Topic: MS390 / MS290 ignition wiring  (Read 12896 times)

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Hi Guys,
I am not getting any fire, I recently had the saw all town down, I have rebuilt these saws before. I know my flywheel is correct. However I am seeing that I have a wire that is running from the side of the jug to my ignition coil. Is this correct. Could someone sketch me a lil wiring diagram? I am using two known good coils.
Thanks

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If you look at the coil there should be three wires coming from the left side under the screw. I am thinking they are green with a gold stripe. Two of these go under the screw on the cylinder. The other which is the only one that will reach goes to the blade terminal on the end of the metal contact in the carb box.

The other wire which I am thinking is black goes from the spade on the ignition coil and the round end slips in to the opening in the switch so that when you switch to off the metal contacts are closed and it shorts out the coil.

If the black wire or whichever wire you used to the blade on the coil is contacting the metal crankcase any way the ignition is shorted all the time so it will not spark.

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This is what you need

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yes! thank you.
It turns out that my coil bolt was a little loose allowing my coil to move.

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Where did you find such a great illustration?

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Where did you find such a great illustration?

I just modified this ;D

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That's a helpful diagram. Thanks for posting it.

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Too many chainsaws, not enough wood.
Stihl, Husky, Craftsman, Mac, Homelite, Poulan. Some live here, some just passing through.

Is this the same as a MS391? We came upon a problem in one last week that was not getting any spark. Several wires were not connected. One wire placement I'm not sure on.

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Many Stihl Saws-16"-60"
"Go Ask The Other Master Chief"
18-Wheeler Driver for Crete

The 391 is a completely different saw but not sure if the wiring is similar or not.

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Too many chainsaws, not enough wood.
Stihl, Husky, Craftsman, Mac, Homelite, Poulan. Some live here, some just passing through.

Is this the same as a MS391? We came upon a problem in one last week that was not getting any spark. Several wires were not connected. One wire placement I'm not sure on.

This is the basic ignition wiring of the 391. Connect them, disconnect any other wires and try it.

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Thanks for the diagram, joe! That's what I was thinking.

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Many Stihl Saws-16"-60"
"Go Ask The Other Master Chief"
18-Wheeler Driver for Crete

I was a little curious why they put the redundant green wire in the 029-039 to start with.

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If I am not mistaken that extra green wire is additional ground on the cylinder for the spark

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If I am not mistaken that extra green wire is additional ground on the cylinder for the spark

Yes, but it is redundant. If you just throw the extra short green wire in the trash it works exactly the same.

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Yes, but it is redundant. If you just throw the extra short green wire in the trash it works exactly the same.

Agreed (it works exactly the same).
I did it on the first 029 (1995) that I repaired (1995), threw out that extra green wire.
But the engineer from Stihl told me it provided some safety for the ignition system with that extra ground. Maybe better grounding for the HT current? Maybe to prevent arcing between the switch terminals. I dont know.
But I do know that if it was not essential Stihl  would have stopped having that extra wire there.
They would have saved some costs in production without that wire in more than 2 decades of 029/039/290/390. It definitely serves some purpose.

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Well, that is the circuit as opposed to a saw where the module is screwed directly to a metal crank case. So, two wires would be less resistance than one although in that length I'm not sure you could measure it without some sophisticated test equipment.

But, like you say they didn't do it for no reason.

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It is there to ensure that the coil and engine/sparkplug have the same ground reference value.
Plastic saw frames need this.

It will find the ground thru the other wire but a short direct route is the best way.

Look at 120v house wiring, ground and neutral end up at the same place in the breaker panel but you have to have both to pass code.

It may not make sense but in certain conditions that second wire is needed for safe and relyable operation.
Keep the connections tight and clean with a little lube to reduce oxidation and go make chips.

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It will find the ground thru the other wire but a short direct route is the best way.

Check out the first diagram Joe posted. The lengths are exactly the same.

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The same, if you don't consider that one of the two has a longer wire attached to it to "drain" noise, etc. away.
Grounds are tricky issues, IIRC.

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south central Wisconsin
 It may be that my sole purpose in life is simply to serve as a warning to others

The same, if you don't consider that one of the two has a longer wire attached to it to "drain" noise, etc. away.
Grounds are tricky issues, IIRC.

The thing is, the other wire does not attach or ground to anything until the switch is closed.

I was in the electrical industry for 30 years and have been a ham radio operator longer than that. I work on ham radio equipment and vintage stereo equipment, and chainsaws LOL. I can see where doubling the wire might give you a minuscule improvement in ground path, and cheaper in manufacturing to double it than making that end of the pigtail a larger wire size.

I have left that short wire out and if it makes any difference I can't tell it.

Could be a retired NASA engineer went to work for Stihl or something, I mean things can be overdesigned.

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