No spark!

Joeygentry · Jun 29, 2018

I have a Stihl fs45 trimmer and it has no spark. I replaced the spark plug, tested the coil with a multimeter and a spark tester and both of them said that it was good, and I used a multimeter to test the kill switch and it checked out. If you look at the magnet on the flywheel it’s not dirty. Please help

Russ2251 · Jun 29, 2018

What procedure are you using to check for spark?

EngineMan · Jun 29, 2018

Your post says No spark and yet you say you have tested it if a spark tester and you have a spark...!

A quick test with the multimeter to show if there is continuity between the low tension terminal (which is used to switch off the engine) and earth, if there is its bad..!

Joeygentry · Jun 29, 2018

Russ2251 said:
What procedure are you using to check for spark?

I have taken the coil off and it was clean( looked almost new) I have replaced the plug even tho the plug that was on it looked like it was brand new. I have tested the resistance of the coil to the end of the spark wire and it was like 2.7 which I heard was good. If I hook up a spark tester (with a light on it) to it it lights up like it has spark but if I ground the plug to the engine and crank it I don’t get any spark. So basically I get spark from the coil but not to the plug but it’s a brand new plug.

Joeygentry · Jun 29, 2018

EngineMan said:
Your post says No spark and yet you say you have tested it if a spark tester and you have a spark...!

A quick test with the multimeter to show if there is continuity between the low tension terminal (which is used to switch off the engine) and earth, if there is its bad..!

Can you explain to me how I do this?

Boudreaux In Eunice La. · Jun 29, 2018

There are 2 ways to check a coil..... There is a thread on here for that ........... One is the primary and then the secondary.....

One way is to put 1 lead on the plug lead and then the body of the coil....... ETC ETC..............

Here is a video link........Let us know Mon Ami ~!~!

https://youtu.be/LbL_P_2LvmU

bertsmobile1 · Jun 30, 2018

You can check the secondary windings ( plug to ground ) but you can not test the primary on most because the chip on some is in line with the primary and on others it is parallel to the coil.
The chip measures minute voltage rises then grounds the primary to allow the spark to escape.
These are very tiny voltages and circuits than can not carry any current at all.
Thus you need a chip testing meter as most standard multimeters will fry the chip.
Try testing it in the dark.
I think you will see the outside of the wire flash cause the voltage is going down the inside of the wire then back up the outside.

Joeygentry · Jun 30, 2018

I’m just getting a new coil. I’ll let you know if that fixed the problem. Just out of curiosity what is my multimeter supposed to read when I test resistance for the primary and secondary of the coil?

Boudreaux In Eunice La. · Jun 30, 2018

That info is in the video link that I posted....... There are other videos on you tube also..... On this forum there is info on that also if I remember right ...........

bertsmobile1 · Jun 30, 2018

There are no figures for primary resistance due to the Hall Effect Timing Chip Embedded in the coil.
As for secondary resistance Stihl do not publish it.
The magnetos are either a pass or fail test.
The actual resistance is determined by the number of turns of wire and will be different for each & every coil
Rule of thumb is 1 to 20 Ohms.
Accuracy of most multimeters that cost less than $ 200 is +/- 10 ohms.

Multimeters work by sending a voltage down the wires and measuring what comes back compared to what went in so it is a Voltage reading.
In fact just about every function is a modified voltage reading.
The signal voltage sent down the primary wire on cheaper meter is enough to fry many of the chips just the same as hooking up a 12 V battery.

Remember point ?
Well the chip replaces the points and just like points, has a finite life and is easily damaged, in fact they are lot easier to damage than points.

No spark!

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