it takes all of about 15 min and it will almost immediately rule out the single most expensive part in the system.
Step 3) Return the black boxes to there original locations but before doing so, grab a small ice pick and disconnect the red wires coming into the rear of the engine. Then switch these wires a perform a test run. Ask your self does the problem switch sides? These wires are what sends the AC voltage developed by the stator on the rear of the engine to the black boxes to be directed to the spark plugs. If the problem switches sides then it is an issue with the stator ( check wire resistance to ground 3.2-4.5ohms) if the problem doesn't switch sides then the issue is "down stream " of the stator and is either in the black boxes ( which you have already ruled out) or in the secondary coils and plugs.
Step 4) After determining that the issue lies in the secondary coils then the next logical step is to check the resistance of both sides of the coils the primary side being fed by the black boxes ( 0.1-0.4ohms) then test the secondary.
Step 5) check your triggers, these are located on the back of the engine and tell the black boxes when to fire the spark, there are 4 of them that trigger a respective secondary coil, the resistance should be 220-250 ohms for the new style, these a rarely the problem unless there has been physical damage to them, they are also only replaceable as a set so as much as I would love to sell you a set please be sure that they are not working before ordering... Were trying to be mechanics not parts changers right?
Note: the secondary coil is suppose to be between 6.1-6.7 kohms, to get this reading rather than removing both spark plug caps, since this may hide your problem I would leave both caps connected and test from one cap to the other (1&2T...1&2B ...3&4T...3&4B) each cap is rated at 5kohms so your readings should be between 16.1-16.7k ohms the 0.1 ohm per foot of the spark plug cable should not be a factor.
After all these test have been performed you should of localized the problem to a specific component.
Common issues more or less in order..
1) loose spark plug caps!
2) Bad grounds ( look for corrosion on central grounding point on #1/3 intake manifold)
3) Bad charging coil, usually as a result of a loose cap or running the engine with out spark plus caps installed with mags on
4) ignition switch ( like I said usually only the case with either very slight mag drops or complete failure
5) charging wires there is a SB on older engines but in the case of most CTs it would most likely be related to physical damage ( shorted or nicked wire)
6) Black boxes ( rarely ever fail but when they do it's almost always related to heat under the cowling
7) Triggers, usually as a result of physical damage, be very suspicious of this if your engine has recently been removed
 Timing.....oh wait!? It never changes ( this can pretty much be ruled out every time 
There you go...
A&P
FCCL
Sport Pilot
ROTAX MECHANIC -912 OVERHAUL RATED
Technical Director & Rotax Warranty Rep. for California Power Systems
Nor-Cal Emergency Field Services, LLC -Owner
(209)481-5881 Cell
(510)247-9653 CPS Shop Mon-Fri
Nor_Calefs(at)Yahoo.com
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
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ricklach
Joined: 20 Jun 2006
Posts: 127
Location: Kernville, Calif.
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