Stihl MS390 replacement coil wire count issue.

[trouts2]

My Stihl MS390 had intermittent spark problems. Wiggling the coil wires sometimes would get it to run. If I remember right, disconnecting the ground wire got it to run. When connected spark would return at times after moving the wires.

I could not figure out what was getting shorted so got a replacement coil which comes with wires. The replacement has two wires versus three. I think a short ground wire was eliminated. I googled around the net and for pictures of the Stihl 0000 400 1300 replacement coils. They all had two wires not three. I could not find a picture of an OEM version. Given many makers are making two wire versions that work I think the reason is the elimination of a redundant ground wire. I'm not sure about that though so asking if anyone has come across this two/three wire thing.

The MS390 coil also goes in the following models but I don't know if they are wired the same way between models: 026, MS260, 029, 039, MS290, MS310, MS390, 036, MS360, 038, MS380, 044, MS440

[ZeroJunk]

There is drilled and tapped hole in the cylinder and two wires are under the screw that holds the coil, one to the cylinder and the other to the switch. It is not exactly redundant because the non metallic case is not a sufficient ground to short out the coil.
To turn off a saw all you are doing is shorting the coil to ground. That's it.
If you have an intermittent problem sometimes it can be a bare wire against metal somewhere. Or, just a bad coil.

[trouts2]

ZeroJunk,

  Looked around for what I think was an intermittent short but could not find it.  The issue had to be that so bought a coil with wires to hopefully cure it which I think will.  It will be a while before I can install the coil as I'm at a remote location and not at home with access to my tools.  I wanted to clear up the 2/3 wire thing before I start the repair.  

 I found this post and you, ZeroJunk, are in it.  I think you describe the issue there with the gold green wire which I think goes under the flywheel probably to that screw you mentioned in the cylinder.   I can't remember exactly as it was a few months ago when the coil was removed.   Another person was doing it and me watching.

  If I have this right the gold/green wire was the short one and ran from the coil to the cylinder with another wire.  The short gold/green wire is the missing wire in new replacement coils.   I can use the original wire but that one might be faulty in some way causing the short.  I could make up another when I get home so not a big deal.  I'd just like to have some confirmation on the 2/3 wire issue.  Given that every coil I saw on the net was two long wires and missing the short one I thought there would be other guys posting about that but only found the post I included.

 So the two grounds are coil base to cylinder via the short gold/green wire and another long wire probably going up to the grounding position on the on off run choke lever by the trigger throttle.
http://forestryforum.com/board/index.php?topic=84365.0,
73, N1RBM....  My rigs used to be all AM Johnson Rangers, Viking Valiants, Collins 32V3, 75A4, T368 & etc.  Loved'em all. 

[ZeroJunk]

Yeah, I had forgotten about that.  I just throw the extra wire away, makes no sense to me. And, they cut off just fine.


The left over wire really can't be faulty unless it is broken in two. It is always grounded so if the insulation is bad it is meaningless.

The one that goes to the spade on the coil is the one that must be insulated and only grounded to turn the saw off.


Was just on 75 meters and turned around to the computer to make this reply.

FT1000MP MarkV  and a home brew amplifier, dipole up about 80 feet.

[Mad Professor]

If not the coil could be wires to kill switch are grounding (insulation or switch, to each other if both bare), or bad ground to cylinder (connection or wire).

But if disconnecting switch ground fixes things it must be the former

Did you look over insulation on the wires and test continuity with an ohm meter? Your coil might be O.K.

[Mad Professor]

This might help

[trouts2]

Mad Professor:
  I think the coil is fine and the intermittent issue with the wires.  As I mentioned I can get it to run for a while after wiggling the wires.   The spark tester shows a nice bright spark. 
   Yes, when I disconnect ground and it runs fine.   The ohm readings were fine.  My guess was there the issue was an insulation problem that let the charge ground sometimes.   I debugged with another guy and left it with him.   I later gave him the replacement coil but the missing wire tripped him up so I have the saw now. 

   I just wanted to clear up the 2/3 wire issue with the replacement coil.  I got the replacement to get the harness wires.    I'll probably install the coil and new wires.  I check the short ground wire from the original harness.  If that seems good I'll use it.  If not then make up another.  I'm pretty sure the issue is with the 2 long wires.  I think the grounding wire going to the kill switch is the problem. 

   Thanks for the parts link.  It shows the original 3 wire harness.  All the non-OEM replacement coils for the MS390 I've seen on the net have two wires.  I have not been able to locate pictures of that coil on the net.  Maybe the Stihl OEM version still comes with three wires. 

ZeroJunk:

   I'm feeling more confident about the reinstall working.  It looks like I could go with either 2 or 3 wires and the thing will work. 

   I had about 15 transmitters, 20 receivers and way too many tubes and parts.  All vintage tube gear.  Four very nice 75A4's.    I sold it off and got my money back plus for the many hears of use I got out of all the gear.  Come to think of it I did have an ICOM-730 for mobile and had blast with it.  I rigged up a big Texas Bug Catcher with a 16 inch capacity hat on a VW Golf all held up with ¾ PVC pipes.   Had a 400 watt amp and a 50 pounds of grounding straps all over the car.  It was tuned for 3885 AM.

   I was in Massachusetts with 80 meter dipole up 50 feet.  It was tough to get straight south with it but got Florida a few times.  Can't remember any other coast contact between FLA and VA though.  I did well out to the south west to Texas and few times to southern Calf.   Pretty much all on 3885 and occasionally 160 on the Old Buzzards Net.   Great stuff.   Take care.

[Mad Professor]

Look over the long wires for bad insulation.  If you find a bad spot,  use liquid electrical tape, then shrink wrap the repair.

Make sure the switch is not the problem also.

[ZeroJunk]

Mad Professor:
 I think the coil is fine and the intermittent issue with the wires.  As I mentioned I can get it to run for a while after wiggling the wires.   The spark tester shows a nice bright spark.  
  Yes, when I disconnect ground and it runs fine.   The ohm readings were fine.  My guess was there the issue was an insulation problem that let the charge ground sometimes.   I debugged with another guy and left it with him.   I later gave him the replacement coil but the missing wire tripped him up so I have the saw now.  

  I just wanted to clear up the 2/3 wire issue with the replacement coil.  I got the replacement to get the harness wires.    I'll probably install the coil and new wires.  I check the short ground wire from the original harness.  If that seems good I'll use it.  If not then make up another.  I'm pretty sure the issue is with the 2 long wires.  I think the grounding wire going to the kill switch is the problem.  

  Thanks for the parts link.  It shows the original 3 wire harness.  All the non-OEM replacement coils for the MS390 I've seen on the net have two wires.  I have not been able to locate pictures of that coil on the net.  Maybe the Stihl OEM version still comes with three wires.  

ZeroJunk:

  I'm feeling more confident about the reinstall working.  It looks like I could go with either 2 or 3 wires and the thing will work.  

  I had about 15 transmitters, 20 receivers and way too many tubes and parts.  All vintage tube gear.  Four very nice 75A4's.    I sold it off and got my money back plus for the many hears of use I got out of all the gear.  Come to think of it I did have an ICOM-730 for mobile and had blast with it.  I rigged up a big Texas Bug Catcher with a 16 inch capacity hat on a VW Golf all held up with ¾ PVC pipes.   Had a 400 watt amp and a 50 pounds of grounding straps all over the car.  It was tuned for 3885 AM.

  I was in Massachusetts with 80 meter dipole up 50 feet.  It was tough to get straight south with it but got Florida a few times.  Can't remember any other coast contact between FLA and VA though.  I did well out to the south west to Texas and few times to southern Calf.   Pretty much all on 3885 and occasionally 160 on the Old Buzzards Net.   Great stuff.   Take care.


I remember back in the 80's I could have bought a KWS1 and 74A4  with a matching speaker for $400. Next thing you know the Japanese got infatuated with old Collins stuff and I saw a KWM2A round emblem go for $5000.   Just Wow.

I remember back in the 80's I could have bought a KWS1 and 74A4  with a matching speaker for $400. Next thing you know the Japanese got infatuated with old Collins stuff and I saw a KWM2A round emblem go for $5000.   Just Wow.