Can I bench test the coil on the J4 magneto with a multimeter?

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ThunderCloud
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Can I bench test the coil on the J4 magneto with a multimeter?

Postby ThunderCloud » Wed Oct 11, 2017 6:39 pm

:help:

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John *.?-!.* cub owner
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Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby John *.?-!.* cub owner » Wed Oct 11, 2017 7:47 pm

You can take resistance readings, but they are real low, problem is a multi meter only uses 1.5 volts for low resistance readings, but coils runs at as much as 40,000 volts. A few shorted turns in a winding will affect output, but would take a lab quality meter to read. If a winding is completely open it would show, but a few shorted turns would not.
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Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby inairam » Wed Oct 11, 2017 8:35 pm

you could try to spin the mag drive with a drill or small motor and check the spark.
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Rick Spivey
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Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby Rick Spivey » Wed Oct 11, 2017 9:56 pm

Ue your resistance setting, check from the nub on the coil to the wire end that connects to the points, and then from the same nub to the tang that grounds under the bolt that holds the coil in the housing. In both cases, your reading should be between 9500 and 10500 ohms. An open circuit will show as zero, and anything much over 11,000 has always been a problematic coil, in my experience.
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inairam
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Joined: Wed Jul 08, 2015 10:24 am
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Tractors Owned: 1948 6v - Dozer
1949 with kub klipper belly mower. mag 6v - Mom
1950 with plow, 54 blade, mott mag 6v - Roxanne
1953 54 blade, c22, wood 42 6v
1957 with c-2 mower & c-54 grader blade 6v - barn Queen
1965 lo-boy with c-3 mower 12 v - Loboy
1974 Horse II 12 v
1975 with homemade mower 12 v - Horse
1979 long strip 12 v stuck engine
130 with international 1000 loader 6 v
1969 140 with bush hog tow behind mower 6 v
Memberships: Rough and Tumble Engineers Historical Association;Chapter 8 IH Collectors; IH Collectors Worldwide
Circle of Safety: Y
Location: Glen Mills PA

Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby inairam » Thu Oct 12, 2017 8:01 am

Rick are your instructions for a mag or a coil?
When you only have 9 horsepower you need to know the names of all of the ponies!

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Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby JohnMihevic » Thu Oct 12, 2017 12:14 pm

If you do the above tests and find a bad coil then you know you need a new one. However, if the above tests pass, it is still possible you could have a bad coil. At 20,000 - 40,000 volts, windings can still break down under the stress of high voltage which a multimeter won't show. Also, the windings can start breaking down under higher heat conditions, when the engine has warmed up or is operating under high load.

John M.

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Re: Can I bench test the coil on the J4 magneto with a multimeter?

Postby Rick Spivey » Fri Oct 13, 2017 5:07 am

inairam wrote:Rick are your instructions for a mag or a coil?


This is a quick, but not foolproof, test on a coil from a magneto. I don't know the values for the battery ignition coil. And, as John M. rightly points out, good readings on this test DON'T guarantee the coil is not bad. It is just a first test, because bad readings here DO guarantee a new coil is needed, at minimum.
Rick Spivey
'52 Cub ("Great Personality") 148xxx
'48 Cub with FH ("Gunny Cub") 38xxx
'57 Lambretta (a slow work in progress)
'74 Triumph TR6 (Mama's toy)


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