Stihl FS55 Trimmer Ignition Problem - Confused!

[CaptainBarnacle]

Hi All,

I have an Stihl FS55-R trimmer/brushcutter which has been out of action since last year. After years of reliable service I took it out last September and I just couldn't get the thing to fire up at all. I finally got around to taking a good look at it today and stripped it down to try and fix it.

After reading a bit about what the problem could be I went through these steps:

Disconnected the switch to rule out faulty switch - no difference.
Check the gap between the flywheel and ignition coil - it looked OK but I set it again using a business card as a spacer.
Remove the plug and check for a spark on the plug whilst grounding the body of the plug and cranking the starter - no spark.
Replaced the plug with one that I know to work from my Stihl chainsaw - still no spark.
Removed the plug completely and stuck a steel rod in the spark plug boot. Made sure there was a gap between the rod and the engine body and cranked - no spark.
Checked continuity between the plug boot and the two terminals on the ignition module - no continuity between any two of those three points.

At this point I was all set to go and buy a new ignition module but I though I would try putting my Fluke multimeter on the plug boot to see if there was any voltage when I cranked. I set it to record the min and max voltages. When I did this the Fluke showed the max voltage as OL (over limit, overload?? - more than the 1000V DC that it will measure anyway). So now I am not so sure that the ignition module is broken.

So my questions are:
1. Typically what sort of voltages does the coil supply to the spark plug?
2. Am I doing something wrong in my testing?
3. Are there any other tests that I can do to help determine where the problem is?
4. Should I just get a replacement ignition module?

Thanks guys,
Paul.

 

[FergusonTO35]

An ignition needs a minimum of 8000 volts or so to jump the gap. I would not use you nice Fluke meter for spark testing. It won't work for that purpose and will get fried. Instead get a spark test tool for small engines that looks like a spark plug and clamps to ground. Sounds to me like you have a bad ignition as long as your flywheel magnets are ok.

 

[CaptainBarnacle]

Wow! 8000V!! I had no idea. OK, my shiny new Fluke is not going to be used for that job again

I'll go and pick up a spark test tool and give that a try.

Thanks,
Paul.

 

[CaptainBarnacle]

OK, I got a spark tester today and there doesn't appear to be any sign of a light when I crank the engine. I guess I am looking at a new ignition coil.
Thanks,
Paul.

 

[Fred482]

I went through a similar problem & solution exercise last year with my (fairly new) FS250. Same deal, wouldn't start, no spark, checked everything, finally replaced the module out of desperation. After I got it running, just for fun, I checked spark using my GM STI spark tester, it's a factory test tool designed for vehicles, not small engines. The small engine ignition didn't have enough power to "jump the gap" on the tester.

I checked spark with a new plug, couldn't see the spark very well, turned off the lights in the shop office and checked it "in the dark". Finally, I could see the faint blue spark! This is a bit of a mystery, it's just not a normal hot blue flash like most ignition systems generate. I have ohm checked all components, new module, tool starts easily, runs great, full power, doesn't misfire, everything is fine, except in my mind..:msp_w00t:..I'm still looking for a hotter spark!