Ignition coil resistance

[KidDingo]

Hi All,

Checked the ignition coil resistance today on my '81 FI (looks like it's the original coil).

Primary: 0.002 ohms (specs: 0.75 - 0.81 ohms)
Secondary: 10,300 ohms (specs: 10,000 - 11,000 ohms)

IAP website (http://www.international-auto.com/fiat- ... n-coil.cfm) and manual say that since the primary is out of spec, it should be replaced... but I don't understand why a lower resistance on the primary is cause for concern.

Can anyone offer any explanation?

Thanks.

[seabeelt]

Kind of technical, but I will give it a shot. As current flows through the primary coil winding, it generates an electromagnetic field. Field strength is based upon the resistance of the coil winding and the voltage applied. The field then excites the secondary coil winding and uses the field and resistance of the secondary coil to generate a voltage to create spark. If the resistance of the primary side is next to nothing , the output of the seconday side will be reduced or next to nothing. Think of it a s a power line transformer in reverse.. The power company takes high voltage lines with very little current ( current relates to wire size and power loss of the transmission line) and steps the voltage down via a transformer near your house. The stepped down voltage then is able to carry with it more current to drive all of your household devices. Now back to the coil. high current/low voltage on the input side, low current /high voltage on the output side.

[opus10583]

Kind of technical, but I will give it a shot. As current flows through the primary coil winding, it generates an electromagnetic field. Field strength is based upon the resistance of the coil winding and the voltage applied. The field then excites the secondary coil winding and uses the field and resistance of the secondary coil to generate a voltage to create spark. If the resistance of the primary side is next to nothing , the output of the seconday side will be reduced or next to nothing. Think of it a s a power line transformer in reverse.. The power company takes high voltage lines with very little current ( current relates to wire size and power loss of the transmission line) and steps the voltage down via a transformer near your house. The stepped down voltage then is able to carry with it more current to drive all of your household devices. Now back to the coil. high current/low voltage on the input side, low current /high voltage on the output side.

Hi,

Why would or wouldn't I prefer a coil with 1.5 Ohm or 3.0 Ohm primary coil winding resistance, given both have the same secondary winding resistance and output voltage rating?

Thanks,
Mark

[spider2081]

Just a thought. Most easily purchased digital ohm meters sold today have the lowest scale of approx. 0-200 ohms. They are not very accurate when reading resistance values less than a few ohms. If you car is running good I would not be worried about the reading you are seeing. The 0.75 ohms may actually be there but your meter can't accurately display it.
Generally speaking the resistance you are measuring is the resistance of the length of wire used to make the primary coil. The wire is copper with a thin coat of varnish as an insulator. As the wire is coiled each wrap touches the next wrap. The varnish keeps one wrap from electrically shorting to the wraps it touches. A pure short is 0 (zero) ohms. If the varnish breaks down and the coils short together the resistance of the primary will measure lower than spec. The output voltage at the secondary would be less than it should be. This would be evident by a visually weak spark. If your spark appear sharp and whitish and you hear it snap I would think you are good to go.

[KidDingo]

Thanks, guys. It all makes sense now.

The ohmmeter I use was set to 0 - 2 ohms, but it IS a consumer product so I understand the unreliability issues.

I'll avoid shelling out the $150 - $200 for a coil and put it towards a new starter solenoid and/or ignition switch which seem to be more problematic right now, anyway.

Thanks again.

[majicwrench]

Does it run OK? Then coil is fine.