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1967 - 1972 F-100 & Larger F-Series Trucks Discuss the Bumpsides Ford Truck

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Old 12-31-2012, 11:24 AM
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Ohms in a stock coil

Can anyone tell me something about ohms in a coil? I bought a new coil at a parts store 2 weeks ago. Then I decided to buy an electronic ignition. That company says your coil must have 1.5 ohms of primary resistance. Obviously I don't want to buy another coil. Of course the electronic ignition company wants to sell their coils. I called the parts store to ask them what the ohms was on the coil. They looked in their books but could not find it.

This coil is for a 1970 F350 with a 360. On the coil are printed these words: "12V use with primary resistance wire or external resistor 067." I called the local auto electric shop that rebuilt my starter and told me my old coil was bad. They can not tell what the primary resistance is on a coil, only if it is good or bad. He recommended looking in a 1970 truck shop manual.

Can anyone help me? A Pertronix Flame Thrower coil costs $31.95. I don't know if I can take my other coil back to the parts store. It has been on the truck but driven only 4 miles......

Thanks and Happy New Year!
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Old 12-31-2012, 11:44 AM
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That coil should have next to no resistance. It's designed for an exrernal resistor.
To measure the coil's resistance is fairly simple. Get an ohmmeter touch one of the probes to the coil tower and the other probe to the + termnal.
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Old 12-31-2012, 11:56 AM
rustywheel68 rustywheel68 is offline
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you won't be able to return a used ignition coil.

i think the stock coils are 3 ohms.
to measure- just put your voltmeter leads across the + and - terminals.

what electronic ignition have you purchased?

i'm sure someone will correct me if i'm talking out of my rear here, but-
i imagine that the the directions from the ignition mean your coil must have *at least* 1.5 ohms of primary resistance.
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Old 12-31-2012, 12:34 PM
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Quote:
Originally Posted by rustywheel68 View Post
you won't be able to return a used ignition coil.

i think the stock coils are 3 ohms.
to measure- just put your voltmeter leads across the + and - terminals.

what electronic ignition have you purchased?

i'm sure someone will correct me if i'm talking out of my rear here, but-
i imagine that the the directions from the ignition mean your coil must have *at least* 1.5 ohms of primary resistance.
I ordered a Hot Spark electronic ignition kit. Everything fits under the Dist cap. On their website it says: "8-cylinder distributors require a coil with a minimum of 1.5 Ohms primary resistance or HS17HEC high-energy coil."

What do you think?
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Old 12-31-2012, 01:12 PM
rustywheel68 rustywheel68 is offline
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i looked up the Hot Spark (hadn't heard of it before, but looks like the Pertronix).

it says "works with stock coil", and also as you noted, "6- and 8-cylinder distributors require a coil with 1.5 or more Ohms primary resistance"

you'll be fine with your stock coil.

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Old 12-31-2012, 02:35 PM
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Quote:
Originally Posted by rustywheel68 View Post
i looked up the Hot Spark (hadn't heard of it before, but looks like the Pertronix).

it says "works with stock coil", and also as you noted, "6- and 8-cylinder distributors require a coil with 1.5 or more Ohms primary resistance"

you'll be fine with your stock coil.

OK, fine. I will borrow a volt meter or an ohm meter anyway. Hopefully the kit will arrive today and I can put it on tomorrow. It is supposed to be about 37 degrees F Wed.... If it works with stock coil, great; if not I guess I have to buy one from them.

thanks!
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Old 12-31-2012, 11:28 PM
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how to check ur coil
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Old 01-01-2013, 10:40 AM
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Quote:
Originally Posted by mikeo0o0o0 View Post
That coil should have next to no resistance. It's designed for an exrernal resistor.
To measure the coil's resistance is fairly simple. Get an ohmmeter touch one of the probes to the coil tower and the other probe to the + termnal.
that would measure secondary resistance- but the OP is looking for primary resistance. secondary resistance would be extremely high- more than 10,000 ohms.
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Old 01-01-2013, 03:19 PM
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I checked the 1967 Ford Truck Manual, Part 9-7, ignition system specifications.
Coil Primary Resistance 1.40- 1.54 ohms.
Coil Secondary Resistance 7600-8800 ohms.
Both at room temperature.
Being a transformer, the isolation resistance between primary and secondary should be infinite.

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Old 01-01-2013, 05:51 PM
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Originally Posted by CougarJohn View Post
I checked the 1967 Ford Truck Manual, Part 9-7, ignition system specifications.
Coil Primary Resistance 1.40- 1.54 ohms.
Coil Secondary Resistance 7600-8800 ohms.
Thanks, that is what I was looking for, something out of the shop manual from Ford. With that and the fact that Hot Spark says you can use the stock coil (if it is 1.5 ohms) I feel safe using it. I am stuck with it since I installed it on the truck. It is only about 2 weeks old and only driven 4 miles.
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Old 01-01-2013, 05:51 PM
 
 
 
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