The ignition system on an older Makita 520i gave out on me in the middle of a cut. I never saw a happy saw go so quiet so fast as that. Usually, it just sets right up, and starts to aringing and asinging, when I put it in some wood ...)
A quick check showed no spark, so I changed plugs, to no effect. Then, I pulled an ignition coil (with newer leadwire and with springclip and rubber cap attached) off a spare 540 and mounted it onto the 520 ... it started right up. Change of plugs had no effect.
So far, it seems simple ... I need a replacement ignition coil ($70); but, when I compared the two coils with an ohmmeter, to check if there was a knowable short on the winding side, I don't get a significant difference between the two ... the windings of the newer good coil measure 8.23K ohm ... on the older dead coil, 8.75K ohm. Judging from internal resistance, the older coil, a Ducati brand, seems to have more "wire" in it, than the newer Makita brand. According to the ohmmeter, neither has a short in it, and the winding resistances seem easily comparable.
(Also, neither has any damage from friction heat, such as from screws coming loose and the coil grinding on the flywheel & magnets ... and I redid the connections to the leadwire. The old leadwire also had some resistance, but I forgot to write it down. Also, interesting sideline, I measure internal resistance of a new NGK-BPMR7A spark plug at 9.74K ohm (tip to tip), same plug but slightly used at 9.12K ohm, and well used, but still running great with no evidence of detonation or overheating at 7.58Kohm -- seems as plugs get older, their internal resistance drops.)
Any chance it might be the copper in old leadwire has "hardened up" electronically, instead of the coil going bad? If it's definitely the coil, any idea of what is happening, since it seems neither is shorted out.
A quick check showed no spark, so I changed plugs, to no effect. Then, I pulled an ignition coil (with newer leadwire and with springclip and rubber cap attached) off a spare 540 and mounted it onto the 520 ... it started right up. Change of plugs had no effect.
So far, it seems simple ... I need a replacement ignition coil ($70); but, when I compared the two coils with an ohmmeter, to check if there was a knowable short on the winding side, I don't get a significant difference between the two ... the windings of the newer good coil measure 8.23K ohm ... on the older dead coil, 8.75K ohm. Judging from internal resistance, the older coil, a Ducati brand, seems to have more "wire" in it, than the newer Makita brand. According to the ohmmeter, neither has a short in it, and the winding resistances seem easily comparable.
(Also, neither has any damage from friction heat, such as from screws coming loose and the coil grinding on the flywheel & magnets ... and I redid the connections to the leadwire. The old leadwire also had some resistance, but I forgot to write it down. Also, interesting sideline, I measure internal resistance of a new NGK-BPMR7A spark plug at 9.74K ohm (tip to tip), same plug but slightly used at 9.12K ohm, and well used, but still running great with no evidence of detonation or overheating at 7.58Kohm -- seems as plugs get older, their internal resistance drops.)
Any chance it might be the copper in old leadwire has "hardened up" electronically, instead of the coil going bad? If it's definitely the coil, any idea of what is happening, since it seems neither is shorted out.
