Johnson P35 – high ohm readings

[stanley]

Just checking out a new acquisition (P35)and am getting a reading of 22,000 ohms on the secondary winding at the coil.Using the same setting,checking several other engines,I get around 8,000.All the engines have good spark.The P35 has what looks like the original coil and has a spark that will jump more than 1/4". What is going on?

[jcrigan]

I have an Evinrude Zephyr that has coils that read around 16000 ohms. They seem to work just fine when running the motor in a barrel. The secondaries have thousands of turns of very small wire of around 40-44 guage. Age may have a factor as well as moisture.

[Randy in Tampa]

high readings like those are common on PO coils and also on many of the 1929-34 external rotary valve Johnsons and i’m not sure why that is but they seem to work just fine …. 🙂

[lyks2tinkr]

That high of a reading on the secondary could mean that the solder joints may be "cold" but still conductive or that it’s the real reading based on the amount of wire in the winding. At that reading it should give a really great spark.

[garry-in-michigan]

Those pre WW2 antiques had soft iron magnets in the flywheel which were not as robust as the Alnico magnets in later magnetos. More windings were put in the early coils to compensate for degraded magnetism. . . 😉

[Buccaneer]

That would explain why my Johnson PO-15 coil tested at
17.9 ohms on secondary, .2 ohm on primary. Seems to
have good spark, but haven’t tried running it yet!
Thanks for the explanation Garry!

Prepare to be boarded!

[bob-d]

Garry, how can you tell a Alnico magnet from the soft iron?
I have a few motors that were built right after the war (don’t know if they used prewar parts), and would like to know which one is in the motors.
Thanks.

[Author]

Posts