Signs of bad coils?

[erkishhorde]

Out of random curiosity, what are the signs that your coils are going bad? On my '95 m900 the coils are those two big black cylinders behind the battery, right?

[Ddan]

Yes on description and location.  I would think you'd have a weak spark or none at all, misfires, backfires, maybe worse at higher revs.

[H-2 CHARLIE]

  All coils i have had go bad and have never intermittaly  worked or even from  over heated and cooled never worked .... they go bad for me all the way .

[Howie]

Is there a problem that prompted this question?

  All coils i have had go bad and have never intermittaly  worked or even from  over heated and cooled never worked .... they go bad for me all the way .

  All coils i have had go bad and have never intermittaly  worked or even from  over heated and cooled never worked .... they go bad for me all the way .

Intermittent coil problems are common. 

[erkishhorde]

After my last post about dropping a cylinder, I replaced the oily plugs and topped off my oil. All seemed fine for 150mi I had no significantly increased oil consumption. Then on Thursday night I went out to a bike night and had some power loss on the freeway about 10mi before I got there (had traveled 40mi already). Slowing down with traffic allowed the bike to recover and I made it there safely. When leaving, the bike warmed up fine but didn't like hitting the freeway. It started getting very loud like a dirt bike and lost power. It quickly lost all power and died. Tried to start up again while on the side of the road and it sounded like I was pulling the cord on my lawn mower and it wouldn't start. Towed it home and pulled the plugs. They look fine. Dad suggested coils might be going bad. He mentioned that one of his cars would lose power at high rpms when the coils went bad and when I think about it, I think the bike has been kind of weak on the freeway when I go 80 but when I keep it casual around 70 it seems to be alright.

[Howie]

More likely a bad pick up or two.  Or possibly fuel starvation.  Actually, I think fuel starvation (or tank venting) is more likely since it is unlikely both pick ups or bothy coils are crapping out at the same time.

Anyway, coils and pick ups are too expensive to check as a guess.  Carry a multimeter with you.  When the bike looses power check the pick ups by disconnecting and testing with the ohmmeter.  You should read 95+5 ohms.  Coil Primary and secondary resistance can also be can also be measured.  I don't have the spec available at the present, so hopefully someone else does.  Both the pick ups and coils can be bench tested, but since it is a bit of work to remove the pick ups...  Anyway, the trick to bench testing is to simulate the condition when the problem happens, which is heat and vibration.  Oh, bench testing is a lot safer than riding around, stalling and trouble shooting on the side of the road.