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Collection, Preservation and Display of Old Lawn Mowers

Briggs and Stratton Armature Ignition Coil

Armature ignition coil part number 298506 as fitted to 3.5HP Briggs and Stratton engine type 92908.

Wire A - connects to earth
Wire B - connects to capacitor/points
Wire C - connects to spark plug.

Does anyone know the resistance values for the coil alone between
A and B
A and C
B and C



wristpin Sun, 12/04/2015

Briggs are a bit coy about publishing those values, in fact I'm not sure that they do. They have always relied on the simple test of whether the coil will produce a spark strong enough to jump a given gap in free air which would then be strong enough to jump a smaller gap under compression.

While looking through manuals etc to see whether I could find the answer to your question I did find this which may be of interest even though it refers to the later Magnetron electronic ignition rather than your points and condenser system.…

There's also a service bulletin MS-7862 which may be of interest, but again it does not directly answer your question. Possibly the way to go is to make your own measurements from a "known good" coil and use them as a comparison.

supremetwo Sun, 12/04/2015

Thanks for the theory link.

I had already measured 2.5kohms between the HT lead and the earth lead and the article confirms in the right ball park.

The resistance between the earth lead and the points lead is very low, perhaps to be expected if that winding has very few turns.

I was hoping someone with a meter has a known good coil in their spares box.

Edit:- Using a low range meter, I measured the resistance between the earth and points lead to be around 0.5 ohms.

supremetwo Tue, 14/04/2015

Useful discussion with member Jon Cruse of who said that the magneto coils rarely fail and that the points and associated connections were the most-frequent cause of ignition problems.

Thanks, Jon.

So cleaned everything up, new sleeving on all the wires and the spark is back. Motor started on the third pull.

Wristpin was right about the coyness of B&S - their response to my resistance query via their USA web site was to forward the email to the UK site who had no resistance details and advised me to take the machine to an authorised dealer for tests.

Just to sum up, my high resistance coil was 2.5 kohm and the low resistance 0.5 ohm.

The capacitor should be around 0.2 µF. If you do not have a capacitance meter, connect the capacitor to 12Vdc and check for a slow voltage decay of a high-impedance volt meter when the battery is disconnected.

gtc Wed, 15/04/2015

I find with B&S that if information isn't somewhere on their website, then it's very difficult to find other than out there in "user land".