Old American Bosch Coil Polarity

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Anyone know the polarity of the inputs on the above coil? My Car has one and the terminals aren’t marked.

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I have a photo of another, newer Bosch coil and the positive terminal is on the left with the logo upright. I am assuming this as it is all the information I have.

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Follow polarity. When I used to work in a car repair shop, many decades ago, we used an oscilloscope to look at the spark line, dwell, voltage to each spark plug.

One day, I put the scope on a small truck and the pattern on the scope was odd, at first I couldn’t figure out what I was looking at. Then I saw it: the pattern was inverted, upside down. The coil was wired backwards.

What I also saw was that the voltage to each spark plug was less when the polarity was reversed. Once I rewired the coil to correct polarity, the spark footage was greater, I don’t remember by how much, I’ll say 25% or so greater.

So, on your positive ground cars, the points are the positive pole, the ignition power wire to the ignition switch is the negative.

Hope this helps..

Greg

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Randy, all good info, but to answer your original question you can test for correct polarity of the ignition coil by using a voltmeter. Connect the negative lead to the (-) negative terminal and the positive lead to the engine block.

Volt Meter : Set the meter on the highest volt range (these connections are the same whether you have a positive ground or negative ground electrical system). The secondary winding’s polarity, which you are testing, is determined by the combined hookup of the battery and primary windings.

Crank the engine over (do not start it) and the needle of the voltmeter should show an upward swing to the plus or positive side (do not worry about taking a reading). If volt the needle swings down to the negative side and gives a negative reading, your coil is hooked up backwards. To correct the polarity, simply reverse coil primary leads.

A coil with reversed polarity will have about a twenty percent lower output which may not show up at idle and low rpms, but can cause an engine to miss or stumble under load and at higher engine rpms. Model A supplier Snyders makes a “tool”” for this they sell for $25 . part number A-12116 in there online catalog in case you want to start checking a lot of them!!! Karl”

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Great post!

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Karl,

Very Krouch-like, clear and concise!

Peter

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Thank you Karl

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The Snyders $25 unit is a nice item. HOWEVER Postage to Australia is from $62 – $110 !!!!! In Aussie Dollars that is 77 to 137!!!!! I think I will stick to Karl’s voltmeter test.

There is also a test using the point of a pencil held between the plug & HT wire. The shape of the spark tells you the polarity.

Jak.

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