Briggs won't shut down

[Dale_W]

128T05-5268-B1
Mounted on a craftsman mower. When u let go of the safety bail the engine still runs. I found the tiny kill switch. Even when I ground the black wire to the block manually it will not kill the engine. What gives?

Thanks.

[KE4AVB]

Bad coil...Or the wire not attached to the coil's kill terminal. I had several coils like this over the years. Both 2 and 4 cycles.

[Dale_W]

Thank you. The black wire was secure to the terminal. So it must be time for a coil. Thank you for the speedy reply!

[Walt 2002]

Not so fast, use a jumper wire and jump directly from the coil kill wire terminal to the engine block. In 65 years of "tinkering" with small engines, I have never had that results from a bad coil.

Walt Conner

[bgsengine]

Not so fast, use a jumper wire and jump directly from the coil kill wire terminal to the engine block. In 65 years of "tinkering" with small engines, I have never had that results from a bad coil.

Walt Conner

Good thought - I *have* had wires that broke within the insulation - visually looked fine, but on testing, no continuity

[KE4AVB]

Not so fast, use a jumper wire and jump directly from the coil kill wire terminal to the engine block. In 65 years of "tinkering" with small engines, I have never had that results from a bad coil.

Walt Conner

Then Walt you haven't been in my shop then. I have replace about 10 units since '09 for failed kill terminals but as BGS said wires do break or are chew in two sometimes. That don't count the few where the engine idle fine and then the air filter off or set them fire and then get the random misfiring. The one that setting that was setting the air filter on fire was a Homelite chainsaw then was there was the Husky 570 that would shoot a fire out the intake on acceleration last year gone thing I was wearing a cotton long shirt at the time.

I also replace two Honda coils on a GXV610 last year for failing shutdown the engine. That customer been to several shops and they all said the coils could not fail this way.

So to me it is very possible.

[Fulltilt]

Yep, I've seen several that have failed in that way. Most "modules" /coils have a diode in the kill circuit. Open diode = no kill.

[KE4AVB]

Yep, I've seen several that have failed in that way. Most "modules" /coils have a diode in the kill circuit. Open diode = no kill.

If I remember correctly the diodes are external to Briggs coil and not used on single cylinders. There is not one in coil's internal trigger circuit which once was nothing more than a trigger coil, a resistor, a PNP darlington transistor and a capacitor (to provide proper Dwell time). The diode only prevents one coil's trigger coil from shutting down the other one thru it low resistance.

Some might think that the steering diode would prevent +12 vdc from applied to coil's electronic trigger but it doesn't as the coil's kill terminal is at a negative potential which the drains to ground when the diode's anode is grounded; hence, shutting down the darlington trigger transistor switching. Even with the diode in place the full positive voltage goes thru the trigger coil which not design for much current at all it burned up and sometimes even the rest of the trigger circuit as well. This is why a bad ignition switch can destroy coils.

What is happening on single Magnetron coils is that the physical connection from the kill terminal and internal electronics is breaking down; probably from a bad solder connection.

Now these electronic triggers are way more complicated nowadays with the addition of spark advance/delay timing circuits.

Now with these diodes in the kill circuit there can other problems too. One coil killing the other is a good example. I had one diode harness that had one shorted diode and the other one was leaky. The leaky diode tested good with a DVMM but showed on a regular transistor/diode tester. It was just leaky enough to cause a random misfire on a v-twin.