Measuring secondary resistance on a coil

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Fuzehead
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Joined: Tue Jan 15, 2019 2:46 pm

Measuring secondary resistance on a coil

Post by Fuzehead »

I am trying to diagnose an issue with an EFI on a dirt bike. I have investigate numerous areas and now I am looking hard at the ignition. Right now, it is the ignition coil.

This is not a CDI system, it is inductive discharge.

On the primary side, the ECU controls the negative, while the other side is tied to positive.

Here is what baffles me. I cannot get a reading off the secondary of the coil at all. From any points on the primary. From the mounting locations. Nothing, nowhere. Primary is fine, and the bike does run.

A buddy with the same bike (it is working too) did a reading off the secondary. He got infinite resistance one way, then 28M ohms with the polarity reversed.

Its like there is some electronics on the secondary side or an open gap (by design).
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TwoWaves
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Joined: Thu Apr 24, 2014 8:32 pm

Re: Measuring secondary resistance on a coil

Post by Technician »

If I am remembering this correctly from the early days of yesterday. We used to measure the primary resistance of the ignition coils and then a resistance check between primary and secondary. We used to do this on the tower type coils back in the day when we had points and condensers, ballast resistors etc, however, then came along the waist spark ignition systems, and each time I tried to establish a connection between primary and secondary, it proved impossible, however, the coils function was perfectly normal.

So again assuming I'm remembering this correctly, measure the resistance between the primary terminals of the coil, typical readings were around 0.6 Ohms I think, and then using the meter across the secondary towers, 1 & 4 and 2 & 3 we'd see a higher resistance possibly around 10K or more.

I'll have a look into my archives and see what I can dig up for a single outlet coil tower but IMHO you'd be much better using the scope to test the coils. :)

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Autonerdz
TwoWaves
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Joined: Wed Nov 20, 2002 4:19 pm
Location: Washington State USA

Re: Measuring secondary resistance on a coil

Post by Autonerdz »

Some coils have a 'tapped primary' and others do not. Typically, waste spark coils do not need a connection between the primary and secondary as the secondary is a complete circuit from one plug to the other and the engine block.

As for electronics in the secondary, yes. Many automotive COP systems (inductive discharge) have a diode in the secondary. This is to prevent the secondary from an undesired firing at the wrong time with the expanding magnetic field during coil charge. The diode makes sure the coil only fires on field collapse.

That being said, resistance checks on coils have no real practical diagnostic purpose. Coils are best tested by validating primary drive with a scope.

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