Narrow Case AC vs DC coil

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Moto Chuck
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Joined: Sun May 18, 2014 4:16 pm
Location: Kansas, USA

Narrow Case AC vs DC coil

Postby Moto Chuck » Tue Jul 03, 2018 10:51 pm

I know that the red label coil is a DC coil for a battery setup and the green label coil is an AC coil for a magneto setup. I have several coils in the parts bin with no labels. How do you tell the difference? Have seen that there is a difference in the secondary resistance but don't know how to test the difference. Which terminals do I check?

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Joined: Thu Nov 11, 2010 1:12 am
Location: Northern Plains, USA

Re: Narrow Case AC vs DC coil

Postby Rick » Wed Jul 04, 2018 6:28 pm

The primary coil is between the 2 battery posts- probably marked '1' and '15', and the secondary is between one end of the primary and the spark plug output. The battery '+' usually goes to '15', so measure the primary between '1' and '15' and the secondary between '15' and the spark plug output. Depending on where the primary and secondary are tied together you may be adding the primary coil to the secondary resistance, but it won't be much- you can measure between the spark output and both coil posts- if one reading is significantly smaller then that's probably the secondary coil alone.

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Re: Narrow Case AC vs DC coil

Postby HMBAtrail » Wed Jul 04, 2018 7:08 pm

Aren't the AC coil terminals marked 1 and 15 and the DC terminals marked + and -?

Posts: 56
Joined: Wed Nov 28, 2012 9:53 pm

Re: Narrow Case AC vs DC coil

Postby Samurai » Wed Jul 11, 2018 5:03 pm

I've just purchased a DC coil from Classic Ducati here in the UK and the posts are indeed marked + and -, the same as the original Ducati coil that I had with the bike (175TS).

Hope I'm right in connecting the ignition feed to the + and the points connection to the -. The coil was never attached when I bought it as a project, but it seems to run OK?

What are the implications in accidentally swapping the connections over, or doesn't it really matter?

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