Ignition Coil Failures

[KE4AVB]

I know we have discuss this before but I had my first case of this myself where ignition switch causes magneto coil failures.

I been working on a Giant Vac Giant Mow where it has a completely reworked electrical harness. Previous wiring harness had a complete meltdown. I personally did not do the electrical rewire.

The mower was brought into my shop after it was pulled from another shop. I manage to get it running after carb repairs. It just came back in after several weeks where it would not start. Looked like a simple coil failure on an opposed engine. Replaced the coil, started the engine, and called the customer for pickup. Well the customer came by Wednesday morning started the mower and it ran well like crap. I went over to see what over and it just keep getting worse, started shooting out flames from the exhaust.

I apologize for problem and got the customer leave it me to find what was going on as I had another customer on the yard at the time. Well when I got back to it I found the ignition was out again. Double checked by unplugging the kill terminal. I set out looking for a possible cause remembering that a voltage surge via the kill terminal can cause this failure on new coils.

I did find a voltage problem but not the usual monetary application of 12vdc switching contacts. I actually had a 1.5 vdc voltage on the kill wire when the ignition switch was on and normal ground in off position. I made sure there no voltage was on the line with the ignition switch unplugged. This was a high resistance voltage leakage as I was able to pull it to ground; just enough current to take out the coil's electronics. Strange that a new switch would cause this but I suspect rain water had somehow gotten into the switch.

Just a reminder to me at least to check the kill wire for voltage when we have a magneto coil failure.

I still have a charging in my opinion but I unable to remove the flywheel due previous removal where and hammer and chisel was used then an impact to tighten the nut. Normally I can get most stubborn flywheels off but not this one. The customer will just need to recharge the battery as needed.

[hanz63]

You mentioned opposed engine. Did you use the OEM coil perchance? The reason I asked is last year I had 2 new Briggs OP twin coils fail. I figured that there was a voltage leak from the switch or ? as well, but could never find a blip on the screen. Of course, these things are on ancient machines with ancient harnesses. I think that I have one OEM left in stock and when that goes, I'll try try the dreaded aftermarket coils on these. I think the new style Chineseum Briggs coils have absolutely no tolerance for a small voltage surge where the old ones did. At least my opinion. Briggs won't fess up about it.

[wristpin]

Perhaps coils can be protected from kill wire voltages with a diode ?

[KE4AVB]

Should work. In theory it will work. I give it a try this weekend on a donor unit by applying a full 12 battery to the circuit them switch to kill mode while the engine is running.

[KE4AVB]

Reversed bias a 1N4004. Coil kill terminal to ground resistance was one ohm on the test coil. No voltage present at the coil. Well worth it put in a $0.14 diode, a couple crimp connector and section heat shrink. I price out the diode on the higher volume that the factory would be using and would cost them less than 0.02 USD each diode to add them per coil.

But then they won't be selling as many replacement coils. It all comes down to making more money.

[wristpin]

Thanks for testing out the theory. When I was in business I once sold two coils to a guy for a chipper and it was only when he came back for a third,that I refused to sell him until we had checked the machine over, that a test on the starter switch showed a voltage at the kill terminal after 20+ turns of the key.

[Walt 2002]

I just had a coil on a Kohler CV14s fail. After replacing I found voltage on kill wire. I removed cooling shroud, unplugged the kill wire at the coil terminal and still had 2.5V AC on the coil terminal?

Walt Conner

[bgsengine]

I just had a coil on a Kohler CV14s fail. After replacing I found voltage on kill wire. I removed cooling shroud, unplugged the kill wire at the coil terminal and still had 2.5V AC on the coil terminal?

Walt Conner

AC Volts? Were magnets lined up with the coil?

[KE4AVB]

If this was with the engine running it should be there. Here is basic diagram how Magnatrons are wired. While running there will an AC voltage across the pickup (trigger) coil. But should not be any voltage across it when the engine is not running. Now in a very rare case it could with the right component failure go into self oscillation. The kill terminal connects to the darlington emmitter effectively shorting out the primary winding and the pickup coil.